PSEB 12th Class Physics Solutions Chapter 10 Wave Optics

Punjab State Board PSEB 12th Class Physics Book Solutions Chapter 10 Wave Optics Textbook Exercise Questions and Answers.

PSEB Solutions for Class 12 Physics Chapter 10 Wave Optics

PSEB 12th Class Physics Guide Wave Optics Textbook Questions and Answers

Question 1.
Monochromatic light of wavelength 589 nm is incident from air on a water surface. What are the wavelength, frequency and speed of (a) reflected, and (b) refracted light? The Refractive index of water is 1.33.
Answer:
Wavelength of incident monochromatic light, λ = 589 nm = 589 x 10-9 m
Speed of light in air, c = 3 x 108 m/s
Refractive index of water, µ = 1.33

(a) The ray will reflect back in the same medium as that of the incident ray. Hence, the wavelength, speed and frequency of the reflected ray will be the same as that of the incident ray.
Frequency of light is given by the relation,
v = \(\frac{c}{\lambda}=\frac{3 \times 10^{8}}{589 \times 10^{-9}}\)
= 5.09 x 1014 Hz
Hence, the speed, frequency, and wavelength of the reflected light are 3 x 108 m/s, 5.09 x 1014 Hz, and 589 nm respectively.

(b) Frequency of light does not depend on the property of the medium in which it is travelling. Hence, the frequency of the refracted ray in water will be equal to the frequency of the incident or reflected light in air.
Refracted frequency, v = 5.09 x 1014 Hz
Speed of light in water is related to the refractive index of water as
vw = \(\frac{c}{\mu}\)
vw = \(\frac{3 \times 10^{8}}{1.33} \) = 2.26 x 108 m/s
Wavelength of light in water is given by the relation,
λ = \(\frac{v_{w}}{v}=\frac{2.26 \times 10^{8}}{5.09 \times 10^{14}}\)
= 444.007 x 10-9 m
= 444.01 nm
Hence, the speed, frequency and wavelength of refracted light are 2.26 x 108 m/s, 5.09 x 1014 Hz
and 444.01 nm respectively.

Question 2.
What is the shape of the wavefront in each of the following cases:
(a) Light diverging from a point source. ;
(b) Light emerging out of a convex lens when a point source is placed at its focus.
(c) The portion of the wavefront of light from a distant star intercepted hy the Earth.
Answer:
(a) The shape of the wavefront in case of a light diverging from a point source is spherical. The wavefront emanating from a point source is shown in the given figure
PSEB 12th Class Physics Solutions Chapter 10 Wave Optics 1
(b) The shape of the wavefront in case of a light emerging out of a convex lens when a point source is placed at its focus is a plane or a parallel grid. This is shown in the given figure
PSEB 12th Class Physics Solutions Chapter 10 Wave Optics 2
(c) The portion of the wavefront of light from a distant star intercepted by the Earth is a plane.

PSEB 12th Class Physics Solutions Chapter 10 Wave Optics

Question 3.
(a) The refractive index of glass is 1.5. What is the speed of light in glass? (Speed of light in vacuum is 3.0x 108 ms-1). Is the speed of light in glass independent of the colour of light? If not, which of the two colours red and violet travels slower in a glass prism?
Answer:
(a) Refractive index of glass, µ = 1.5
Speed of light, c = 3 x 108 m/s
Speed of light in glass is given by the relation,
v = \(\frac{c}{\mu}=\frac{3 \times 10^{8}}{1.5} \) = 2 x 108 m/s
Hence, the speed of light in glass is 2 x 108 m/s.

(b) The speed of light in glass is not independent of the colour of light.
The refractive index of a violet component of white light is greater than the refractive index of a red component. Hence, the speed of violet light is less than the speed of red light in glass. Hence, violet light travels slower than red light in a glass prism.

Question 4.
In a Young’s double-slit experiment, the slits are separated by 0.28 mm and the screen is placed 1.4 m away. The distance between the central bright fringe and the fourth bright fringe is measured to be 1.2 cm. Determine the wavelength of light used in the experiment.
Answer:
Distance between the slits, d = 0.28 mm = 0.28 x 10-3 m
Distance between the slits and the screen, D = 1.4m
Distance between the central fringe and the fourth (n = 4) fringe, u = 1.2 cm = 1.2 x 10-2 m
In case of a constructive interference, we have the ‘relation for the distance between the two fringes as
u = \(n \lambda \frac{D}{d}\)

where, n = order of fringes = 4 = 4λ= wavelength of light used
∴ λ = \(\frac{u d}{n D}\)
= \(\frac{1.2 \times 10^{-2} \times 0.28 \times 10^{-3}}{4 \times 1.4}\)
= 6 x 10-7 = 600 nm
Hence, the wavelength of the light is 600 nm.

Question 5.
In Young’s double-slit experiment using monochromatic light of wavelength λ, the intensity of light at a point on the screen where path difference is λ is K units. What is the intensity of light at a point where path difference is λ / 3?
Answer:
Here, I =K when path difference = λ
I’ = ? when path difference = \(\frac{\lambda}{3}\)
We know that the intensity I is given by
I = 2I0(1 + cosΦ) ………………………….. (1)
When Φ = phase difference

When path difference is λ, let Φ be the phase difference.
∴ From relation,
Φ’ = \(\frac{2 \pi}{\lambda}\) x, we get
Φ’ = \(\frac{2 \pi}{\lambda} \cdot \lambda\) = 2π
∴From eqn.(1),
K = 2I0 (1+ cos 2π) (∵ cos 2π =1)
= 2I0(1+1)
or K = 4I0
or I0 = \(\frac{K}{4}\) ……………………………… (2)
Let Φ, be the phase difference for a path difference \(\frac{\lambda}{3}\)
∴ Φ1 = \(\frac{2 \pi}{\lambda} \times \frac{\lambda}{3}\)
= \(\frac{2 \pi}{3}\)
∴ I’ = 2I0(1+cosΦ1)
PSEB 12th Class Physics Solutions Chapter 10 Wave Optics 3
PSEB 12th Class Physics Solutions Chapter 10 Wave Optics

Question 6.
A beam of light consisting of two wavelengths, 650 mn and 520 nm, is used to obtain interference fringes in a Young’s double-slit experiment.
(a) Find the distance of the third bright fringe on the screen from the central maximum for wavelength 650 nm.
(b) What is the least distance from the central maximum where the bright fringes due to both the wavelengths coincide?
Answer:
First wavelength of the light beam, λ1 = 650 nm
Second wavelength of the light beam, λ2 = 520 nm
Distance of the slits from the screen = D
Distance between the two slits = d
(a) Distance of the nth bright fringe on the screen from the central maximum is given by the relation,
x = nλ1\(\left(\frac{D}{d}\right)\)
For third bright fringe, n = 3
∴ x = 3x 650\(\left(\frac{D}{d}\right)\) = 1950\(\left(\frac{D}{d}\right)\) nm

(b) Let the nth bright fringe due to wavelength λ2 and (n – 1)th bright fringe due to wavelength λ1 coincide on the screen. We can equate the conditions for bright fringes as nλ2 = (n-1)λ
520 n = 650 n -650
650 = 130 n
∴ n = 5
Hence, the least distance from the central maximum can be obtained by the relation
x = nλ2\(\left(\frac{D}{d}\right)\) = 5 x 520\(\left(\frac{D}{d}\right)\) = 2600\(\left(\frac{D}{d}\right)\) nm
Note : The value of d and D are not given in the question.

Question 7.
In a double-slit experiment, the angular width of a fringe is found to be 0.2° on a screen placed 1 m away. The wavelength of light used is 600 nm. What will be the angular width of the fringe if the entire experimental apparatus is immersed in water? Take refractive index of water to be 4/ 3.
Answer:
Distance of the screen from the slits, D = 1 m
The wavelength of light used, λ1 = 600 nm
Angular width of the fringe in air, θ1=0.2°
Angular width of the fringe in water = θ2
Refractive index of water, µ = \(\frac{4}{3}\)
Refractive index is related to angular width as
PSEB 12th Class Physics Solutions Chapter 10 Wave Optics 4
Therefore, the angular width of the fringe in water will reduce to 0.15°.

Question 8.
What is the Brewster angle for air to glass transition? (Refractive index of glass = 1.5)
Answer:
Refractive index of glass, µ = 1.5
Brewster angle = θ
Brewster angle is related to refractive index as
tanθ = µ
θ= tan-1 (1.5)=56.31°
Therefore, the Brewster angle for air to glass transition is 56.3 1°.

Question 9.
Light of wavelength 5000 A falls on a plane reflecting surface. What are the wavelength and frequency of the reflected light? For what angle of incidence is the reflected ray normal to the incident ray?
Answer:
Wavelength of incident light, λ = 5000 Å = 5000 x 10-10 m
Speed of light, c =3 x 108 m
Frequency of incident light is given by the relation,
v = \(\frac{c}{\lambda}=\frac{3 \times 10^{8}}{5000 \times 10^{-10}}\) = 6 x 1010 Hz

The wavelength and frequency of incident light is the same as that of reflected ray. Hence, the wavelength of reflected light is 5000 Å and its frequency is 6 x 1014 Hz. When reflected ray is normal to incident ray, the sum of the angle of incidence, ∠i and angle of reflection, ∠r is 90°.

According to the law of reflection, the angle of incidence is always equal to the angle of reflection. Hence, we can write the sum as
∠i + ∠r =90
∠i + ∠i=90
∠i = \( \frac{90}{2}\) = 45°
Therefore, the angle of incidence for the given condition is 45°.

PSEB 12th Class Physics Solutions Chapter 10 Wave Optics

Question 10.
Estimate the distance for which ray optics is a good approximation for an aperture of 4 mm and wavelength 400 nm.
Answer:
Fresnel’s distance (ZF) is the distance for which the ray optics is a good approximation. It is given by the relation,
ZF = \(\frac{a^{2}}{\lambda}\)
where,
aperture width, a = 4 mm = 4 x 10-3m
wavelength of light, λ = 400 nm = 400 x 10-9 m
ZF = \(\frac{\left(4 \times 10^{-3}\right)^{2}}{400 \times 10^{-9}}\) = 40 m
Therefore, the distance for which the ray optics is a good approximation is 40 m.

Additional Exercises

Question 11.
The 6563 Å Hα line emitted by hydrogen in a star is found to be red-shifted by 15 Å. Estimate the speed with which the star is receding from the Earth.
Answer:
Wavelength of Hα line emitted by hydrogen, λ = 6563 Å
= 6563 x 10-10 m.
Star’s red-shift, (λ’ – λ) = 15 Å = 15 x 10-10 m
Speed of light, c = 3 x 108 m/s
Let the velocity of the star receding away from the Earth be v.
The redshift is related with velocity as
PSEB 12th Class Physics Solutions Chapter 10 Wave Optics 5
Therefore, the speed with which the star is receding away from the Earth is 6.87 x105 m/s.

Question 12.
Explain how corpuscular theory predicts the speed of light in a medium, say, water, to be greater than the speed of light in vacuum. Is the prediction confirmed by experimental determination of the speed of light in water? If not, which alternative picture of light is consistent with experiment?
Answer:
According to Newton’s corpuscular theory of light, when light corpuscles strike the interface of two media from a rarer (air) to a denser (water) medium, the particles experience forces of attraction normal to the surface. Hence, the normal component of velocity increases while the component along the surface remains unchanged.
Hence, we can write the expression
c sin i = v sin r …………………………… (1)
where i = Angle of incidence
r = Angle of reflection
c = Velocity of light in air
v = Velocity of light in water

We have the relation for a relative refractive index of water with respect to air as
μ = \(\frac{v}{c}\)
Hence, equation (1) reduces to
\(\frac{v}{c}=\frac{\sin i}{\sin r}\) = μ
But, μ > 1
Hence, it can.be inferred from equation (2) that v > c. This is not possible since this prediction is opposite to the experimental results of c > v. The wave picture of light is consistent with the experimental results.

Question 13.
You have learnt in the text how Huygen’s principle leads to the laws of reflection and refraction. Use the same principle to deduce directly that a point object placed in front of a plane mirror produces a virtual image whose distance from the mirror is equal to the object’s distance from the mirror.
Answer:
Let an object at 0 be placed in front of a plane mirror MO’ at a distance r (as shown in the given figure).
PSEB 12th Class Physics Solutions Chapter 10 Wave Optics 6
A circle is drawn from the centre (0) such that it just touches the plane mirror at point 0′. According to Huygen’s principle, XY is the wavefront of incident light. If the mirror is absent, then a similar wavefront X’ Y’ (as XT) would form behind 0′ at distance r (as shown in the given figure).
PSEB 12th Class Physics Solutions Chapter 10 Wave Optics 7
X’ Y’ can be considered as a virtual reflected ray for the plane mirror. Hence, a point object placed in front of the plane mirror produces a virtual image whose distance from the mirror is equal to the object distance (r).

PSEB 12th Class Physics Solutions Chapter 10 Wave Optics

Question 14.
Let us list some of the factors, which could possibly influence the speed of wave propagation :
(i) nature of the source.
(ii) direction of propagation.
(iii) motion of the source and/or observer.
(iv) wavelength.
(v) intensity of the wave.
On which of these factors, if any, does
(a) the speed of light in vacuum,
(b) the speed of light in a medium (say, glass Or water), depend?
Answer:
(a) The speed of light in a vacuum i. e., 3 x 108 m/s (approximately) is a universal constant. It is not affected by the motion of the source, the observer, or both. Hence, the given factor does not affect, the speed of light in a vacuum.
(b) Out of the listed factors, the speed of light in a medium depends on the wavelength of light in that medium.

Question 15.
For sound waves, the Doppler formula for frequency shift differs slightly between the two situations : (i) source at rest; observer moving, and (ii) source moving; observer at rest. The exact Doppler formulas for the case of light waves in a vacuum are, however, strictly identical for these situations. Explain why this should be so. Would you expect the formulas to be strictly identical for the two situations in the case of light travelling in a medium?
Answer:
No, sound waves can propagate only through a medium. The two given situations are not scientifically identical because the motion of an observer relative to a medium is different in the two situations. Hence, the Doppler formulas for the two situations cannot be the same.

In the case of light waves, sound can travel in a vacuum. In a vacuum, the above two cases are identical because the speed of light is independent of the motion of the observer and the motion of the source. When light travels in a medium, the above two cases are not identical because the speed of light depends on the wavelength of the medium.

Question 16.
In a double-slit experiment using light of wavelength 600 nm, the angular width of a fringe formed on a distant screen is 0.1°. What is the spacing between the two slits?
Answer:
Wavelength of light used, λ = 600 nm = 600 x 10-9 m
Angular width of fringe, θ = 0.1° = 0.1 x \(\frac{\pi}{180}=\frac{3.14}{1800}\)rad
Angular width of a fringe is related to slit spacing (d) as
θ = \(\frac{\lambda}{d}\)
PSEB 12th Class Physics Solutions Chapter 10 Wave Optics 8
Therefore, the spacing between the two slits is 3.44 x 10-4 m.

Question 17.
Answer the following questions:
(a) In a single slit diffraction experiment, the width of the slit is made double the original width. How does this affect the size and intensity of the central diffraction band?
(b) In what way is diffraction from each slit related to the interference pattern in a double-slit experiment?
(c) When a tiny circular obstacle is placed in the path of light from a distant source, a bright spot is seen at the centre of the shadow of the obstacle. Explain why?
(d) Two students are separated by a 7 m partition wall in a room 10 m high. If both light and sound waves can bend around obstacles, how is it that the? students are unable to see each other even though they can converse easily.
(e) Ray optics is based on the assumption that light travels in a straight line. Diffraction effects (observed when light propagates through small apertures/slits or around small obstacles) disprove this assumption. Yet the ray optics assumption is so commonly used in an understanding of location and several other properties of images in optic instruments. What is the justification?
Answer:
(a) In a single slit diffraction experiment, if the width of the slit is made double the original width, then the size of the central diffraction band reduces to half and the intensity of the central diffraction band increase up to four times.

(b) The interference pattern in a double-slit experiment is modulated by diffraction from each slit. The pattern is the result of the interference of the diffracted wave from each slit.

(c) When a tiny circular obstacle is placed in the path of light from a distant source, a bright spot is seen at the centre of the shadow of the obstacle. This is because light waves are diffracted from the edge of the circular obstacle, which interferes constructively at the centre of the shadow. This constructive interference produces a bright spot.

(d) Bending of waves by obstacles by a large angle is possible when the size of the obstacle is comparable to the wavelength of the waves. On the one hand, the wavelength of the light waves is too small in comparison to the size of the obstacle. Thus, the diffraction angle will be very small. Hence, the students are unable to see each other. On the other hand, the size of the wall is comparable to the wavelength of the sound waves. Thus, the bending of the waves takes place at a large angle. Hence, the students are able to hear each other.

(e) The justification is that in ordinary optical instruments, the size of the aperture involved is much larger than the wavelength of the light used.

Question 18.
Two towers on top of two hills are 40 km apart. The line joining them passes 50 m above a hill halfway between the towers. What is the longest wavelength of radio waves, which can be sent between the towers without appreciable diffraction effects?
Answer:
Distance between the towers, d = 40 km
Height of the line joining the hills, d = 50 m
Thus, the radial spread of the radio waves should not exceed 50 km.
Since the hill is located halfway between the towers, Fresnel’s distance can be obtained as
ZP = 20 km = 20 x 103m
Aperture can be taken as
a = d= 50 m

Fresnel’s distance is given by the relation,
Zp = \(\frac{a^{2}}{\lambda}\)
where, λ = wavelength of radio waves
∴ λ = \(\frac{a^{2}}{Z_{P}}\)
= \(\frac{(50)^{2}}{20 \times 10^{3}}\) = 1250 x 10-4 = 0.1250 m
= 12.5 cm
Therefore, the wavelength of the radio waves is 12.5 cm.

PSEB 12th Class Physics Solutions Chapter 10 Wave Optics

Question 19.
A parallel beam of light of wavelength 500 nm falls on a narrow slit and the resulting diffraction pattern is observed on a screen 1 m away. It is observed that the first minimum is at a distance of 2.5 mm from the centre of the screen. Find the width of the slit.
Answer:
Wavelength of light beam, λ = 500 nm = 500 x 10-9 m
Distance of the screen from the slit, D=1m
For first minima, n = 1
Distance between the slits = d
Distance of the first minimum from the centre of the screen can be obtained as
x = 2.5mm = 2.5 x 10-3 m
It is related to the order of minima as
PSEB 12th Class Physics Solutions Chapter 10 Wave Optics 9
Therefore, the width of the slits is 0.2 mm.

Question 20.
Answer the following questions :
(a) When a low flying aircraft passes overhead, we sometimes notice a slight shaking of the picture on our TV screen. Suggest a possible explanation.
(b) As you have learnt in the text, the principle of linear superposition of wave displacement is basic to understanding intensity distributions in diffraction and interference patterns. What is the justification of this principle?
Answer:
(a) Weak radar signals sent by a low flying aircraft can interfere with the TV signals received by the antenna. As a result, the TV signals may get distorted. Hence, when a low flying aircraft passes overhead, we sometimes notice a slight shaking of the picture on our TV screen.

(b) The principle of linear superposition of wave displacement is essential to our understanding of intensity distributions and interference patterns. This is because superposition follows from the linear character of a differential equation that governs wave motion. If y1 and y2 are the solutions of the second-order wave equation, then any linear combination of y± and y2 will also be the solution of the wave equation.

Question 21.
In deriving the single slit diffraction pattern, it was stated that the intensity is zero at angles of n λ/a. Justify this by suitably dividing the slit to bring out the cancellation.
Answer:
Consider that a single slit of width d is divided into n smaller slits.
∴ Width of each slit, d’ = \(\frac{d}{n}\)
Angle of diffraction is given by the relation,
θ = \(\frac{\frac{d}{d^{\prime}} \lambda}{d}=\frac{\lambda}{d^{\prime}} \)
Now, each of these infinitesimally small slit sends zero intensity in direction θ. Hence, the combination of these slits will give zero intensity.

PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter

Punjab State Board PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter Important Questions and Answers.

PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter

Very short answer type questions

Question 1.
What is the angle of dip at a place where the horizontal and vertical components of the earth’s magnetic field are equal?
Answer:
The angle of dip is given by
θ = tan-1 (\(\frac{B_{V}}{B_{H}}\))
BV = vertical component of the earth’s magnetic field.
BH = horizontal component of the earth’s magnetic field.
So, as BV = BH
Then, θ = tan-1 (1) = 45°
∴ The angle of dip will be θ = 45°.

Question 2.
The motion of copper plate is damped when it is allowed to oscillate between the two poles of a magnet. What is the cause of this damping?
Answer:
As the copper plates oscillate in the magnetic field between the two plates of the magnet, there is a continuous change of magnetic flux linked with the pendulum. Due to this, eddy currents are set up in the copper plate which try to oppose the motion of the pendulum according to the Lenz’s law and finally bring it to rest.

PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter

Question 3.
Relative permeability of a material μr = 0.5. Identify the nature of the magnetic material and write its relation of magnetic susceptibility.
Answer:
The nature of magnetic material is a diamagnetic.
μr = 1 + χm

Question 4.
Which of the following substances are diamagnetic?
Bi, Al, Na, Cu, Ca and Ni
Answer:
Diamagnetic substances are (i) Bi (ii) Cu.

Question 5.
The susceptibility of a magnetic material is -4.2 × 10-6. Name the type of magnetic material, it represents.
Answer:
Negative susceptibility represents diamagnetic substance.

Question 6.
What are permanent magnets? Give one example.
Answer:
Substances that retain their attractive property for a long period of time at room temperature are called permanent magnets.
Examples: Those pieces which are made up of steel, alnico, cobalt and ticonal.

PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter

Question 7.
Why is the core of an electromagnet made of ferromagnetic materials?
Answer:
Ferromagnetic material has a high retentivity. So on passing current through windings it gains sufficient magnetism immediately.

Question 8.
From molecular view point, discuss the temperature dependence of susceptibility for diamagnetism, paramagnetism and ferromagnetism. (NCERT Exemplar)
Answer:
Diamagnetism is due to orbital motion of electrons developing magnetic moments opposite to applied field and hence is not much affected by temperature.

Paramagnetism and ferromagnetism is due to alignments of atomic magnetic moments in the direction of the applied field. As temperature increases, this alignment is disturbed and hence susceptibilities of both decrease as temperature increases.

Question 9.
A ball of superconducting material is dipped in liquid nitrogen and placed near a bar magnet.
(i) In which direction will it move?
(ii) What will be the direction of its magnetic moment? (NCERT Exemplar)
Answer:
When a diamagnetic material is dipped in liquid nitrogen, it again behaves as a diamagnetic material. Thus, superconducting material will again behave as a diamagnetic material. When this diamagnetic material is placed near a bar magnet, it will be feebly magnetised opposite to the direction of magnetising field.

(i) Thus, it will be repelled.
(ii) Also its direction of magnetic moment will be opposite to the direction of magnetic field of magnet.

PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter

Question 10.
Consider the plane S formed by the dipole axis and the axis of earth. Let P be point on the magnetic equator and in S. Let Q be the point of intersection of the geographical and magnetic equators. Obtain the declination and dip angles at P and Q. (NCERT Exemplar)
Answer:
In adjoining figure:
PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter 1
(i) P is in S (needle will point both north)
Declination = 0
P is also on magnetic equator.
∴ Dip = 0

(ii) Q is on magnetic equator.
∴ Dip = 0
But declination = 11.3.

Short answer type questions

Question 1.
Explain the following:
(i) Why do magnetic field lines form continuous closed loops?
(ii) Why are the field lines repelled (expelled) when a
diamagnetic material is placed in an external uniform magnetic field?
Answer:
PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter 2
(i) Magnetic lines of force form continuous closed loops because a magnet is always a dipole and as a result, the net magnetic flux of a magnet is always zero.
(ii) When a diamagnetic substance is placed in an external magnetic field, a feeble magnetism is induced in opposite direction. So, magnetic lines of force are repelled.

Question 2.
How does a circular loop carrying current behaves as a magnet?
Answer:
The current round in the face of the coil is in anti-clockwise direction, then this behaves like a North pole, whereas when it viewed from other scale, then current round in it is in clockwise direction necessarily forming South pole of magnet.
PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter 3
Hence, current loop have both magnetic poles and therefore, behaves like a magnetic dipole.

PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter

Question 3.
Give two points to distinguish between a paramagnetic and diamagnetic substance.
Answer:

Paramagnetic substance Diamagnetic substance
1. A paramagnetic substance is feebly attracted by magnet. A diamagnetic substance is feebly repelled by a magnet.
2. For a paramagnetic substance, the intensity of magnetisation has a small positive value. For a diamagnetic substance, the intensity of magnetism has a small negative value.

Question 4.
(a) How is an electromagnet different from a permanent magnet?
Write two properties of a material which makes it suitable for making (i) a permanent magnet, and (ii) an electromagnet.
Answer:
(a) An electromagnet consists of a core made of a ferromagnetic material placed inside a solenoid. It behaves like a strong magnet when current flows through the solenoid and effectively loses its magnetism when the current is switched off.

A permanent magnet is also made up of a ferromagnetic material but it retains its magnetism at room temperature for a long time after being magnetised one.

(b) Properties of material are as below:
(i) Permanent magnet

  • Retentivity and coercivity should be large
  • Magnetically hard

(ii) An electromagnet

  • Magnetically soft
  • Coercivity should be low.

Question 5.
A bar magnet of magnetic moment M and moment of inertia I (about centre, perpendicular to length) is cut into two equal pieces, perpendicular to length. Let T be the period of oscifiations of the original magnet about an axis through the mid-point, perpendicular to length, in a magnetic field B. What would be the similar period T’ for each piece? (NCERT Exemplar)
Answer:
Given, I = moment of inertia of the bar magnet
m = mass of bar magnet
l = length of magnet about an any passing through its centre and perpendicular to its length
M = magnetic moment of the magnet
B = uniform magnetic field in which magnet is oscillating, we get time period of oscillation is
T = 2π\(\sqrt{\frac{I}{M B}}\)
Here I = \(\frac{m l^{2}}{12}\)
When magnet is cut into two equal pieces, perpendicular to length, then moment of inertia of each piece of magnet about an axis perpendicular to length passing through its centre is
PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter 4

PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter

Question 6.
A uniform conducting wire of length 12 a and resistance R is wound up as a current carrying coil in the shape of (i) an equilateral triangle of side a; (ii) a square of sides a and, (iii) a regular hexagon of sides a. The coil is connected to a voltage source V0. Find the magnetic moment of the coils in each case. (NCERT Exemplar)

(i) Area of equilateral triangle, A = \(\frac{\sqrt{3}}{4}\) a2
(ii) Area of square, A = a2
PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter 5

Long answer type questions

Question 1.
Verify the Ampere’s law for magnetic field of a point dipole of dipole moment M = Mk̂. Take C as the closed curve running clockwise along
(i) the z-axis from z a > 0 to z = R,
(ii) along the quarter circle of radius R and centre at the origin in the first quadrant of vz-plane,
(iii) along the x-axis from x = R to x = a, and
(iv) along the quarter circle of radius a and centre at the origin
in the first quadrant of xz-plane (NCERT Exemplar)
Answer:
From P to Q, every point on the z-axis lies at the axial line of magnetic
dipole of moment \(\vec{M}\). Magnetic field induction at a point distance z from the magnetic dipole of moment is
PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter 6

(ii) Along the quarter circle QS of radius R as-.given in the figure below
PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter 7
The point A lies on the equatorial line of the magnetic dipole of moment M sin0. Magnetic field at point A on the circular arc is
B = \(\frac{\mu_{0}}{4 \pi} \frac{M \sin \theta}{R^{3}}\) ; \(\overrightarrow{d l}\) = Rdθ

PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter
PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter 8
(iii) Along x-axis over the path ST, consider the figure given below.
PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter 9
From figure, every point lies on the equatorial line of magnetic dipole. Magnetic field induction at a point distance x from the dipole is
PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter 10
(iv) Along the quarter circle TP of radius a. Consider the figure given below
PSEB 12th Class Physics Important Questions Chapter 5 Magnetism and Matter 11

PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter

Punjab State Board PSEB 12th Class Physics Book Solutions Chapter 5 Magnetism and Matter Textbook Exercise Questions and Answers.

PSEB Solutions for Class 12 Physics Chapter 5 Magnetism and Matter

PSEB 12th Class Physics Guide Magnetism and Matter Textbook Questions and Answers

Question 1.
Answer the following questions regarding earth’s magnetism:
(a) A vector needs three quantities for its specification. Name the three independent quantities conventionally used to specify the earth’s magnetic field.

(b) The angle of dip at a location in southern India is about 18°
Would you expect a greater or smaller dip angle in Britain?

(c) If you made a map of magnetic field lines at Melbourne in Australia, would the lines seem to go into the ground or come out of the ground?

(d) In which direction would a compass free to move in the vertical plane point to, if located right on the geomagnetic north or south pole?

(e) The earth’s field, it is claimed, roughly approximates the field due to a dipole of magnetic moment 8 × 1022 JT-1 located at its centre. Check the order of magnitude of this number in some way.

(f) Geologists claim that besides the main magnetic N-S poles, there are several local poles on the earth’s surface oriented in different directions. How is such a thing possible at all?
Answer:
(a) The three independent quantities conventionally used for specifying earth’s magnetic field are magnetic declination, angle of dip and horizontal component of earth’s magnetic field.

(b) The angle of dip at a point depends on how far the point is located with respect to the North Pole or the South Pole. The angle of dip would be greater in Britain (it is about 70°) than in southern India because the location of Britain on the globe is closer to the magnetic North Pole.

(c) It is hypothetically considered that a huge bar magnet is dipped inside earth with its North Pole near the geographic South Pole and its South Pole near the geographic North Pole.

Magnetic field lines emanate from a magnetic North Pole and terminate at a magnetic South Pole. Hence, in a map depicting earth’s magnetic field lines, the field lines at Melbourne, Australia would seem to come out of the ground.

(d) If a compass is located on the geomagnetic North Pole or South Pole, then the compass will be free to move in the horizontal plane while earth’s field is exactly vertical to the magnetic poles. In such a case, the compass can point in any direction.

(e) Magnetic moment, M = 8 × 1022 JT-1
Radius of earth, r = 6.4 × 106 m
Magnetic field strength, B = \(\frac{\mu_{0} M}{4 \pi r^{3}}\)
∴ B = \(\frac{4 \pi \times 10^{-7} \times 8 \times 10^{22}}{4 \pi \times\left(6.4 \times 10^{6}\right)^{3}}\) = 0.3G
This quantity is of the order of magnitude of the observed field on earth.

(f) Yes, there are several local poles on earth’s surface oriented in different directions. A magnetised mineral deposit is an example of a local N-S pole.

PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter

Question 2.
Answer the following questions :
(a) The earth’s magnetic field varies from point to point in space. Does it also change with time? If so, on what time scale does it change appreciably?

(b) The earth’s core is known to contain iron. Yet geologists do not regard this as a source of the earth’s magnetism. Why?

(c) The charged currents in the outer conducting regions of the earth’s core are thought to be responsible for earth’s magnetism. What might be the ’battery’ (i.e., the source of energy) to sustain these currents?

(d) The earth may have even reversed the direction of its field several times during its history of 4 to 5 billion years. How can geologists know about the earth’s field in such distant past?

(e) The earth’s field departs from its dipole shape substantially at large distances (greater than about 30,000 km). What agencies may be responsible for this distortion?

(f) Interstellar space has an extremely weak magnetic field of the order of 10-12 T. Can such a weak field he of any significant consequence? Explain.
[Note: Exercise 5.2 is meant mainly to arouse your curiosity. Answers to some questions above are tentative or unknown. Brief answers wherever possible are given at the end. For details, you should consult a good text on geomagnetism.]
Answer:
(a) Earth’s magnetic field changes with time. It takes a few hundred years to change by an appreciable amount. The variation in earth’s magnetic field with the time cannot be neglected.

(b) Earth’s core contains molten iron. This form of iron is not ferromagnetic. Hence, this is not considered as a source of earth’s magnetism.

(c) The radioactivity in earth’s interior is the source of energy that sustains the currents in the outer conducting regions of earth’s core. These charged currents are considered to be responsible for earth’s magnetism.

(d) Earth reversed the direction of its field several times during its history of 4 to 5 billion years. These magnetic fields got weakly recorded in rocks during their solidification. One can get clues about the geomagnetic history from the analysis of this rock magnetism.

(e) Earth’s field departs from its dipole shape substantially at large distances (greater than about 30,000 km) because of the presence of the ionosphere. In this region, earth’s field gets modified because of the field of single ions. While in motion, these ions produce the magnetic field associated with them.

(f) An extremely weak magnetic field can bend charged particles moving in a circle. This may not be noticeable for a large radius path. With reference to the gigantic interstellar space, the deflection can affect the passage of charged particles.

Question 3.
A short bar magnet placed with its axis at 30° with a uniform external magnetic field of 0.25 T experiences a torque of magnitude equal to 4.5 × 10-2 J. What is the magnitude of magnetic moment of the magnet?
Answer:
Magnetic field strength, B = 0.25 T
Torque on the bar magnet, τ = 4.5 × 10-2J
Angle between the bar magnet and the external magnetic field, θ = 30°
Torque is related to magnetic moment (M) as
τ = MB sinθ
∴ M = \(\frac{\tau}{B \sin \theta}\)
= \(\frac{4.5 \times 10^{-2}}{0.25 \times \sin 30^{\circ}}\)
\(\frac{4.5 \times 10^{-2} \times 2}{0.25 \times 1}\)
(∵ sin30° = \(\frac{1}{2}\))
= 0.36 JT-1
Hence, the magnetic moment of the magnet is 0.36 JT-1.

PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter

Question 4.
A short bar magnet of magnetic moment m = 0.32 JT-1 is placed in a uniform magnetic field of 0.15 T. If the bar is free to rotate in the plane of the field, which orientation would correspond to its (a) stable, and (b) unstable equilibrium? What is the potential energy of the magnet in each case?
Answer:
Given, M = 0.32 JT-1, B = 0.15T,U = ?
(a) Stable Equilibrium: The magnetic moment should be parallel to the magnetic field. In this position, the potential energy is
U = -MB cos θ =0.32 × 0.15 × 1
= -0.048 J or-4.8 × 10-2 J\

(b) Unstable Equilibrium: The magnetic moment should be antiparallel to the magnetic field. In this position, the potential energy is
U = -MBcosθ = 0.32 × 0.15 × (-1)
= +0.048 J or + 4.8 × 10-2 J

Question 5.
A closely wound solenoid of 800 turns and area of cross-section 2.5 × 10-4 m2 carries a current of 3.0 A. Explain the sense in which the solenoid acts like a bar magnet. What is its associated magnetic moment?
Answer:
Solenoid acts as a bar magnet, its magnetic moment is along the axis of the solenoid, the direction determined by the sense of flow of current. The magnetic moment of a current carrying loop having N turns
= NIA = 800 × 3 × 2.5 × 10-4
= 6 × 10-1
= 0.60 A-m2
= 0.60 JT-1

Question 6.
If the solenoid in Exercise 5.5 is free to turn about the vertical direction and a uniform horizontal magnetic field of 0.25 T is applied, what is the magnitude of torque on the solenoid when its axis makes an angle of 30° with the direction of applied field?
Answer:
Magnetic field strength, B = 0.25 T
Magnetic moment, M = 0.6 JT-1
The angle θ, between the axis of the solenoid and the direction of the applied field is 30°.
Therefore, the torque acting on the solenoid is given as
τ = MB sinθ
= 0.6 × 0.25 × sin30°
= 0.6 × 0.25 × \(\frac{1}{2}\)
= 0.075 N-m

PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter

Question 7.
A bar magnet of magnetic moment 1.5 J T-1 lies aligned with the direction of a uniform magnetic field of 0.22 T.
(a) What is the amount of work required by an external torque to turn the magnet so as to align its magnetic moment:
(i) normal to the field direction,
(ii) opposite to the field direction?

(b) What is the torque on the magnet in cases (i) and (ii)?
Answer:
Given, M = 1.5 JT-1,B = 0.22 T,θ1 =0°

(a) To align the dipole normal to the field direction θ2 = 90°. Therefore,
W = MB(cosθ1 – cosθ2)
W = 1.5 × 0.22(cos0° – cos90°) = 0.33 J
Also, τ = MB sinθ2
or τ = 1.5 × 0.22sin90° = 0.33 Nm

(b) To align the dipole opposite to the field direction θ2 = 180°. Therefore,
W =MB(cosθ1 – cosθ2)
W = 1.5 × 0.22(cos0° – cos180°) = 0.66 J
Also, τ = MB sinθ2
or τ = 1.5 × 0.22sinl80° = 0 Nm

Question 8.
A closely wound solenoid of2000 turns and area of cross-section 1.6 × 10-4 m2, carrying a current of 4.0 A, is suspended through its centre allowing it to turn in a horizontal plane.
(a) What is the magnetic moment associated with the solenoid?
(b) What is the force and torque on the solenoid if a uniform horizontal magnetic field of 7.5 × 10-2 T is set up at an angle of 30° with the axis of the solenoid?
Answer:
Number of turns on the solenoid, N = 2000
Area of cross-section of the solenoid, A = 1.6 × 10-4 m2</sup
Current in the solenoid, I = 4 A

(a) Let M = magnetic moment of the solenoid.
∴ Using the relation M = NIA, we get
M = 2000 × 4.0 × 1.6 × 10-4</sup
= 1.28 JT-1</sup

The direction of \(\vec{M}\) is along the axis of the solenoid in the direction related to the sense of current according to right-handed screw rule.

(b) Here θ = 30°
\(\vec{B}\) = 7.5 × 10-2 T
Let F = force on the solenoid = ?
τ = torque on the solenoid = ?
The solenoid behaves as a bar magnet placed in a uniform magnetic field, so the force is
F = m \(\vec{B}\) + (-m \(\vec{B}\)) = 0
where m = pole strength of the magnet.
PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter 1
Using the relation, τ = MB sinθ, we get
τ = 1.28 × 7.5 × 10-2 × sin30°
= 1.28 × 7.5 × 10-2 × \(\frac{1}{2}\) = 0.048 J
The direction of the torque is such that it tends to align the axis of the solenoid (i. e., magnetic moment vector \(\vec{M}\)) along \(\vec{B}\).

PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter

Question 9.
A circular coil of 16 turns and radius 10 cm carrying a current of 0.75 A rests with its plane normal to an external field of magnitude 5.0 × 10-2 T. The coil is free to turn about an axis in its plane perpendicular to the field direction. When the coil is turned slightly and released, it oscillates about its stable equilibrium with a frequency of 2.0 s-1. What is the moment of inertia of the coil about its axis of rotation?
Answer:
Number of turns in the circular coil, N = 16
Radius of the coil, r = 10cm = 0.1m
Cross-section of the coil, A = πr2 = π × (0.1)2 m2
Current in the coil, I = 0.75 A
Magnetic field strength, B = 5.0 × 10-2 T
Frequency of oscillations of the coil, v = 2.0 s-1
∴ Magnetic moment, M = NIA = 16 × 0.75 × π × (0.1)2 = 0.377 JT-1
Frequency is given by the relation
v = \(\frac{1}{2 \pi} \sqrt{\frac{M B}{I}}\)
where, I = Moment of inertia of the coil
I = \(\frac{M B}{4 \pi^{2} v^{2}}\) = \(\frac{0.377 \times 5 \times 10^{-2}}{4 \pi^{2} \times(2)^{2}}\)
= 1.19 × 10-4 kg m2
Hence, the moment of inertia of the coil about its axis of rotation is 1.19 × 10-4 kg m2.

Question 10.
A magnetic needle free to rotate in a vertical plane parallel to the magnetic meridian has its north tip pointing down at 22° with the horizontal. The horizontal component of the earth’s magnetic field at the place is known to be 0.35 G. Determine the magnitude of the earth’s magnetic field at the place.
Answer:
Horizontal component of earth’s magnetic field, BH = 0.35 G
Angle made by the needle with the horizontal plane
= Angle of dip = δ = 22°
Earth’s magnetic field strength = B
We can relate B and BH as
BH = B cosδ
∴ B = \(\frac{B_{H}}{\cos \delta}=\frac{0.35}{\cos 22^{\circ}}=\frac{0.35}{0.9272}\) = 0.377 G
Hence, the strength of earth’s magnetic field at the given location is 0.377 G.

Question 11.
At a certain location in Africa, a compass points 12° west of the geographic north. The north tip of the magnetic needle of a dip circle placed in the plane of magnetic meridian points 60° above the horizontal. The horizontal component of the earth’s field is measured to he 0.16 G. Specify the direction and magnitude of the earth’s field at the location.
Answer:
Angle of declination, θ = 12°
Angle of dip, δ = 60°
Horizontal component of earth’s magnetic field, BH = 0.16 G
Earth’s magnetic field at the given location = B
We can relate B and BH as
BH = B cosδ
B= \(\frac{B_{H}}{\cos \delta}\) = \(=\frac{0.16}{\cos 60^{\circ}}\) = \(\frac{0.16}{\left(\frac{1}{2}\right)}\) = 0.16 × 2 = 0.32 G
Earth’s magnetic field lies in the vertical plane, 12° west of the geographic meridian, making an angle of 60° (Upward) with the horizontal direction. Its magnitude is 0.32 G.

PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter

Question 12.
A short bar magnet has a magnetic moment of 0.48 JT-1. Give the direction and magnitude of the magnetic field produced by the magnet at a distance of 10 cm from the centre of the magnet on (a) the axis, (b) the equatorial lines (normal bisector) of the magnet.
Answer:
Magnetic moment of the bar magnet, M = 0.48 JT-1
Distance, d = 10cm = 0.1m

(a) The magnetic field at distance d, from the centre of the magnet on the axis is given by the relation,
B = \(\frac{\mu_{0}}{4 \pi} \frac{2 M}{d^{3}}\)
∴ B = \(\frac{4 \pi \times 10^{-7} \times 2 \times 0.48}{4 \pi \times(0.1)^{3}}\)
= 0.96 × 10-4 T = 0.96 G
The magnetic field is along the S-N direction.

(b) The magnetic field at a distance of 10 cm (i.e., d = 0.1 m) on the equatorial line of the magnet is given as,
B = \(\frac{\mu_{0} \times M}{4 \pi \times d^{3}}\)
∴ B = \(\frac{4 \pi \times 10^{-7} \times 0.48}{4 \pi \times(0.1)^{3}}\) = 0.48G
The magnetic field is along the N-S direction.

Question 13.
A short bar magnet placed in a horizontal plane has its axis aligned along the magnetic north-south direction. Null points are found on the axis of the magnet at 14 cm from the centre of the magnet. The earth’s magnetic field at the place is 0.36 G and the angle of dip is zero. What is the total magnetic field on the normal bisector of the magnet at the same distance as the null-point (i. e., 14 cm) from the centre of the magnet? (At null points, field due to a magnet is equal and opposite to the horizontal component of earth’s magnetic field.)
Answer:
Distance of the null point from the centre of magnet
d = 14 cm = 0.14 m
The earth’s magnetic field where the angle of dip is zero, is the horizontal component of earth’s magnetic field. i.e., H = 0.36 G
Initially, the null points are on the axis of the magnet. We use the formula of magnetic field on axial line (consider that the magnet is short in length).
B1 = \(\frac{\mu_{0}}{4 \pi} \cdot \frac{2 m}{d^{3}}\)
This magnetic field is equal to the horizontal component of earth’s magnetic field.
i.e., B1 = \(\frac{\mu_{0}}{4 \pi} \cdot \frac{2 m}{d^{3}}\) = H ……….(1)
On the equitorial line of magnet at same distance (d) magnetic field due to the magnet
B2 = \(\frac{\mu_{0}}{4 \pi} \cdot \frac{m}{d^{3}}=\frac{B_{1}}{2}=\frac{H}{2}\) …………….. (2)
The total magnetic field on equitorial line at this point (as given in question)
B = B2 + H = \(\frac{H}{2}\) + H = \(\frac{3}{2}\)H = \(\frac{3}{2}\) × 0.36 = 0.54G
The direction of magnetic field is in the direction of earth’s field.

Question 14.
If the bar magnet in exercise 5.13 is turned around by 180°, where will the new null points be located?
Answer:
When the bar magnet is turned by 180°, then the null points are obtained on the equitorial line.
So, magnetic field on the equitorial line at distance d’ is
B’ = \(\frac{\mu_{0}}{4 \pi} \cdot \frac{m}{d^{3}}\)
This magnetic field is equal to the horizontal component of earth’s magnetic field
B’ = \(\frac{\mu_{0}}{4 \pi} \cdot \frac{m}{d^{3}}\) = H ………… (1)
From Q.No. 13 MISS
Magnetic field B1 = \(\frac{\mu_{0}}{4 \pi} \cdot \frac{2 m}{d^{3}}\) = H ………….. (2)
From eqs. (1) and (2), we get
PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter 2
Thus, the null points are located on the equitorial line at a distance of 11.1 cm.

PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter

Question 15.
A short bar magnet of magnetic moment 5.25 × 10-2 JT-1 is placed with its axis perpendicular to the earth’s field direction. At what distance from the centre of the magnet, the resultant field is inclined at 45° with earth’s field on (a) its normal bisector and (b) its axis. Magnitude of the earth’s field at the place is given to be 0.42 G. Ignore the length of the magnet in comparison to the distances involved.
Answer:
Given, magnetic moment m = 5.25 × 10-2 J/T
Let the resultant magnetic field is Bnet. It makes an angle of 45° with Be.
∴ Be = 0.42G =0.42 × 10-4 T
(a) At normal bisector
Let r is the distance between axial line and point P.
The magnetic field at point P, due to a short magnet
B = \(\frac{\mu_{0}}{4 \pi} \cdot \frac{m}{r^{3}}\) …………. (1)
PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter 3
The direction of B is along PB, i.e., along N pole to S pole.
According to the vector analysis,
PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter 4
r = 0.05 m
r = 5 cm

(b) When point lies on axial line
Let the resultant magnetic field Bnet makes an angle 45° from Be. The magnetic field on the axial line of the magnet at a distance of r from the centre of magnet
B’ = \(\frac{\mu_{0}}{4 \pi} \cdot \frac{2 m}{r^{3}}\) (S to N)
PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter 5
Direction of magnetic field is from S to N.
According to the vector analysis,
PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter 6

PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter

Question 16.
Answer the following questions :
(a) Why does a paramagnetic sample display greater magnetisation (for the same magnetising field) when cooled?

(b) Why is diamagnetism, in contrast, almost independent of temperature?

(c) If a toroid uses bismuth for its core, will the field in the core be (slightly) greater or (slightly) less than when the core is empty?

(d) Is the permeability of a ferromagnetic material independent of the magnetic field? If not, is it more for lower or higher fields?

(e) Magnetic field lines are always nearly normal to the surface of a ferromagnet at every point. (This fact is analogous to the static electric field lines being normal to the surface of a conductor at every point). Why?

(f) Would the maximum possible magnetisation of a paramagnetic sample be of the same order of magnitude as the magnetisation of a ferromagnet?
Answer:
(a) Owing to the random thermal motion of molecules, the alignments of dipoles get disrupted at high temperatures. On cooling, this disruption is reduced.’Hence, a paramagnetic sample displays greater magnetisation when cooled.

(b) The induced dipole moment in a diamagnetic substance is always opposite to the magnetising field. Hence, the internal motion of the atoms (which is related to the temperature) does not affect the diamagnetism of a material.

(c) Bismuth is a diamagnetic substance. Hence, a toroid with a bismuth core has a magnetic field slightly greater than a toroid whose core is empty.

(d) The permeability of ferromagnetic materials is not independent of the applied magnetic field. It is greater for a lower field and vice versa.

(e) The permeability of ferromagnetic materials is not less than one. It is always greater than one. Hence, magnetic field lines are always nearly normal to the surface of such materials at every point.

(f) The maximum possible magnetisation of a paramagnetic sample can be of the same order of magnitude as the magnetisation of a ferromagnet. This requires high magnetising fields for saturation.

Question 17.
Answer the following questions:
(a) Explain qualitatively on the basis of domain picture the irreversibility in the magnetisation curve of a ferromagnet.

(b) The hysteresis loop of a soft iron piece has a much smaller area than that of a carbon steel piece. If the material is to go through repeated cycles of magnetisation, which piece will dissipate greater heat energy?

(c) ‘A system displaying a hysteresis loop such as a ferromagnet, is a device for storing memory?’ Explain the meaning of this statement.

(d) What kind of ferromagnetic material is used for coating magnetic tapes in a cassette player,’ or for building ‘memory stores’ in a modern computer?

(e) A certain region of space is to be shielded from magnetic fields. Suggest a method.
Answer:
(a) The hysteresis curve (B-H curve) of a ferromagnetic material is shown in the following figure
PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter 7
It can be observed from the given curve that magnetisation persists even when the external field is removed. This reflects the irreversibility of a ferromagnet.

(b) The dissipated heat energy is directly proportional to the area of a hysteresis loop. A carbon steel piede has a greater hysteresis curve area. Hence, it dissipates greater heat energy.

(c) The value of magnetisation is memory or record of hysteresis loop cycles of magnetisation. These bits of information correspond to the cycle of magnetisation. Hysteresis loops can be used for storing information.

(d) Ceramic is used for coating magnetic tapes in cassette players and for building memory stores in modern computers.

(e) A certain region of space can be shielded from magnetic fields if it is – surrounded by soft iron rings. In such arrangements, the magnetic lines are drawn out of the region.

PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter

Question 18.
A long straight horizontal cable carries a current of 2.5 A in the direction 10° south of west to 10° north of east. The magnetic meridian of the place happens to be 10° west of the geographic meridian. The earth’s magnetic field at the location is 0.33 G, and the angle of dip is zero. Locate the line of neutral points (ignore the thickness of the cable). (At neutral points, magnetic field due to a current-carrying cable is equal and opposite to the horizontal component of earth’s magnetic field.)
Answer:
Given, current in the cable
I = 2.5 A
PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter 8
Magnetic meridian MNMS is 10° west of geographical meridian GNGS earth’s magnetic field R = 0.33 G
= 0.33 × 10-4 T ……….. (1)
Angle of dip δ = S
The neutral point is the point where the magnetic field due to the current carrying cable is equal to the horizontal component of earth’s magnetic field.
Horizontal component of earth’s magnetic field
H = Rcosθ = 0.33 × 10-4 cos0°
= 0.33 × 10-4 T
Using the formula of magnetic field at distance r due to an infinite long current carrying conductor
B = \(\frac{\mu_{0}}{4 \pi} \cdot \frac{2 I}{r}\)
At neutral points,
H = B
0.33 × 10-4 = \(\frac{\mu_{0}}{4 \pi} \cdot \frac{2 I}{r}\)
0.33 × 10-4 = \(\frac{10^{-7} \times 2 \times 2.5}{r}\)
or r = \(\frac{5 \times 10^{-7}}{0.33 \times 10^{-4}}\)
or r = 1.5 × 10-2 m = 1.5cm
Thus, the line of neutral points is at a distance of 1.5 cm from the cable.

Question 19.
A telephone cable at a place has four long straight horizontal wires carrying a current of 1.0 A in the same direction east to west. The earth’s magnetic field at the place is 0.39 G, and the angle of dip is 35°. The magnetic declination is nearly zero. What are the resultant magnetic fields at points 4.0 cm above and below the cable?
Answer:
Number of horizontal wires in the telephone cable, n = 4
Current in each wire, I = 1.0 A
Earth’s magnetic field at the location, H = 0.39 G = 0.39 × 10-4 T
Angle of dip at the location, δ = 35°
Angle of declination, θ = 0°
For a point 4 cm below the cable
Distance, r = 4 cm = 0.04 m
The horizontal component of earth’s magnetic field can be written as
Hh, = Hcosδ – B
where,
B = Magnetic field at 4 cm due to current I in the four wires
= 4 × \(\frac{\mu_{0} I}{2 \pi r}\)
μ0 = 4π × 10-7 TmA-1
∴ B = 4 × \(\frac{4 \pi \times 10^{-7} \times 1}{2 \pi \times 0.04}\)
= 0.2 × 10-4 T = 0.2G
∴ Hh = 0.39 cos35°- 0.2
= 0.39 × 0.819 – 0.2 ≈ 0.12 G

The vertical component of earth’s magnetic field is given as
Hv = H sinδ
= 0.39 sin35°= 0.22 G
The angle made by the field with its horizontal component is given as
θ = tan-1 \(\frac{H_{v}}{H_{b}}\)
= tan-1 \(\frac{0.22}{0.12}\) = 61.39°
The resultant field at the point is given as
H1 = \(\sqrt{\left(H_{v}\right)^{2}+\left(H_{h}\right)^{2}}\)
= \(\sqrt{(0.22)^{2}+(0.12)^{2}}\) = 0.25 G

For a point 4 cm above the cable
Horizontal component of earth’s magnetic field
Hh = Hcosδ +B = 0.39 cos35° + 0.2 = 0.52 G
Vertical component of earth’s magnetic field
Hv = H sinδ
= 0.39 sin35° = 0.22 G
Angle, θ = tan-1 \(\frac{H_{v}}{H_{h}}\) = tan-1\(\frac{0.22}{0.52}\) = 22.90
And resultant field
H2 = \(\sqrt{\left(H_{v}\right)^{2}+\left(H_{h}\right)^{2}}\)
= \(\sqrt{(0.22)^{2}+(0.52)^{2}}\) = 0.56 G

PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter

Question 20.
A compass needle free to turn in a horizontal plane is placed at the centre of circular coil of 30 turns and radius 12 cm. The coil is in a vertical plane making an angle of 45° with the magnetic meridian. When the current in the coil is 0.35 A, the needle points west to east.
(a) Determine the horizontal component of the earth’s magnetic field at the location.
(b) The current in the coil is reversed, and the coil is rotated about its vertical axis by an angle of 90° in the anticlockwise sense looking from above. Predict the direction of the needle. Take the magnetic declination at the places to be zero.
Answer:
Number of turns in the circular coil, N = 30
Radius of the circular coil, r = 12cm = 0.12m
Current in the coil, I = 0.35 A
Angle of dip, δ = 45°

(a) The magnetic field due to current I, at a distance r, is given as
B = \(\frac{\mu_{0} 2 \pi N I}{4 \pi r}\)
∴ B = \(\frac{4 \pi \times 10^{-7} \times 2 \pi \times 30 \times 0.35}{4 \pi \times 0.12}\)
= 5.49 × 10-5 T
The compass needle points from west to east. Hence, the horizontal component of earth’s magnetic field is given as
BH = B sinδ
= 5.49 × 10-5 sin 45°
= 3.88 × 10-5 T = 0.388G

(b) When the current in the coil is reversed and the coil is rotated about its vertical axis by an angle of 90°, the needle will reverse its original direction. In this case, the needle will point from east to west.

Question 21.
A magnetic dipole is under the influence of two magnetic fields. The angle between the field directions is 60°, and one of the fields has a magnitude of 1.2 × 10-2 T. If the dipole comes to stable equilibrium at an angle of 15° with this field, what is the magnitude of the other field?
Answer:
The two fields \(\vec{B}\)1 and \(\vec{B}\)2 are shown in the figure
PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter 9
here in which a magnet is placed s.t.
∠NOB = 15°
∠B1OB2 = 60°
∴ ∠NOB2 = 60 – 15 = 45°
B1 = 1.2 × 10-2 T
B2 = ?
Let θ1 and θ2 he the inclination of the dipole
with \(\vec{B}\)1 and \(\vec{B}\)2 respectively.
∴ θ1 = 15°,θ2 = 45°
If τ1 and τ2 be the torques on the dipole due to \(\vec{B}\)1 and \(\vec{B}\)2 respectively, then
Using the relation,
τ = MB sin θ, we get
τ1 = MB1 sinθ1
and τ2 = MB2 sinθ2
As the dipole is in equilibrium, the torques on the dipole due to \(\vec{B}\)1 and \(\vec{B}\)2 are equal and opposite, i. e.,
PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter 10

PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter

Question 22.
A monoenergetic (18 keV) electron beam initially in the horizontal direction is subjected to a horizontal magnetic field of 0.40 G normal to the initial direction. Estimate the up or down deflection of the beam over a distance of 30 cm (me = 9.11 × 10-31 kg). [Note : Data in this exercise are so chosen that the answer will give you an idea of the effect of earth’s magnetic field on the motion of the electron beam from the electron gun to the screen in a TV set.]
Answer:
Here, energy = E = 18 KeV = 18 × 1.6 × 10-16 J
(∵ 1 KeV=103eV = 103 × 1.6 × 10-19 J)
B = horizontal magnetic field = 0.40 G = 0.40 × 10-4 J
m = 9.11 × 10-31 kg, e = 1.6 × 10-19 C
x = 30 cm = 0.30 m
As the magnetic field is normal to the velocity, the charged particle follows circular path in magnetic field. The centrepetal force \(\frac{m v^{2}}{r}\) required for this purpose is provided by force on electron due to magnetic field i. e., BeV
PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter 11

Question 23.
A sample of paramagnetic salt contains 2.0 × 1024 atomic dipoles each of dipole moment 1.5 × 10-23 JT-1. The sample is placed under a homogeneous magnetic field of 0.64 T, and cooled to a temperature of 4.2 K. The degree of magnetic saturation achieved is equal to 15%. What is the total dipole moment of the sample for a magnetic field of 0.98 T and a temperature of 2.8 K? (Assume Curie’s law)
Answer:
Number of atomic dipoles, n = 2.0 × 1024
Dipole moment of each atomic dipole, M = 1.5 × 10-23 JT-1
When the magnetic field, B1 = 0.64 T
The sample is cooled to a temperature, T1 = 4.2 K
Total dipole moment of the atomic dipole, Mtot = n × M
= 2 × 1024 × 1.5 × 10-23 = 30 JT-1
Magnetic saturation is achieved at 15%.
Hence, effective dipole moment, M1 = \(\frac{15}{100}\) × 30 = 4.5 JT-1
When the magnetic field, B2 = 0.98 T
Temperature, T2 = 2.8 K
Its total dipole moment = M2
According to Curie’s law, we have the ratio of two magnetic dipoles as
\(\frac{M_{2}}{M_{1}}=\frac{B_{2}}{B_{1}} \times \frac{T_{1}}{T_{2}}\)
∴ M2 = \(\frac{B_{2} T_{1} M_{1}}{B_{1} T_{2}}\) = \(\frac{0.98 \times 4.2 \times 4.5}{2.8 \times 0.64}\) = 10 336 JT-1
Therefore, 10.336 J T-1 is the total dipole moment of the sample for a magnetic field of 0.98 T and a temperature of 2.8 K.

Question 24.
A Rowland ring of mean radius 15 cm has 3500 turns of wire wound on a ferromagnetic core of relative permeability 800. What is the magnetic field B in the core for a magnetising current of 1.2 A?
Answer:
Mean radius of the Rowland ring, r = 15 cm = 0.15 m
Number of turns on the ferromagnetic core, N = 3500
Relative permeability of the core material, μr = 800
Magnetising current, I = 1.2 A
The magnetic field is given by the relation
B = \(\frac{\mu_{r} \mu_{0} I N}{2 \pi r}\)
B = \(\frac{800 \times 4 \pi \times 10^{-7} \times 1.2 \times 3500}{2 \pi \times 0.15}\) = 4.48T
Therefore, the magnetic field in the core is 4.48 T.

PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter

Question 25.
The magnetic moment vectors μs and μl associated with the intrinsic spin angular momentum S and orbital angular momentum l, respectively, of an electron are predicted by quantum theory (and verified experimentally to a high accuracy) to be given by
μg = -(e/m)S, μl = -(e/2m)l
Which of these relations is in accordance with the result expected classically? Outline the derivation of the classical result.
Answer:
Out of these two relations, \(\overrightarrow{\mu_{l}}=-\frac{e}{2 m} \vec{l}\) is in accordance with classical physics and can be derived as follows

We know that electrons revolving around the nucleus of an atom in circular orbits behave as tiny current loops having angular momentum \(\vec{\imath}\) given in magnitude as
\(\vec{\imath}\) = mvr …………. (1)
where m = mass of an electron
v = its orbital velocity
r = radius of the circular orbit.
or vr = \(\frac{l}{m}\) …………… (2)

\(\vec{\imath}\) acts along the normal to the plane of the orbit in upward direction. The orbital motion of electron is taken as equivalent to the flow of conventional current I given by
PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter 12
where – ve sign shows that the electron is negatively
charged. The eqn. (3) shows that μe and \(\vec{\imath}\) are opposite to each other i. e., antiparallel and both being normal to the plane of the orbit as shown in the figure
PSEB 12th Class Physics Solutions Chapter 5 Magnetism and Matter 13
∴ \(\overrightarrow{\mu_{l}}=-\frac{e}{2 m} \cdot \vec{l}\)
\(\frac{\mu_{s}}{S}\) in contrast to \(\frac{\mu_{l}}{\vec{l}}\) is \(\frac{e}{m}\) i.e., twice the classically
expected value. This latter result is an outstanding consequence of modern quantum theory and cannot be obtained classically.

PSEB 12th Class Physics Important Questions Chapter 11 Dual Nature of Radiation and Matter

Punjab State Board PSEB 12th Class Physics Important Questions Chapter 11 Dual Nature of Radiation and Matter Important Questions and Answers.

PSEB 12th Class Physics Important Questions Chapter 11 Dual Nature of Radiation and Matter

Very short answer type questions

Question 1.
In the photoelectric effect, why should the photoelectric current increase as the intensity of monochromatic radiation incident on a photosensitive surface is increased? Explain.
Answer:
The photoelectric current increases proportionally with the increase in intensity of incident radiation. Larger the intensity of incident radiation, larger is the number of incident photons and hence larger is the number of electrons ejected from the photosensitive surface.

Question 2.
Define the term ‘threshold frequency’ in relation to photoelectric effect.
Answer:
Threshold frequency is defined as the minimum frequency of incident radiation which can cause photoelectric emission. It is different for different metal.

PSEB 12th Class Physics Important Questions Chapter 11 Dual Nature of Radiation and Matter

Question 3.
WrIte the basic features of photon picture of electromagnetic radiation on which Einstein’s photoelectric equation is based.
Answer:
Features of the photons are as:

  • Photons are particles of light having energy E = hv and momentum p = \(\frac{h}{\lambda}\), where h is Planck’s constant.
  • Photons travel with the speed of light in vacuum, independent of the frame of reference.
  • Intensity of light depends on the number of photons crossing unit area in a unit time.

Question 4.
Define Intensity of radiation on the basis of photon picture of light. Write its SI unit.
Answer:
The amount of light energy or photon energy incident per meter square per second is called intensity of radiation1 Its SI unit is \(\frac{\mathrm{W}}{\mathrm{m}^{2}}\) or J/s m

Question 5.
State de Broglie hypothesis.
Answer:
According to the hypothesis of de Brogue “The atomic particles of matter moving with a given velocity, can display the wave-like properties.” i.e., λ = \(\frac{h}{m v}\) (mathematically)

Question 6.
Write the relationship of de Brogue wavelength λ associated with a particle of mass m terms of its kinetic energy E.
Answer:
Kinetic energy EK = \(\frac{p^{2}}{2 m}\)
where, p = momentum
m = mass and EK = kinetic energy
⇒ p = \(\sqrt{2 m E_{K}} \)
de Brogue wavelength,
λ = \(\frac{h}{p}\)
Where, p = \(\sqrt{2 m E_{K}} \)
⇒ λ = \(\frac{h}{\sqrt{2 m E_{K}}}\)

Question 7.
Name the phenomenon which shows the quantum nature of electromagnetic radiation.
Answer:
Photoelectric effect.

Question 8.
Do all the electrons that absorb a photon comes out as photoelectrons? (NCERT Exemplar)
Answer:
No, most electrons get scattered into the metal. Only a few come out of the surface of the metal.

PSEB 12th Class Physics Important Questions Chapter 11 Dual Nature of Radiation and Matter

Question 9.
There are materials which absorb photons of shorter wavelength and emit photons of longer wavelength. Can there be stable substances which absorb photons of larger wavelengths and emit light of shorter wavelengths? (NCERT Exemplar)
Answer:
In the first case, energy given out is less than the energy supplied. In the second case, the material has to supply the energy as the emitted photon has more energy. This cannot happen for stable substances.

Question 10.
There are two sources of light, each emitting with a power 100W.
One emits X-rays of wavelength 1 nm and the other visible light at 500 nm.
Find the ratio of number of photons of X-rays the photons of visible light of the given wavelength. (NCERT Exemplar) Ans. Total E is constant.
Let n1 and n2 be the number of photons of X-rays and visible region.
n1E2 = n2E2
PSEB 12th Class Physics Important Questions Chapter 11 Dual Nature of Radiation and Matter 1

Short answer type questions

Question 1.
What is meant by work function of a metal? How does the value of work function influence the kinetic energy of electrons liberated during photoelectron emission?
Answer:
Work Function: The minimum energy required to free an electron from metallic surface is called the work function.
Smaller the work function, larger the kinetic energy of emitted electron.

Question 2.
Show mathematically how Bohr’s postulate of quantization of orbital angular momentum in hydrogen atom is explained by de Broglie’s hypothesis.
Answer:
According to de Broglie’s hypothesis,
λ = \(\frac{h}{m v}\) …………………………… (1)
According to de Broglie’s condition of stationary orbits, the stationary orbits are those which contain complete de Broglie wavelength.
2πr = nλ …………………………………… (2)

Substituting value of λ from (1) in (2), we get
2πr = n\(\frac{h}{m v}\)
⇒ mvr = n\(\frac{h}{2 \pi}\) ……………………………………… (3)
This is Bohr’s postulate of quantization of energy levels.

PSEB 12th Class Physics Important Questions Chapter 11 Dual Nature of Radiation and Matter

Question 3.
Write briefly the underlying principle used in the Davison-Germer experiment to verify wave nature of electrons experimentally. What is the de Broglie wavelength of an electron with kinetic energy (EK) 120 eV?
Answer:
Principle: Diffraction effects are observed for beams of electrons scattered by the crystals.
λ = \(\frac{h}{p}=\frac{h}{\sqrt{2 m E_{K}}}=\frac{h}{\sqrt{2 m e V}}\)
= \(\frac{6.63 \times 10^{-34}}{\sqrt{2 \times 9.1 \times 10^{-31} \times 1.6 \times 10^{-19} \times 120}}\)
λ = 0.112 nm.

Question 4.
(i) Describe briefly three experimentally observed features in the phenomenon of photoelectric effect.
(ii) Discuss briefly how wave theory of light cannot explain these features.
Answer:
(i) Three experimentally observed features in the phenomenon of photoelectric effect are as follows :

  • Intensity: When intensity of incident light increases as one photon ejects one electron, the increase in intensity will increase the number of ejected electrons. Frequency has no effect on photoelectron.
  • Frequency: When the frequency of incident photon increases, the kinetic energy of the emitted electrons increases. Intensity has no effect on kinetic energy of photoelectrons.
  • No Time Lag: When energy incident photon is greater than the work function, the photoelectron is immediately ejected. Thus, there is no time lag between the incidence of light and emission of photoelectrons.

(ii) These features cannot be explained in the wave theory of light because wave nature of radiation cannot explain the following :

  • The instantaneous ejection of the photoelectrons.
  • The existence of threshold frequency for a metal surface.
  • The fact that kinetic energy of the emitted electrons is independent of the intensity of light and depends upon its frequency.

Question 5.
A beam of monochromatic radiation is incident on a photosensitive surface. Answer the following questions :
(i) Do the emitted photoelectrons have the same kinetic energy?
(ii) Does the kinetic energy of the emitted electrons depend on the intensity of incident radiation?
(iii) On what factors does the number of emitted photoelectrons depend?
Answer:
In photoelectric effect, an electron absorbs a quantum of energy hv of radiation, which exceeds the work function, an electron is emitted with maximum kinetic energy.
EK max = hv – W
(i) No, all electrons are bound with different forces in different layers of the metal. So, more tightly bound electron will emerge with less kinetic energy. Hence, all electrons do not have same kinetic energy.
(ii) No, because an electron cannot emit out if quantum energy hv is less than the work function of the metal. The KE depends on the energy of each photon.
(iii) Number of emitted photoelectrons depends on the intensity of the radiations provided the quantum energy hv is greater than the work function of the metal.

Question 6.
Define the term “cut-off frequency” in photoelectric emission. The threshold frequency of a metal is f. When the light of frequency 2f is incident on the metal plate, the maximum velocity of photoelectrons is v1. When the frequency of the incident radiation is increased to 5f, the maximum velocity of photoelectrons is v2. Find the ratio v1: v2.
Answer:
Cut-off Frequency: It is that maximum frequency of incident radiation below which no photoemission takes place from a photoelectric material. According to Einstein’s photoelectric equation
EK max = \(\frac{h c}{\lambda}-\phi \) = \(h v-\phi\)
Given that threshold frequency of the metal is f. If light of frequency, 2f is incident on metal plate, maximum velocity of photoelectron is v1 then,
\(\frac{1}{2} m v_{1}^{2}\) = h (2f-f)
⇒ \(\frac{1}{2} m v_{1}^{2}\) = hf …………………….. (1)

If light of frequency, 5f is incident and maximum velocity of photoelectron is v2.
\(\frac{1}{2} m v_{1}^{2}\) = h(5f-f)
⇒ \(\frac{1}{2} m v_{1}^{2}\) = 4hf ………………………………… (2)
Dividing (1) by (2), we get
\(\left(\frac{v_{1}}{v_{2}}\right)^{2}=\frac{1}{4}\)
⇒ \(\frac{v_{1}}{v_{2}}=\frac{1}{2}\)
∴ v1 = v2 = 1:2

PSEB 12th Class Physics Important Questions Chapter 11 Dual Nature of Radiation and Matter

Question 7.
Two monochromatic beams A and B of equal intensity I hit a screen. The number of photons hitting the screen by beam A is twice that by beam B. Then what inference can you make about their frequencies? (NCERTExemplarl
Answer:
Let no. of photons falling per second of beam A = nA
No. of photons falling per second of beam B = nB
Energy of beam A = hvA
Energy of beam B = h vB

According to question, I = nAvA = nBvB
\(\frac{n_{A}}{n_{B}}=\frac{v_{B}}{v_{A}}\) or \(\frac{2 n_{B}}{n_{B}}=\frac{v_{B}}{v_{A}}\)
⇒ vB = 2 vA
The frequency of beam B is twice that of A.

Question 8.
Consider Fig. for photoemission.
PSEB 12th Class Physics Important Questions Chapter 11 Dual Nature of Radiation and Matter 2
How would you reconcile with momentum- conservation? No light (Photons) have momentum in a different direction than the emitted electrons. (NCERT Exemplar)
Answer:
The momentum is transferred to the metal. At the microscopic level, atoms absorb the photon and its momentum is transferred mainly to the nucleus and electrons. The excited electron is emitted. Conservation of momentum needs to be accounted for the momentum transferred to the nucleus and electrons.

Long answer type questions

Question 1.
Describe Davisson and Germer’s experiment to demonstrate the wave nature of electrons. Draw a labeled diagram of apparatus used.
Answer:
Davisson and Germer Experiment: In 1927 Davisson and Germer performed a diffraction experiment with electron beam in analogy with X-ray diffraction to observe the wave nature of matter.
Apparatus: It consists of three parts
(i) Electron gun : It gives a fine beam of electrons, de Brogue used electron beam of energy 54 eV. de Brogue wavelength associated with this beam
PSEB 12th Class Physics Important Questions Chapter 11 Dual Nature of Radiation and Matter 3
λ = \(\frac{h}{\sqrt{2 m E_{k}}}\)
Here, m = mass of electron = 9.1 x 10-31 kg
EK = Kinetic energy of electron = 54eV
= 54 x 1.6 x 10-19 J = 86.4 x 10-19 J
∴ λ = \(\frac{6.63 \times 10^{-34}}{\sqrt{2 \times 9.1 \times 10^{-31} \times 86.4 \times 10^{-19}}}\)
= 1.66 x 10-10 = 1.66 Å
(ii) Nickel crystal: The electron beam was directed on nickel crystal against the electron detector. The smallest separation between nickel atoms is O.914Å. Nickel crystal behaves as diffraction grating.

(iii) Electron detector: It measures the intensity of electron beam diffracted from nickel crystal. It may be an ionization chamber fitted with a sensitive galvanometer. The energy of electron beam, the angle of incidence of beam on nickel crystal and the position of detector can all be varied.

Method: The crystal is rotated in small steps to change the angle (α say) between incidence and scattered directions and the corresponding intensity (I) of scattered beam is measured. The variation of the intensity (I) of the scattered electrons with the angle of scattering a is obtained for different accelerating voltages.

The experiment was performed by varying the accelerating voltage from 44 V to 68 V. k was noticed that a strong peak appeared in the intensity (I) of the scattered electron for an accelerating voltage of 54 V at a scattering angle α = 50°.
PSEB 12th Class Physics Important Questions Chapter 11 Dual Nature of Radiation and Matter 4
From Bragg’s law, 2d sinθ = nλ
Here, n =1,d =0.914 Å,θ =65°
∴ λ = \(\frac{2 d \sin \theta}{n}=\frac{2 \times(0.914 \AA) \sin 65^{\circ}}{1}\)
=2 x 0.914 x 0.9063Å =1.65Å
The measured wavelength is in close agreement with the estimated de Broglie wavelength. Thus the wave nature of electrons is verified. Later on G.P. Thomson demonstrated the wave nature of fast electrons. Due to their work Davisson and G.P. Thomson were awarded Nobel Prize in 1937.

PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter

Punjab State Board PSEB 12th Class Physics Book Solutions Chapter 11 Dual Nature of Radiation and Matter Textbook Exercise Questions and Answers.

PSEB Solutions for Class 12 Physics Chapter 11 Dual Nature of Radiation and Matter

PSEB 12th Class Physics Guide Dual Nature of Radiation and Matter Textbook Questions and Answers

Question 1.
Find the
(a) maximum frequency, and
(b) minimum wavelength of X-rays produced by 30 kV electrons.
Answer:
Potentialoftheelectrons, V=30 kV= 3O x 103 V=3 x 104 V
Hence, energy of the electrons, E = 3 x 104 eV
where, e = Charge on an electron = 1.6 x 10-19C
(a) Maximum frequency produced by the X-rays = v
The energy of the electrons is given by the relation
E=eV=hv
where, h = Planck’s constant = 6.63 x 10-34 Js
∴ v = \(\frac{e V}{h} \) (∵ E = eV)
= \(\frac{1.6 \times 10^{-19} \times 3 \times 10^{4}}{6.63 \times 10^{-34}}\) = 7.24 x 1018 Hz
Hence, the maximum frequency of X-rays produced is 7.24 x 1018 Hz

(b) The minimum wavelength produced by the X-rays is given as
λ = \(\frac{c}{v}\)
= \(\frac{3 \times 10^{8}}{7.24 \times 10^{18}}\)
= 0.414 x 10-10
= 0.0414 x 10-9 m
= 0.0414 nm
Hence, the minimum wavelength of X-rays produced is 0.0414 nm.

Question 2.
The work function of caesium metal is 2.14 eV. When light of frequency 6 x 1014 Hz is incident on the metal surface, photoemission of electrons occurs.
What is the
(a) maximum kinetic energy of the emitted electrons,
(b) stopping potential, and
(c) maximum speed of the emitted photoelectrons?
Answer:
Work function of caesium metal, Φ0 = 2.14 eV
Frequency of light, v = 6.0 x 1014 Hz
The maximum kinetic energy is given by the photoelectric effect as
K = hv- Φ0
where, h = Planck’s constant = 6.63 x 10-34 Js .
∴ k = \(\frac{6.63 \times 10^{-34} \times 6 \times 10^{14}}{1.6 \times 10^{-19}}-2.14 \)
( ∵ e=1.6 x 10-19)
= 2.485-2.140 =0.345eV
Hence, the maximum kinetic energy of the emitted electrons is 0.345 eV.

(b) For stopping potential V0, we can write the equation for kinetic energy
as K=eV0
∴ V0 = \(\frac{K}{e}\) (∵ e=1.6×1019)
= \(\frac{0.345 \times 1.6 \times 10^{-19}}{1.6 \times 10^{-19}}\) =0.345V
Hence, the stopping potential of the material is 0.345 V.

(c) Maximum speed of the emitted photoelectrons = y
Hence, the relation for kinetic energy can be written as
K = \(\frac{1}{2}\) mv2
where, m = mass of an electron = 9.1 x 10-31 kg
(∴ e=1.6 x 10-19)
= \(\frac{2 \times 0.345 \times 1.6 \times 10^{-19}}{9.1 \times 10^{-31}}\) = 0.1104 x 1012
∴ v = 3.323 x 105 m/s = 332.3 km/s
Hence, the maximum speed of the emitted photoelectrons is 332.3 km/s.

PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter

Question 3.
The photoelectric cut off voltage n a certain experiment is 1.5 V. What is the maximum kinetic energy of photoelectrons emitted?
Answer:
Cut-off voltage, V0 = 1.5 V
Maximum kinetic energy of photoelectrons
EK =eV0 =1.5eV=1.5 x 1.6 x 10-19J
=2.4 x 10-19J.

Question 4.
Monochromatic light of wavelength 632.8 mn is produced by a helium-neon laser. The power emitted is 9.42 mW.
(a) Find the energy and momentum of each photon in the light beam,
(b) How many photons per second, on the average, arrive at a target irradiated by this beam? (Assume the beam to have uniform cross-section which is less than the target area), and
(c) How fast does a hydrogen atom have to travel in order to have the same momentum as that of the photon?
Answer:
Wavelength of the monochromatic light, 632.8 nm = 632.8 x 10-9 m
Power emitted by the laser, P = 9.42 mW = 9.42 x 10-3 W
Planck’s constant, h = 6.63 x 10-34Js
Speed of light, c=3 x 108 m/s
Mass of a hydrogen atom, m =1.66 x 10-27 kg
(a) The energy of each photon is given as
E = \(\frac{h c}{\lambda}\)
= \(\frac{6.63 \times 10^{-34} \times 3 \times 10^{8}}{632.8 \times 10^{-9}}\)
= 3.141 x 10-19

The momentum of each photon is given as
p = \(\frac{h}{\lambda}\)
= \(\frac{6.63 \times 10^{-34}}{632.8 \times 10^{-9}}=1.047 \times 10^{-27} \mathrm{~kg} \mathrm{~ms}^{-1} \)

(b) Number of photons arriving per second, at a target irradiated by the beam = n.
Assume that the beam has a uniform cross-section that is less than the
target area.
Hence, the equation for power can be written as
P=nE
∴ n= \(\frac{P}{E}\)
= \(\frac{9.42 \times 10^{-3}}{3.141 \times 10^{-19}}\) = 3 x 1016

(c) Momentum of the hydrogen atom is the same as the momentum of the photon, .
p=1.047 x 1027 kgms-1
Momentum is given as
p = mv
where, v = speed of the hydrogen atom
v = \(\frac{p}{m}\)
= \(=\frac{1.047 \times 10^{-27}}{1.66 \times 10^{-27}}\) = 0.630m/s

Question 5.
The enery flux of sunlight reaching the surface of the earth is 1.388 x 103 W/m2.
How many photons (nearly) per square metre are incident on the Earth per second? Assume that the photons in the sunlight have an average wavelength of 550 rims.
Answer:
Energy flux of sunlight reaching the surface of earth,
Φ = 1.388 x 103 W/m2
Hence, power of sunlight per square metre, P = 1.388 x 103W
Speed of light, c = 3 x 108 m/s
Planck’s constant, h = 6.63 x 10-34 Js
Average wavelength of photons present in sunlight, λ = 550 nm.
=550 x 10-9m
Number of photons per square metre incident on earth per second = n

Hence, the equation for power can be written as
P = nE
∴ n = \(\frac{P}{E}=\frac{P \lambda}{h c}=\frac{1.388 \times 10^{3} \times 550 \times 10^{-9}}{6.63 \times 10^{-34} \times 3 \times 10^{8}}\)
= 3.84 x 1021 photons/m2/s
Therefore, every second, 3.84×1021 photons are incident per square metre on earth.

PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter

Question 6.
In an experiment on photoelectric effect, the slope of the cut-off voltage versus frequency of incident light is found to be 4.12 x 10-15 Vs. Calculate the value of Planck’s constant.
Answer:
The slope of the cut-off voltage (V) versus frequency (v) of an incident light is given as
\(\frac{V}{v}\) = 4.12 x 10-15 Vs
V is related to frequency by the equation
hv = eV

where, e = charge on an electron = 1.6 x 10-19
h = Planck’s constant
∴ h = e x \(\frac{V}{v}\)
= 1.6 x 10-19 x 4.12 x 10-15
= 6.592 x 10-34 Js
Therefore, the value of Planck’s constant is 6.592 x 10-34 Js.

Question 7.
A 100 W sodium lamp radiates energy uniformly in all directions. The lamp is located at the centre of a large sphere that absorbs all the sodium light which is incident on it. The wavelength of the sodium light is 589 nm.
(a) What is the energy per photon associated with the sodium light?
(b) At what rate are the photons delivered to the sphere?
Answer:
Power of the sodium lamp,. P = 100 W
Wavelength of the emitted sodium light, λ = 589 nm = 589 x 109 m
Planck’s constant, h = 6.63 x 10-34 Js
Speed of light, c = 3 x 108 m/s
The energy per photon associated with the sodium light is given as
E= \(\frac{h c}{\lambda}\)
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 1
(b) Number of photons delivered to the sphere = n
The equation for power can be written as
P=nE
∴ n = \(\frac{P}{E}=\frac{100}{3.37 \times 10^{-19}}\) = 2.96 x 1020 photons/s
Therefore, every second, 2.96 x 1020 photons are delivered to the sphere.

Question 8.
The threshold frequency for a certain metal is 3.3 x 1014 Hz. If light of frequency 8.2 x 1014 Hz is incident on the metal, predict the cut-off voltage for the photoelectric emission.
Answer:
Threshold frequency of the metal, v0 = 3.3 x 1014 Hz
Frequency of light incident on the metal, v = 82 x 1014 Hz
Charge on’an electron, e = 1.6 x 10-19 C .
Planck’s constant, h = 6.63 x 10-34 Js
Cut-off voltage for the photoelectric emission from the metal = V0
The equation for the cut-off energy is given as
eV0 = h(v-v0)
Vo = \(\frac{h\left(v-v_{0}\right)}{e}\)
= \(\frac{6.63 \times 10^{-14} \times\left(8.2 \times 10^{14}-3.3 \times 10^{14}\right)}{1.6 \times 10^{-19}}\)
= 2.0292 V
Therefore, the cut-off voltage for the photoelectric emission is 2.0292 V.

Question 9.
The work function for a certain metal Is 4.2 eV. Will this metal give photoelectric emission for incident radiation of
wavelength 330 nm?
Answer:
The energy of incident radiations
E = \(\frac{h c}{\lambda}\) = \(\frac{6.63 \times 10^{-34} \times 3 \times 10^{8}}{330 \times 10^{-9}}\) J
= 6.03 x 10-19 J
= \(\frac{6.03 \times 10^{-19}}{1.6 \times 10^{-19}}\)eV = 3.77 eV
The work function of photometal, Φ0 = 4.2 eV
As energy of incident photon is less than work function, photoemission is not possible.

PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter

Question 10.
Light of frequency 7.21 x 1014 Hz is incident on a metal surface. Electrons with a maximum speed of 6.0 x10s m/s are ejected from the surface. What is the threshold frequency for photoemission of electrons?
Answer:
Frequency of light incident on the metal surface, v = 7.21 x 1014 Hz
Maximum speed of the electrons, v = 6.0 x 105 m/s
Planck’s constant, h = 6.63 x 10-34 Js
Mass of an electron, m = 9.1 x 10-31 kg
For threshold frequency v0, the relation for kinetic energy is written asFor threshold frequency y0, the relation for kinetic energy is written as
\(\frac{1}{2} m v^{2}\) = h(v-v0)
v0 = v – \(\frac{m v^{2}}{2 h}\)
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 2
Therefore, the threshold frequency for the photoemission of electrons is 4.738 x 1014 Hz.

Question 11.
Light of wavelength 488 mn is produced by an argon laser which is used in the photoelectric effect. When light from this spectral line is incident on the emitter, the stopping (cut-off) potential of photoelectrons is 0.38 V. Find the work function of the material from which the emitter is made.
Answer:
Wavelength of light produced by the argon laser,
λ = 488 nm = 488 x 10-9 m
Stopping potential of the photoelectrons, V0 = 0.38 V
1 eV=l.6 x 10-19 J
∴ V0= \(\frac{0.38}{1.6 \times 10^{-19}}\) eV
Planck’s constant, h = 6.63 x 10-34 Js
Charge on an electron, e = 1.6 x 10-19 C
Speed of light, c =3 x 10 m/s
From Einstein’s photoelectric effect, we have the relation involving the work function Φ0 of the material of the emitter as
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 3
Therefore, the material with which the emitter is made has the work function of 2.16 eV.

Question 12.
Calculate the
(a) momentum, and
(b) de Broglie wavelength of the electrons accelerated through a potential difference of 56 V.
Answer:
Potential difference, V = 56 V
Planck’s constant, h = 6.63 x 10-34 Js
Mass of an electron, m = 9.1 x 10-31 kg
Charge on an electron, e = 1.6 x 10-19 C
(a) At equilibrium, the kinetic energy of each electron is equal to the accelerating potential, i.e., we can write the relation for velocity (v) of each electron as
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 4
The momentum of each accelerated electron is given as
P = mv
= 9.1 x 10-31 x 4.44 x 106
= 4.04 x 10-24 kg m s-1
Therefore, the momentum of each electron is 4.04 x 10-24 kg m s-1.

(b) de Broglie wavelength of an electron accelerating through a potential V is given by the relation
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 5
Therefore, the de Broglie wavelength of each electron is 0.1639 nm.

PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter

Question 13.
What is the
(a) momentum,
(b) speed, and
(c) de Broglie wavelength of an electron with kinetic energy of 120 eV.
Answer:
Kinetic energy of the electron, Ek = 120 eV
Planck’s constant, h = 6.63 x 10-34 Js
Mass of an electron, m = 9.1 x 10-31
Charge on an electron, e = 1.6 x 10-19 C

(a) For the electron, we can write the relation for kinetic energy as
Ek = \(\frac{1}{2}\) mv2
where, v = speed of the electron
∴ v2 = \(\sqrt{\frac{2 e E_{k}}{m}}=\sqrt{\frac{2 \times 1.6 \times 10^{-19} \times 120}{9.1 \times 10^{-31}}}\)
= \(\sqrt{42.198 \times 10^{12}}\) = 6.496 x 106 m/s
Momentum of the electron, p = mv = 9.1 x 10-31 x 6.496 x 106
=5.91 x 1024 kg ms-1
Therefore, the momentum of the electron is 5.91 x 1024 kg ms-1.

Question 14.
The wavelength of light from the spectral emission line of sodium is 589 nm. Find the kinetic energy at which an electron, and a neutron, would have the same de Broglie wavelength.
Answer:
Wavelength of light of sodium line, λ = 589 nm = 589 x 10-9 m
Mass of an electron, me = 9.1 x 10-31 kg
Mass of a neutron, mn = 1.66 x 10-27 kg
Planck’s constant, h = 6.63 x 10-34 Js

(a) For the kinetic energy K, of an electron accelerating with a velocity v, we have the relation
K = \(\frac{1}{2}\) mev2 ………………………… (1)
We have the relation for de Broglie wavelength as
λ = \(\frac{h}{m_{e} v}\)
∴ v2 = \(\frac{h^{2}}{\lambda^{2} m_{e}^{2}}\) ………………………… (2)
Substituting equation (2) in equation (1), we get the relation
K = \(\frac{1}{2} \frac{m_{e} h^{2}}{\lambda^{2} m_{e}^{2}}=\frac{h^{2}}{2 \lambda^{2} m_{e}}\) ………….. (3)
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 6
Hence, the kinetic energy of the electron is 6.9 x 10-25 J or 4.31 µeV.

(b) Using equation (3), we can write the relation for the kinetic energy of the neutron as = \(\frac{h^{2}}{2 \lambda^{2} m_{n}}\)
= \(\frac{\left(6.63 \times 10^{-34}\right)^{2}}{2 \times\left(589 \times 10^{-9}\right)^{2} \times 1.66 \times 10^{-27}}\)
= 3.78 x 10-28
= \(\frac{3.78 \times 10^{-28}}{1.6 \times 10^{-19}} \) = 2.36 x 10-9 eV
= 2.36 neV
Hence, the kinetic energy of the neutron is 3.78 x 10-28 J or 2.36 neV.

Question 15.
What is the de Broglie wavelength of
(a) a bullet of mass 0.040 kg travelling at the speed of 1.0 km/s,
(b) a ball of mass 0.060 kg moving at a speed of 1.0 m/s, and
(c) a dust particle of mass 1.0 x 10-9 kg drifting with a speed of 2.2 m/s?
Answer:
(a) Mass of the bullet, m = 0.040 kg
Speed of the bullet, v = 1.0 km/s = 1000 m/s
Planck’s constant, h = 6.63 x 10-34 Js
de Broglie wavelength of the bullet is given by the relation
λ = \(\frac{h}{m v}\)
= \(\frac{6.63 \times 10^{-34}}{0.040 \times 1000} \) = 1.65 x 10-35 m

(b) Mass of the ball, m = 0.060 kg
Speed of the ball, v =1.0 m/s
de Brogue wavelength of the ball is given by the relation
λ = \(\frac{h}{m v}\)
= \(\frac{6.63 \times 10^{-34}}{0.060 \times 1}\) = 1.1 x 10-32 m

(c) Mass of the dust particle, m = 1 x 10-9 kg
Speed of the dust particle, v = 2.2 m/s
de Brogue wavelength of the dust particle is given by the relation
λ = \(\frac{h}{m v}\)
= \(\frac{6.63 \times 10^{-34}}{2.2 \times 1 \times 10^{-9}}\) = 3.0 x 10-25 m.

PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter

Question 16.
An electron and a photon each have a wavelength of 1.00 run. Find
(a) their momenta,
(b) the energy of the photon, and
(c) the kinetic energy of electrons.
Answer:
Wavelength of an electron (λe) and a photon (λp),λe = λp = λ = 1 nm
= 1 x 10-9 m
Planck’s constant, h = 6.63 x 10-34 Js

(a) The momentum of an elementary particle is given by de Broglie relation
λ = \(\frac{h}{p}\)
p = \(\frac{h}{\lambda}\)
It is clear that momentum depends only on the wavelength of the particle. Since the wavelengths of an electron and a photon are equal, both have an equal momentum.
∴ p= \(\frac{6.63 \times 10^{-34}}{1 \times 10^{-9}} \) =6.63 x 10-25 kgms-1

(b) The energy of a photon is given by the relation
E= \(\frac{h c}{\lambda}\)
where, speed of light, c =3 x 108 m/s
∴ E = \(\frac{6.63 \times 10^{-34} \times 3 \times 10^{8}}{1 \times 10^{-9} \times 1.6 \times 10^{-19}}\)
= 1243.1 eV = 1.243 keV
Therefore, the energy of the photon is 1.243 keV.

(c) The kinetic energy (K) of an electron having momentum p, is given by the relation
K = \(\frac{1}{2} \frac{p^{2}}{m}\)
where, m = mass of the electron = 9.1 x 10-31 kg;
p = 6.63 x 10-25 kgm s-1

∴ K = \(\frac{1}{2} \times \frac{\left(6.63 \times 10^{-25}\right)^{2}}{9.1 \times 10^{-31}}\) = 2.415 x 10-19 J
= \(\frac{2.415 \times 10^{-19}}{1.6 \times 10^{-19}}\) = 1.51 eV
Hence, the kinetic energy of the electron is 1.51 eV.

Question 17.
(a) For what kinetic energy of a neutron will the associated de Broglie wavelength be 1.40x 10-10 m?
(b) Also, find the de Brogue wavelength of a neutron, in thermal equilibrium with matter, having an average kinetic energy of (3/2) kT at 300 K.
Answer:
(a) de Brogue wavelength of the neutron, λ =1.40 x 10-10 m
Mass of a neutron,mn =1. 66 x 10-27 kg
Planck’s constant, h = 6.63 x 10-34 Js
Kinetic energy (K) and velocity ( v) are related as
K = \( \frac{1}{2} m_{n} v^{2}\) ……………………………… (1)
de Brogue wavelength (λ) and velocity (v) are related as
λ= \(\frac{h}{m_{n} v}\) ……………………………….. (2)
Using equation (2) in equation (1), we get
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 7
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 8
Hence, the kinetic energy of the neutron is 6.75 x 10-21 J or 4.219 x 10-2 eV.

(b) Temperature of the neutron, T = 300 K
Boltzmann’s constant, k =1.38 x 10-23 kg m2 s-2 K-1
Average kinetic energy of the neutron,
K’ = \(\frac{3}{2} \) kT.
= \(\frac{3}{2} \) x 1.38 x 10-23 x 300 = 6.21 x 10-21 J
The relation for the de Broglie wavelength is given as
λ ‘ = \(\frac{h}{\sqrt{2 K^{\prime} m_{n}}}\)

where, mn = 1.66 x 10-27 kg
h = 6.63 x 10-34 Js
K’ = 6.21 x 10-21 J
∴ λ’ = \(\frac{6.63 \times 10^{-34}}{\sqrt{2 \times 6.21 \times 10^{-21} \times 1.66 \times 10^{-27}}}\)
=1.46 x 10-10
m = 0.146 nm
Therefore, the de Broglie wavelength of the neutron is 0.146 nm.

Question 18.
Show that the wavelength cf electromagnetic radiation is equal to the de Broglie wavelength of its quantum (photon).
Answer:
The momentum of a photon having energy (hv) is given as ’
p = \(\frac{h v}{c}=\frac{h}{\lambda}\)
λ = \(\frac{h}{p}\) ………………………….. (1)
where, λ = wavelength of the electromagnetic radiation
c = speed of light
h = Planck’s constant
de Broglie wavelength of the photon is given as
λ = \(\frac{h}{m v}\)
But p = mv
∴ λ =\(\frac{h}{p}\) ………………………………….. (2)
where, m = mass of the photon
v = velocity of the photon
Hence, it can be inferred from equations (1) and (2) that the wavelength of the electromagnetic radiation is equal to the de Broglie wavelength of the photon.

Question 19.
What is the de Broglie wavelength of a nitrogen molecule in air at 300 K?
Assume that the molecule is moving with the root mean square speed of molecules at this temperature. (Atomic mass of nitrogen = 140076 u)
Answer:
Temperature of the nitrogen molecule, T = 300 K
Atomic mass of nitrogen = 14.0076 u
Hence, mass of the nitrogen molecule, m = 2 x 14.0076 = 28.0152 u
But 1 u = 1.66 x 10-27 kg
∴ m=28.0152 x 1.66 x 10-27 kg
Planck’s constant, h = 6.63 x 10-34 Js
Boltzmann’s constant, k = 1.38 x 10-23 K-1
We have the expression that relates mean kinetic energy \(\left(\frac{3}{2} k T\right)\) of the nitrogen molecuLe with the root mean square speed (vrms) as
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 9
Hence, the de Broglie wavelength of the nitrogen molecule is given as
λ = \(\frac{h}{m v_{\text {rms }}}=\frac{h}{\sqrt{3 m k T}}\)
= \(\frac{6.63 \times 10^{-34}}{\sqrt{3 \times 28.0152 \times 1.66 \times 10^{-27} \times 1.38 \times 10^{-23} \times 300}}\)
= 0.028 x 10-9 m
= 0.028 nm
Therefore, the de Broglie wavelength of the nitrogen molecule is 0.028 nm.

PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter

[Additional Exercises]

Question 20.
(a) Estimate the speed with which electrons emitted from a heated emitter of an evacuated tube impinge on the collector maintained at a potential difference of 500 V with respect to the emitter.
Ignore the small initial speeds of the electrons. The specific charge of the electron, i.e., its elm, is given to be 1.76 x 1011 Ckg-1.
(b) Use the same formula you employ in (a) to obtain electron speed for an collector potential of 10 MV. Do you see what is wrong? In what way is the formula to be modified?
Answer:
(a) Potential difference across the evacuated tube, V = 500 V
Specific charge of the electron, e/m = 1.76 x 1011 C kg-1
The speed of each emitted electron is given by the relation for kinetic energy as

KE = \(\frac{1}{2}\) mv2
Therefore, the speed of each emitted electron is
KE =\(\frac{1}{2}\) mv2 = eV
∴ v = \(\left(\frac{2 e V}{m}\right)^{1 / 2}=\left(2 V \times \frac{e}{m}\right)^{1 / 2} \)
= (2x 500 xl.76 x 1011)1/2
= 1.327 x 107 m/s

(b) Potential of the anode, V = 10 MV = 10 x 106 = 107 V
The speed of each electron is given as
v = \(\left(2 V \frac{e}{m}\right)^{1 / 2}\)
= (2 x 107x 1.76 x 1011)1/2
= 1.88 x 109 m/s .
This result is wrong because nothing can move faster than light. In the above formula, the expression (mv2 / 2) for energy can only be used in the non-relativistic limit, i. e., for v < < c.
For very high speed problems, relativistic equations must be considered for solving them. In the relativistic limit, the total energy is given as E = mc2
where, m = relativistic mass
m0 = \(\left(1-\frac{v^{2}}{c^{2}}\right)^{1 / 2}\) = mass of the particle at rest
Kinetic energy is given as
K = mc2 – m0c2

Question 21.
(a) A monoenergetic electron beam with electron speed of 5.20x 106 ms-1 is subject to a magnetic field of 1.30 x 10 4T normal to the beam velocity. What is the radius of the circle traced by the beam, given elm for electron equals 1.76x 1011 C kg-1
(b) Is the formula you employ in (a) valid for calculating radius of the path of a 20 MeV electron beam? If not, in what way is it modified? [Note: Exercises 11.20 (b) and 11.21 (b) take you to relativistic mechanics which is beyond the scope of this book. They have been inserted here simply to emphasise the point that the formulas you use in part (a) of the exercises are not valid at very high speeds or energies. See answers at the end to know what ‘very high speed or energy’ means.]
Answer:
(a) Speed of the electron, v = 5.20 x 106 m/s
Magnetic field experienced by the electron, B = 1.30 x 10-4 T
Specific charge of the electron, e/m = 1.76 x 1011 C kg’
where, e = charge on the electron = 1.6 x 10-19 C
m = mass of the electron = 9.1 x 10-31 kg
The force exerted on the electron is given as
F = e\(|\vec{v} \times \vec{B}|\)
= evBsinθ
θ = angle between the magnetic field and the beam velocity.

The magnetic field is normal to the direction of beam.
∴ θ = 90°
F = evB ……………………………. (1)
The beam traces a circular path of radius, r. It is the magnetic field, due to its bending nature, that provides the centripetal force \(\left(F=\frac{m v^{2}}{r}\right)\) for the
beam.
Hence, equation (1) reduces to
evB = \(\frac{m v^{2}}{r}\)
∴ r = \(\frac{m v}{e B}=\frac{v}{\left(\frac{e}{m}\right) B}=\frac{5.20 \times 10^{6}}{\left(1.76 \times 10^{11}\right) \times 1.30 \times 10^{-4}}\)
= 0.227 m
= 0.227 x 100 cm = 22.7 cm
Therefore, the radius of the circular path is 22.7 cm.

(b) Energy of the electron beam, E = 20 MeV = 20 x 106 x 1.6 x 10-19 J
The energy of the electron is given as
E = \(\frac{1}{2} \) mv2
∴ v = \(\left(\frac{2 E}{m}\right)^{1 / 2}\)
= \(\sqrt{\frac{2 \times 20 \times 10^{6} \times 1.6 \times 10^{-19}}{9.1 \times 10^{-31}}}\) = 2.652 x 109 m/s

This result is incorrect because nothing can move faster than light. In the above formula, the expression (mv2 / 2) for energy can only be used in the non-relativistic limit, i. e., for v« c.

When very high speeds are concerned, the relativistic domain comes into consideration. In the relativistic domain, mass is given as
m = m0 \(\left[1-\frac{v^{2}}{c^{2}}\right]^{1 / 2}\)
where, m0 = mass of the particle at rest

Hence, the radius of the circular path is given as
r = mv/eB = \(\frac{m_{0} v}{e B \sqrt{\frac{c^{2}-v^{2}}{c^{2}}}}\).

Question 22.
An electron gun with its collector at a potential of 100 V fires out electrons in a spherical hulh containing hydrogen gas at low pressure (~ 10-2 nun of Hg). A magnetic field of 2.83 x 10-4 T curves the path of the electrons in a circular orbit of radius 12.0 cm. (The path can be viewed because the gas ions in the path focus the beam by attracting electrons, and emitting light by electron capture; this method is known as the ‘fine beam tube’ method). Determine elm from the data.
Answer:
Potential of the anode, V = 100 V
Magnetic field experienced by the electrons, B = 2.83 x 10,sup>-4 T
Radius of the circular orbit r = 12.0 cm = 12.0 x 10-2 m
Mass of each electron = m
Charge on each electron = e ‘
Velocity of each electron = v

The energy of each electron is equal to its kinetic energy, i. e.,
\(\frac{1}{2}\) mv2 = eV
v2 = \(\frac{2 e V}{m}\)
It is the magnetic field, due to its bending nature, that provides the \(\left(F=\frac{m v^{2}}{r}\right) \) for the beam.
Hence, we can write Centripetal force = Magnetic force mv2
\(\frac{m v^{2}}{r}\) = evB
eB = \(\frac{m v}{r}\)
v = \(\frac{e B r}{m}\) ………………………………. (2)
Putting the value of y in equation (1), we get
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 10
Therefore, the specific charge ratio (e/ m) is 1.73 x 1011 C kg-1.

Question 23.
(a) An X-ray tube produces a continuous spectrum of radiation with its short wavelength end at 0.45 A. What is the maximum energy of a photon in the radiation?
(b) From your answer to (a), guess what order of accelerating voltage (for electrons) is required in such a tube?
Answer:
(a) Wavelength produced by the X-ray tube, λ= 0.45Å= 0.45 x 10-10 m
Planck’s constant, h = 6.63 x 10-34 Js
Speed of light, c = 3 x 108 m/s
The maximum energy of a photon is given as
E = \(=\frac{h c}{\lambda}\frac{6.63 \times 10^{-34} \times 3 \times 10^{8}}{0.45 \times 10^{-10} \times 1.6 \times 10^{19}} \) = 27.6 x 103 eV = 27.6 keV
Therefore, the maximum energy of the X-ray photon is 27.6 keV.

(b) Accelerating voltage provides energy to the electrons for producing X-rays.
To get an X-ray of 27.6 keV, the incident electrons must possess at least 27.6 keV of kinetic electric energy. Hence, an accelerating voltage of the order of 30 keV is required for producing X-rays.

PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter

Question 24.
In an accelerator experiment on high-energy collisions of electrons with positrons, a certain event is interpreted as an annihilation of an electron-positron pair of total energy 10.2 BeV into two y-rays of equal energy. What is the wavelength associated with each γ-ray? (1 BeV = 109 eV)
Answer:
Total energy of two γ-rays
E = 10.2 BeV
= 10.2 x 109 eV
= 10.2 x 109 x 1.6 x 10-19 J
= 10.2 x 1.6 x 10-10

Hence, the energy of each γ-ray
E’ = \(\frac{E}{2}\)
= \(\frac{10.2 \times 1.6 \times 10^{-10}}{2}\)
= 8.16 x 10-10 J
Planck’s constant, h = 6.63 x 10-34 Js
Speed of light, c = 3 x 108 m/s
Energy is related to wavelength as
E ‘ = \(\frac{h c}{\lambda}\)
∴ λ = \(\frac{h c}{E^{\prime}}=\frac{6.63 \times 10^{-34} \times 3 \times 10^{8}}{8.16 \times 10^{-10}}\)
= 2.436 x 10-16 m
Therefore, the wavelength associated with each γ-ray is 2.436 x 10-16 m.

Question 25.
Estimating the following two numbers should be interesting. The first number will tell you why radio engineers do not need to worry much about photons. The second number tells you why our eye can never ‘count photons’, even in barely detectable light.
(a) The number of photons emitted per second by a Mediumwave transmitter of 10 kW power, emitting radio waves of wavelength 500 m.
(b) The number of photons entering the pupil of our eye per second corresponding to the minimum intensity of white light that we humans can perceive (~ 10-10 W m-2). Take the area of the pupil to be about 0.4 cm2, and the average frequency of white light to be about 6 x 1014 Hz.
Answer:
(a) Power of the medium wave transmitter,
P = 10kW = 104 W = 104 J/s
Hence, the energy emitted by the transmitter per second, E = 104
The wavelength of the radio wave, λ = 500 m
The energy of the wave is given as E1 = \(\frac{h c}{\lambda}\)
where, h = Planck’s constant = 6.63 x 10-34 Js
c = speed of light = 3 x 108 m/s
∴ E1 = \(\frac{6.63 \times 10^{-34} \times 3 \times 10^{8}}{500}\) = 3.96 x 10-28

Let n be the number of photons emitted by the transmitter.
∴ nE1 = E
n = \(\frac{E}{E_{1}}\) =\(\frac{10^{4}}{3.96 \times 10^{-28}}\) = 2.525 x 1031
≈ 3 x 1031
The energy (E1) of a radio photon is very less, but the number of photons (n) emitted per second in a radio wave is very large. The existence of a minimum quantum of energy can be ignored and the total energy of a radio wave can be treated as being continuous.

(b) Intensity of light perceived by the human eye, I = 10-10 W m-2
Area of the pupil, A = 0.4 cm2 = 0.4 x 10-4 m2
Frequency of white light, v = 6 x 1014 Hz
The energy emitted by a photon is given as
E = hv
where, h = Planck’s constant = 6.63 x 10-34 Js
∴ E = 6.63 x 10-34 x 6 x 1014
= 3.96 x 10-19 J
Let n be the total number of photons falling per second, per unit area of the pupil. The total energy per unit for n falling photons is given as
E = n x 3.96 x 10-19 Js-1 m-2
The energy per unit area per second is the intensity of light.
∴ E = I
n x 3.96 x 10-19 = 10-10
n = \(\frac{10^{-10}}{3.96 \times 10^{-19}}\)
= 2.52 x 108 m2 s-1

The total number of photons entering the pupil per second is given as
nA =n x A
= 2.52 x 108 x 0.4 x 10-4
= 1.008 x 104 s-1
This number is not as large as the one found in problem (a), but it is large enough for the human eye to never see the individual photons.

Question 26.
Ultraviolet light of wavelength 2271 Å from a 100 W mercury source irradiates a photo-cell made of molybdenum metal. If the stopping potential is -1.3 V, estimate the work function of the metal. How would the photo-cell respond to a high intensity (~105 Wm-2) red light of wavelength 6328 Å produced by a He-Ne laser?
Answer:
Wavelength of ultraviolet light, λ = 2271 Å = 2271 x 10-10 m
Stopping potential of the metal, V0 = 1.3 V
Planck’s constant, h = 6.63 x 10-34 Js
Charge on an electron, e = 1.6 x 10-19 C
Work function of the metal = Φ0
Frequency of light = v
We have the photo-energy relation from the photoelectric effect as
Φ0 =hv0
v0 = \(\frac{\phi_{0}}{h}\)
= \(\frac{6.64 \times 10^{-19}}{6.63 \times 10^{-34}} \)
= 1.006 x 1015 Hz = 4.15 eV
Let v0 be the threshold frequency of the metal.
∴ Φ0 = hv0
v0 = \(\frac{\phi_{0}}{h}\)
= \(\frac{6.64 \times 10^{-19}}{6.63 \times 10^{-34}}\)
= 1.006 x 1015 Hz

Wavelength of red light, λr = 6328 Å = 6328 x 10-10
∴ Frequency of red light, vr = \(\frac{c}{\lambda_{r}}=\frac{3 \times 10^{8}}{6328 \times 10^{-10}} \)
= 4.74 x 1014 Hz
Since v0 > vr, the photocell will not respond to the red light produced by the laser.

PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter

Question 27.
Monochromatic radiation of wavelength 640.2 nm (1 nm = 10-9 m) from a neon lamp irradiates photosensitive material made of caesium on tungsten. The stopping voltage is measured to be 0.54 V. The source is replaced by an iron source and its 427.2 nm line irradiates the same photo-cell. Predict the new stopping voltage.
Answer:
Wavelength of the monochromatic radiation, λ = 640.2 nm = 640.2 x 10-9 m
Stopping potential of the neon lamp, V0 = 0.54 V
Charge on an electron, e = 1.6 x 10-19 C
Planck’s constant, h = 6.63 x 10-34 Js
Let Φ0 the work function and v be the frequency of emitted light. We have the photo-energy relation from the photoelectric effect as
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 11
Wavelength of the radiation emitted from an iron source, λ’ = 427.2 nm = 427.2 x 10-9 m
Let V’0 be the new stopping potential. Hence, photo-energy is given as
eV’0 = \(\frac{h c}{\lambda^{\prime}}-\phi_{0}\)
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 12
Hence, the new stopping potential is 1.50 eV.

Question 28.
A mercury lamp is a convenient source for studying frequency dependence of photoelectric emission, since it gives a number of spectral lines ranging from the UV to the red end of the visible spectrum. In our experiment with rubidium photo-cell, the following lines from a mercury source were used :
λ1 =3650 Å,
λ2 = 4047 Å,
λ3 =4358 Å,
λ4 =5461 Å,
λ5 =6907 Å,
The stopping voltages, respectively, were measured to be :
V01 = 1.28 V,
V02 = 0.95 V,
V03 = 0.74 V,
V04 = 0.16 V,
V05 = 0 V.
Determine the value of Planck’s constant h, the threshold frequency and work function for the material.
[Note: You will notice that to get h from the data, you will need to know e (which you can take to be 1.6 x 10-19 C). Experiments of this kind on Na, Li, K, etc. were performed by Millikan, who, using his own value of e (from the oil-drop experiment) confirmed Einstein’s photoelectric equation and at the same time gave an independent estimate of the value of h.]
Answer:
Given, the following wavelength from a mercury source were used
λ1 =3650 Å, = 3650 x 10-10 m
λ2 = 4047 Å, = 4047 x 10-10m
λ3 =4358 Å, = 4358 x 10-10 m
λ4 =5461 Å, = 5461 x 10-10 m
λ5 =6907 Å, = 6907 x 10-10m
The stopping voltages are as follows
V01 =1.28 V,
V02 = 0.95V,
V03 =0.74V,
V04 =0.16 V,
V5 =0

Frequencies corresponding to wavelengths
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 13
As we know that
eV0 = hv-Φ0
v0 = \(\frac{h v}{e}-\frac{\phi_{0}}{e}\)
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 14
As the graph between V0 and frequency v is a straight line.
The slope of this graph gives the values of \(\frac{h}{e} \)
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 15
(b) Work function, Φ0 = hv0
= 6.574 x 10-34 x 5 x 1014
= 32.870 x 10-20J
= 2.05eV.

PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter

Question 29.
The work function for the following metals is given: Na: 2.75 eV; K: 2.30 eV; Mo: 4.17 eV; Ni: 5.15 eV.
Which of these metals will not give photoelectric emission for a radiation of wavelength 3300 Å from a He-Cd laser placed 1 m away from the photo-cell? What happens if the laser is brought nearer and placed 50 cm away?
Answer:
Mo and Ni will not show photoelectric emission in both cases.
Wavelength for the radiation, λ – 3300 Å = 3300 x 10-10 m
Speed of light, c = 3 x 108 m/s
Planck’s constant, h = 6.63 x 10-34 Js
The energy of incident radiation is given as
E = \(\frac{h c}{\lambda}\)
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 16
= 3.758eV
It can be observed that the energy of the incident radiation is greater than the work function of Na and K only. It is less for Mo and Ni. Hence, Mo and Ni will not show photoelectric emission. If the source of light is brought near the photocells and placed 50 cm away from them, then the intensity of radiation will increase. This does not affect the energy of the radiation. Hence, the result will be the same as before. However, the photoelectrons emitted from Na and K will increase in proportion to intensity.

Question 30.
Light of intensity 10-5 W m-2 falls on a sodium photo-cell of surface area 2 cm2. Assuming that the top 5 layers of sodium absorb the incident energy, estimate time required for photoelectric emission in the wave-picture of radiation. The work function for the metal is given to be about 2 eV. What is the implication of your answer?
Answer:
Given, intensity of light = 10-5 W/m2
Area = 2 cm2 = 2 x 10-4 m2
Work function for the metal Φ0 = 2 eV
Let t be the time.
The effective atomic area of Na = 10-20 m2 and it contains one conduction electron per atom.
Number of conduction electrons in five layers
= \(\frac{5 \times \text { Area of one layer }}{\text { Effective atomic area }}=\frac{5 \times 2 \times 10^{-4}}{10^{-20}}\)
= 107
We know that sodium has one free electron (or conduction electron) per atom.
Incident power on the surface area of photocell
= Incident intensity x Area on the surface area of photocell
= 10-5 x 2 x 10-4
= 2 x 10-9W
The electron present in all the 5 layers of sodium will share the incident energy equally.
Energy absorbed per second per electron,
E = \(\frac{\text { Incident power }}{\text { Number of electrons in five layers }} \)
= \(\frac{2 \times 10^{-9}}{10^{17}}\) = 2 x 10-26W
Time required for emission by each electron, t = \(\frac{\text { Energy required per electron }}{\text { Energy absorbed per second }} \) = \(\frac{2 \times 1.6 \times 10^{-19}}{2 \times 10^{-26}} \) = 1.6 x 107s
which is about 0.5 yr.
The answer obtained implies that the time of emission of electrons is very large and is not agreement with the observed time of emission. There is no time lag between the incidence of light and the emission of photoelectrons. Thus, it is implied that the wave theory cannot be applied in this experiment.

Question 31.
Crystal diffraction experiments can be performed using X-rays, or electrons accelerated through appropriate voltage. Which probe has greater energy? (For quantitative comparison, take the wavelength of the probe equal to 1 Å, which is of the order of interatomic spacing in the lattice) (me = 9.11 x 10-31 kg).
Answer:
An X-ray probe has a greater energy than an electron probe for the same wavelength.
Wavelength of light emitted from the probe, λ = 1 Å = 10-10 m
Mass of an electron, me = 9.11 x 10-31 kg
Planck’s constant, h = 6.63 x 10-34 Js
Charge on an electron, e = 1.6 x 10-19 C
The kinetic energy of the electron is given as
K = \(\frac{1}{2} m_{n} v^{2}\)
mnv = \(\sqrt{2 K m_{n}}\)
where, v = velocity of the electron
mnv = momentum (p) of the electron

According to the de Brogue principle, the de Brogue wavelength is given as
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 17
The energy of a photon,
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 18
Hence, a photon has a greater energy than an electron for the same wavelength.

Question 32.
(a) Obtain the de Broglie wavelength of a neutron of kinetic energy 150 eV. As you have seen in Exercise 11.31, an electron beam of this energy is suitable for crystal diffraction experiments. Would a neutron beam of the same energy be equally suitable? Explain. (m = 1.675x 10-27 kg)
(b) Obtain the de Brogue wavelength associated with thermal neutrons at room temperature (27C). Hence, explain why a fast neutron beam needs to be thermalised with the environment before it can be used for neutron diffraction experiments.
Answer:
(a) de Brogue wavelength= 2.327 x 10-12 m; neutron is not suitable for
the diffraction experiment Kinetic energy of the neutron, K = 150 eV
= 150 x 1.6 X 10-19
=2.4 x 10-17 J
Mass of a neutron, mn = l.675 x 10-27 kg
The kinetic energy of the neutron is given by the relation
K = \(\frac{1}{2} m_{n} v^{2}\)
mnv = \(\sqrt{2 K m_{n}}\)
where, y = velocity of the neutron
mnv = momentum of the neutron

deBroglie wavelength of the neutron is given as
λ = \(\frac{h}{m_{n} v}=\frac{h}{\sqrt{2 K m_{n}}}\)
It is clear that wavelength is inversely proportional to the square root of mass.
Hence, wavelength decreases with increase in mass and vice versa.
∴ λ = \(\frac{6.63 \times 10^{-34}}{\sqrt{2 \times 2.4 \times 10^{-17} \times 1.675 \times 10^{-27}}}\)
= 2.327 x 10-12 m
It is given in the previous problem that the interatomic spacing of a crystal is about 1 Å, i.e., 10-10 m.
Hence, the interatomic spacing is about a hundred times greater. Hence, a neutron beam of energy 150 eV is not
suitable for diffraction experiments.

(b) de Brogue wavelength =1.447 x 10-10 m
Room temperature, T = 27°C = 27+ 273 = 300 K
The average kinetic energy of the neutron is given as
E=\(\frac{3}{2}\) kT
where, k = Boltzmann’s constant = 1.38 x10-23 J mol-1K-1
The wavelength of the neutron is given as
λ = \(\frac{h}{\sqrt{2 m_{n} E}}=\frac{h}{\sqrt{3 m_{n} k T}}\)
= \(\frac{6.63 \times 10^{-34}}{\sqrt{3 \times 1.675 \times 10^{-27} \times 1.38 \times 10^{-23} \times 300}}\)
=1.447 x 10-10 m
This wavelength is comparable to the interatomic spacing of a crystal. Hence, the high energy neutron beam should first be thermalised, before using it for diffraction.

Question 33.
An electron microscope uses electrons accelerated by a voltage of 50 kV. Determine the de Brogue wavelength associated with the electrons, if other factors (such as numerical aperture, etc.) are taken to be roughly the same, how does the resolving power of an electron microscope compare with that of an optical microscope which uses yellow light?
Answer:
Electrons are accelerated by a voltage, V = 50 kV = 50 x 103 V
Charge on an electron, e 1.6 x 10-19 C
Mass of an electron, me = 9.11 x 10-31 kg
Wavelength of yellow light = 5.9 x 10-7 m

The kinetic energy of the electron is given as
E=eV
=l.6 x 10-19x 50x 103
= 8 x 10-15 J
de Brogue wavelength is given by the relation
λ = \(\frac{h}{\sqrt{2 m_{e} E}}\)
= \(\frac{6.63 \times 10^{-34}}{\sqrt{2 \times 9.11 \times 10^{-31} \times 8 \times 10^{-15}}}\)
= 5.467 x 10-12 m
This wavelength is nearly 105 times less than the wavelength of yellow light. The resolving power of a microscope is inversely proportional to the wavelength of light used. Thus, the resolving power of an electron microscope is nearly 105 times that of an optical microscope.

PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter

Question 34.
The wavelength of a probe is roughly a measure of the size of a structure that it can probe in some detail. The quark structure of protons and neutrons appears at the minute length scale of 10-15 m or less. This structure was first, probed in early 1970s using high energy electron beams produced by a linear accelerator at Stanford, USA. Guess what might have been the order of energy of these electron beams. (Rest mass energy of electron = 0.511 MeV.)
Answer:
Wavelength of a proton or a neutron, λ ≈ 10-15 m
Rest mass energy of an electron,
m0c2 =0.511 MeV
= 0.511 x 106 x 1.6 x 10-19
= 0.8176 x 10-13 J
Planck’s constant, h = 6.63 x 10-34 Js
Speed of light, c = 3 x 108 m/s ,

The momentum of a proton or a neutron is given as
p = \(\frac{h}{\lambda}\)
= \(\frac{6.63 \times 10^{-34}}{10^{-15}}\) = 6.6 x 10-19 kg m/s
The relativistic relation for energy (E) is given as
E2 = p2c2 +m02c4
= (6.6x 10-19 x 3 x 108)2 + (0.8176 x 10-13)2
= 392.04 x 10-22 +0.6685 x 10-26
≈ 392.04 x 10-22
∴ E = 1.98x 10-10 J
Thus, the electron energy emitted from the accelerator at Stanford, USA might be of the order of 1.24 BeV.

Question 35.
Find the typical de Broglie wavelength associated with a He atom in helium gas at room temperature (27°C) and 1 atm pressure, and compare it with the mean separation between two atoms under these conditions.
Answer:
de Broglie wavelength associated with He atom = 0.7268 x 10-10 m .
Room temperature, T = 27°C =27 + 273 = 300 K
Atmospheric pressure, P = 1 atm = 1.01 x 105 Pa
Atomic weight of a He atom = 4
Avogadro’s number, NA = 6.023 x 1023
Boltzmann’s constant, k = 1.38 x 10-23 J mol-1 K-1

Average energy of a gas at temperature T, is given as
E = \(\frac{3}{2}\) kT
de Broglie wavelength is given by the relation
λ = \(\frac{h}{\sqrt{2 m E}}\)
where, m = mass of a He atom
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 19
We have the ideal gas formula
PV = RT
PV = kNT
∵ \(\frac{V}{N}=\frac{k T}{P}\)
where V = volume of the gas
N = number of moles of the gas
Mean separation between two atoms of the gas is given by the relation
r = \(\left(\frac{V}{N}\right)^{1 / 3}=\left(\frac{k T}{P}\right)^{1 / 3}=\left[\frac{1.38 \times 10^{-23} \times 300}{1.01 \times 10^{5}}\right]^{1 / 3}\)
= 3.35 x 10-9 m
Hence, the mean separation between the atoms is much greater than the de Broglie wavelength.

Question 36.
Compute the typical de Broglie wavelength of an electron in a metal at 2 7° C and compare it with the mean separation between two electrons in a metal which is given to be about 2 x 1010 m. [Note: Exercises 11.35 and 11.36 reveal that while the wave-packets associated with gaseous molecules under ordinary conditions are non-overlapping, the electron wave- packets in a metal strongly overlap with one another. This suggests that whereas molecules in an ordinary gas can be distinguished apart, electrons in a metal cannot be distinguished apart from one another. This indistinguishability has many fundamental implications which you will explore in more advanced Physics courses.]
Answer:
Temperature, T = 27°C = 27 +273 = 300 K
Mean separation between two electrons, r = 2 x 10-10 m
de Broglie wavelength of an electron is given as
λ = \(\frac{h}{\sqrt{3 m k T}}\)
where,
h = Planck’s constant = 6.63 x 10-34 Js
m = Mass of an electron = 9.11 x 10 -31 kg
k = Boltzmann’s constant = 1.38 x 10-23 J mol-1 K-1
∴ λ = \(\frac{6.63 \times 10^{-34}}{\sqrt{3 \times 9.11 \times 10^{-31} \times 1.38 \times 10^{-23} \times 300}}\)
= 6.2 x 109 m
Hence, the de Broglie wavelength is much greater than the given inter-electron separation.

Question 37.
Answer the following questions :
(a) Quarks inside protons and neutrons are thought to carry fractional charges [(+2/3)e ; (-1/3)e]. Why do they not show up in Millikan’s oil-drop experiment?
(b) What is so special about the combination e/m? Why do we not simply talk of e and m separately?
(c) Why should gases be insulators at ordinary pressures and start conducting at very low pressures?
(d) Every metal has a definite work function. Why do all photoelectrons not come out with the same energy if incident radiation is monochromatic? Why is there an energy distribution of photoelectrons?
(e) The energy and momentum of an electron are related to the frequency and wavelength of the associated matter wave by the relations :
E = hv, p = \(\frac{\boldsymbol{h}}{\lambda}\)
But while the value of λ is physically significant, the value of v (and therefore, the value of the phase speed vλ) has no physical significance. Why?
Answer:
(a) Quarks inside protons and neutrons carry fractional charges. This is because nuclear force increases extremely if they are pulled apart. Therefore, fractional charges may exist in nature; observable charges are siti the integral multiple of an electrical charge.

(b) Thè basic relations for electric field and magnetic field are \(\left(e V=\frac{1}{2} m v^{2}\right)\) and \(\left(e B v=\frac{m v^{2}}{r}\right) \) respectively.

These relations include e (electric charge), y (velocity), m (mass), V (potential), r (radius), and B (magnetic field)._These relations give the value of velocity of an electron as \(\left(v=\sqrt{2 v\left(\frac{e}{m}\right)}\right) \text { and }\left(v=B r\left(\frac{e}{m}\right)\right)\) respectively. It can be observed from these relations that the dynamics of an electron is determined not by e and m separately, but by the ratio e / m.

(c) At atmospheric pressure, the ions of gases have no chance of reaching their respective electrons because of collision and recombination with other gas molecules. Hence, gases are insulators at atmospheric pressure. At low pressure, ions have a chance of reaching their respective electrodes and constitute a current. Hence, they conduct electricity at these pressures.

(d) The work function of a metal is the minimum energy required for a conduction electron to get out of the metal surface. All the electrons in an atom do not have the same energy level. When a ray having some photon energy is incident on a metal surface, the electrons come out from different levels with different energies. Hence, these emitted electrons show different energy distributions.

PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter

(e) The absolute value of energy of a particle is arbitrary within the additive constant. Hence, wavelength (λ) is significant, but the frequency (v) associated with an electron has no direct physical significance. Therefore, the product vλ (phase speed) has no physical significance. Group speed is given as
PSEB 12th Class Physics Solutions Chapter 11 Dual Nature of Radiation and Matter 20
This Quantity has a physical meaning.

PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism

Punjab State Board PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism Important Questions and Answers.

PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism

Very short answer type questions

Question 1.
Using the concept of force between two infinitely long parallel current carrying conductors define one ampere of current.
Answer:
One ampere is that value of current which flows through two straight, parallel infinitely long current carrying conductors placed in air or. vacuum at a distance of 1 m and they experience a force of attractive or repulsive nature of magnitude 2 × 10-7 N/m on their unit length.

Question 2.
State Ampere’s circuit law.
Answer:
It states that the line integral of the magnetic field \(\vec{B}\) around any closed circuit is equal to p0 times the total current passing through this closed circuit.
\(\oint \vec{B} \cdot \overrightarrow{d l}\) = μ0 I

PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism

Question 3.
A narrow beam of protons and deuterons, each having the same momentum, enters a region of uniform magnetic field directed perpendicular to their direction of momentum. What would be the ratio of the radii of the circular path described by them?
Answer:
For the given momentum of charge particle, radius of circular paths depends on charge and magnetic field as
r = \(\) ⇒ r ∝ \(\)
For given momentum,
∴ rproton : rdeuteron = 1 : 1
As they have same momentum and charge moving in a small magnetic field.

Question 4.
Write the expression, in a vector form, for the Lorentz magnetic force \(\overrightarrow{\boldsymbol{F}}\) due to a charge moving with velocity \(\vec{v}\) in a magnetic field \(\overrightarrow{\boldsymbol{B}}\). What is the direction of the magnetic force?
Answer:
Force, \(\vec{F}=q(\vec{v} \times \vec{B})\)
Obviously, the force on charged particle is perpendicular to both velocity \(\vec{v}\) and magnetic field \(\vec{B}\).

Question 5.
When a charged particle moving with velocity \(\vec{v}\) is subjected to magnetic field \(\overrightarrow{\boldsymbol{B}}\), the force acting on it is non-zero. Would the particle gain any energy?
Answe:
No. (i) This is because the charge particle moves on a circular path.
(ii) \(\vec{F}=q(\vec{\nu} \times \vec{B})\)
and power dissipated p = \(\vec{F} \times \vec{V}\)
= q \((\vec{v} \times \vec{B}) \times \vec{y}\) = p\((\vec{v} \times \vec{v}) \times \vec{B}\)
The particle does not gain any energy.

Question 6.
A square coil OPQR of side a carrying a current 7, is placed in the Y-Z plane as shown here. Find the magnetic moment associated with this coil.
PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism 1
Answer:
The magnetic moment associated with the coil, is \(\vec{\mu}\)m = Ia2î

PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism

Question 7.
Under what condition is the force acting on a charge moving through a uniform magnetic field is minimum?
Answer:
Fm = qvB sinθ; for minimum force sinθ = 0. i.e., force is minimum when charged particle move parallel or anti-parallel to the field.

Question 8.
What is the nature of magnetic field in a moving coil galvanometer?
Answer:
The nature of magnetic field in a moving coil galvanometer is radial.

Question 9.
Verify that the cyclotron frequency ω = eB/m has the correct dimensions of [T]-1. (NCERT Exemplar)
Or A charged particle of charge e and mass m is moving in an electric field E and magnetic field B. Construct dimensionless quantities and quantities of dimension [T]-1. (NCERT Exemplar)
Answer:
For a charge particle moving perpendicular to the magnetic field, the magnetic Lorentz forces provides necessary centripetal force for revolution.
\(\frac{m v^{2}}{R}\) = qvB
On simplifying the terms, we have
∴ \(\frac{q B}{m}=\frac{v}{R}\) = ω
Finding the dimensional formula of angu
∴ [ω] = \(\left[\frac{q B}{m}\right]=\left[\frac{v}{R}\right]\) = [T] -1

PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism

Question 10.
Show that a force that does no work must be a velocity dependent force. (NCERT Exemplar)
Answer:
Let no work is done by a force, so we have
dW = F.dl = 0
⇒ F. v dt = 0 (Since, dl = v dt and dt ≠ 0)
⇒ F.v = 0
Thus, F must be velocity dependent which implies that angle between F and v is 90°. If v changes (direction), then (directions) F should also change so that above condition is satisfied.

Question 11.
The magnetic force depends on v which depends on the inertial frame of reference. Does then the magnetic force differ from inertial frame to frame? Is it reasonable that the net acceleration has a different value in different frames of reference? (NCERT Exemplar|
Answer:
Yes, the magnetic force differ from inertial frame to frame. The magnetic force is frame dependent.
The net acceleration which comes into existing out of this is however, frame independent (non-relativistic physics) for inertial frames.

Question 12.
An electron enters with a velocity υ = υ0î into a cubical region (faces parallel to coordinate planes) in which there are uniform electric and magnetic fields. The orbit of the electron is found to spiral down inside the cube in plane parallel to the x-y plane. Suggest a configuration of fields E and B that can lead to it. (NCERT Exemplar)
Answer:
Considering magnetic field B = B0k̂, and an electron enters with a velocity v = v0î into a cubical region (faces parallel to coordinate planes). The force on electron, using magnetic Lorentz force, is given by
F = -e(v0î x B0k̂) = ev0B0î
which revolves the electron in x-y plane.
The electric force F = -eE0k̂ accelerates e along z-axis which in turn increases the radius of circular path and hence particle traversed on spiral path.

PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism

Short answer type questions

Question 1.
Write any two important points of similarities and differences each between Coulomb’s law for the electrostatic field and . Biot-Savart’s law for the magnetic field.
Answer:
Similarities: Both electrostatic field and magnetic field

  • follows the principle of superposition.
  • depends inversely on the square of distance from source to the point of interest.

Differences I

  • Electrostatic field is produced by a scalar source (q) and the magnetic
    field is produced by a vector source (I\(\overrightarrow{d l}\)).
  • Electrostatic field is along the displacement vector between source and point of interest; while magnetic field is perpendicular to the plane, containing the displacement vector and vector source.
  • Electrostatic field is angle independent, while magnetic field is angle
    dependent between source vector and displacement vector.

Question 2.
State the underlying principle of a cyclotron. Write briefly how this machine is used to accelerate charged particles to high energies.
Or State the principle of the working of a cyclotron. Write two uses of this machine.
Answer:
The combination of crossed electric and magnetic fields is used to increase the energy of the charged particle. Cyclotron uses the fact that the frequency of revolution of the charged particle in a magnetic field is independent of its energy.
PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism 2

Inside the dees the particle is shielded from the electric field and magnetic field acts on the particle and makes it to go round in a circular path inside a dee.

Every time, the particle moves from one dee to the other it comes under the influence of electric field which ensures to increase the energy of the particle as the sign of the electric field changed alternately.
The increased energy increases the radius of the circular path so the accelerated particle moves in a spiral path.
Since, radius of trajectory
r = \(\frac{v m}{q B}\)
∴ v = \(\frac{r q B}{m}\)
Hence, the kinetic energy of ions
= \(\frac{1}{2}\)mv2 = \frac{1}{2}\(\)m\(\frac{r^{2} q^{2} B^{2}}{m^{2}}\)
⇒ KE = \(\frac{1}{2}\)\(\frac{r^{2} q^{2} B^{2}}{m}\)

PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism

Question 3.
(i) State Ampere’s circuital law expressing it in the integral form.
(ii) Two long co-axial insulated solenoids S1 and S2 of equal length are wound one over the other as shown in the figure. A steady current I flows through the inner solenoid S1 to the other end B which is connected to the outer solenoid S2 through which the some current I flows in the opposite direction so, as to come out at end A. If n1 and n2 are the number of turns per unit length, find the magnitude and direction of the net magnetic field at a point
(a) inside on the axis and
(b) outside the combined system.
PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism 3
Answer:
(i) Ampere’s circuital law states that the line integral of magnetic field
(B) around any closed path in vacuum is μ0 times the net current (I) threading the area enclosed by the curve.
Mathematically, \(\oint \vec{B} \cdot d \vec{l}\) = μ0I
Ampere’s law is applicable only for an Amperian loop as the Gauss’s law is used for Gaussian surface in electrostatics.

(ii) According to Ampere’s circuital law, the net magnetic field is given by
B = μ0nî.
(a) The net magnetic field is given by
Bnet = B2 – B1
μ0n2I20n1I1
= μ0I(n2 – n1)
The direction is from B to A.

(b) As the magnetic field due to Sx is confined solely inside S1 as the solenoids are assumed to be very long. So, there is no magnetic field outside S1 due to current in S1, similarly there is no field outside S2.
Bnet = 0

Question 4.
(a) State Biot-Savart law and express this law in the vector
form.
(b) Two identical circular coils, P and Q each of radius R, carrying currents 1 A and √3 A, respectively, are placed concentrically and perpendicular to each other lying in the XY and YZ planes. Find the magnitude and direction of the net magnetic field at the centre of the coils. ’
Answer:
(a) According to Biot-Savart’s law the magnitude of the magnetic field \(\overrightarrow{d B}\) due to a small element of length dl of a current carrying wire at a point P, is proportional to the current I, the element length dl and is inversly proportional to the square of the distance r. It is also proportional to sinθ,

where θ is the angle between \(\overrightarrow{d l}\) and \(\vec{r}\).
PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism 4
Its direction is perpendicular to the plane containing \(\overrightarrow{d l}\) and \(\vec{r}\) in vector form
\(\overrightarrow{d B}\) ∝ \(\frac{I \overrightarrow{d l} \times \vec{r}}{r^{3}}\)
⇒ \(\overrightarrow{d B}\) = \(\frac{\mu_{0}}{4 \pi} \cdot \frac{I d \vec{l} \cdot \vec{r}}{r^{3}}\)
(\(\overrightarrow{d l}\) is directed along the length of the wire in the direction of current and
\(\vec{r}\) is the vector joining the centre of current element to the point P) (b) Field due to current in coil P is
\(\vec{B}\)2 = \(\frac{\mu_{0} I_{1}}{2 R}\) .k̂
(Assuming current to be anticlockwise as seen form + ve Z-axis) and that due to current in coil Q is
\(\vec{B}\)2 = \(\frac{\mu_{0} I_{2}}{2 R} \hat{i}\)
(Assuming current to be anticlockwise as seen form positive X-axis)
PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism 5

PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism

Question 5.
Explain, giving reasons, the basic difference in converting a galvanometer into (i) a voltmeter and (ii) an ammeter.
Answer:
(i) In converting a galvanometer into a voltmeter, a very high suitable resistance is connected in series to its coil. So, the galvanometer gives full scale deflection.
(ii) In converting a galvanometer into an ammeter, a very small suitable resistance is connected in parallel to its coil. The remaining pair of the current i.e., (I – Ig) flows through the resistance. Here I = Circuit current
and Ig = Current through galvanometer.

Question 6.
A current carrying loop consists of 3 identical quarter circles of radius R, lying in the positive quadrants of the x-y, y-z and z-x planes with their centres at the origin, joined together. Find the direction and magnitude of B at the origin. (NCERT Exemplar)
Answer:
For the current carrying loop quarter circles of radius R, lying in the positive quadrants of the x-y plane
B1 = \(\frac{\mu_{0}}{4 \pi} \frac{I(\pi / 2)}{R}\) = k̂\(\frac{\mu_{0}}{4} \frac{I}{2 R}\) k̂
For the current carrying loop quarter circles of radius R, lying in the positive quadrants of the y-z plane
B2 = \(\frac{\mu_{0}}{4} \frac{I}{2 R}\)î
For the current carrying loop quarter circles of radius R, lying in the positive quadrants of the z-x plane
B3 = \(\frac{\mu_{0}}{4} \frac{I}{2 R}\)
Current carrying loop consists of 3 identical quarter circles of radius R, lying in the positive quadrants of the x-y, y-y and z-z planes with their centres at the origin, joined together is equal to the vector sum of magnetic field due to each quarter and given by,
B = \(\frac{1}{4 \pi}\) (î + ĵ + k̂)\(\frac{\mu_{0} I}{2 R}\)

PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism

Question 7.
A long straight wire carrying current of 25 A rests on a table as shown in figure. Another wire PQ of length 1 m, mass 2.5 g carries the same current but in the opposite direction.
The wire PQ is free to slide up and down. To what height will PQ rise? (NCERT Exemplar)
PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism 6
Answer:
The magnetic field produced by long straight wire carrying current of 25 A rests on a table on small wire
B = \(\frac{\mu_{0} I}{2 \pi h}\)
The magnetic force on small conductor is , F = BIl sin θ = BIl
Force applied on PQ balance the weight of small current carrying wire.
PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism 7

Long answer type questions

Question 1.
Derive an expression for the force per unit length between two long straight parallel current carrying conductors. Hence define SI unit of current (ampere).
Answer:
Suppose two long thin straight conductors (or wires) PQ and RS are placed parallel to each other in vacuum (or air) carrying currents I1 and Isub>2 respectively. It has been observed experimentally that when the currents in the wire are in the same direction, they experience an attractive force (fig. a) and when they carry currents in opposite directions, they experience a repulsive force (fig. b).

Let the conductors PQ and RS carry currents I1 and I2 in same direction and placed at separation r.

Consider a current-element ‘ab’ of length ΔL of wire RS. The magnetic field produced by current- carrying conductor PQ at the location of other wire RS.
B1 = \(\frac{\mu_{0} I_{1}}{2 \pi r}\) ………….(1)

According to Maxwell’s right hand rule or right hand palm rule number 1, the direction of B1 will be perpendicular to the plane of paper and directed downward. Due to this magnetic field, each element of other wire experiences a force. The direction of current element is perpendicular to the magnetic field; therefore the magnetic force on element ab of length ΔL
ΔF = B1I2ΔL sin90° = \(\frac{\mu_{0} I_{1}}{2 \pi r}\) I2 ΔL
∴ The total force on conductor of length L will be
F = \(\frac{\mu_{0} I_{1} I_{2}}{2 \pi r}\) ΔΣL = \(\frac{\mu_{0} I_{1} I_{2}}{2 \pi r}\)L
∴ Force acting per unit length of conductor
f = \(\frac{F}{L}\) = \(\frac{\mu_{0} I_{1} I_{2}}{2 \pi r}\) N/M …………… (2)

According to Fleming’s left hand rule, the direction of magnetic force will be towards PQ, i.e., the force will be attractive.

On the other hand if the currents I1 and I2 in wires are in opposite directions, the force will be repulsive. The magnitude of force in each case remains the same.

Definition of SI Unit of Current (Ampere) : In SI system of fundamental unit of current ‘ampere’ is defined assuming the force between the two current carrying wires as standard.
PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism 8
The force between two parallel current carrying conductors of separation r is
\(\frac{F}{L}\) = \(\frac{\mu_{0} I_{1} I_{2}}{2 \pi r}\) N/M

If I1 = I2 = 1A, r = lm, then
f = \(\frac{\mu_{0}}{2 \pi}\) = 2 x 10-7 N/m
Thus, 1 ampere is the current which when flowing in each of parallel conductors placed at separation 1 m in vacuum exert a force of 2 x 10-7 on 1 m length of either wire.

PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism

Question 2.
Draw the labelled diagram of a moving coil galvanometer. Prove that in a radial magnetic field, the deflection of the coil is directly proportional to the current flowing in the coil.
Or (a) Draw a labelled diagram of a moving coil galvanometer.
Describe briefly its principle and working.
(b) Answer the following:
(i) Why is it necessary to introduce a cylindrical soft iron core inside the coil of a galvanometer?
(ii) Increasing the current sensitivity of a galvanometer may not necessarily increase its voltage sensitivity. Explain, giving reason.
Or Explain, using a labelled diagram, the principle and working of a moving coil galvanometer. What is the function of (i) uniform radial magnetic field, (ii) soft iron core?
Or Define the terms (i) current sensitivity and (ii) voltage sensitivity of a galvanometer. Why does increasing the current sensitivity not necessarily increase voltage sensitivity?
Answer:
(a) Moving Coil Galvanometer: A galvanometer is used to detect current in a circuit.

Construction: It consists of a rectangular coil wound on a non-conducting metallic frame and is suspended by phosphor bronze strip between the pole-pieces (N and S) of a strong permanent magnet. A soft iron core in cylindrical form is placed between the coil.

One end of coil is attached to suspension wire which also serves as one terminal (Tx) of galvanometer. The other end of coil is connected to a loosely coiled strip, which serves as the other terminal (T2). The other end of the suspension is attached to a torsion head which can be rotated to set the coil in zero position. A mirror (M) is fixed on the phosphor bronze strip by means of which the deflection of the coil is measured by the lamp and scale arrangement. The levelling screws are also provided at the base of the instrument.

The pole pieces of the permanent magnet are cylindrical so that the magnetic field is radial at any position of the coil.
PSEB 12th Class Physics Important Questions Chapter 4 Moving Charges and Magnetism 9

Principle and Working : When current (I) is passed in the coil, torque τ acts on the coil, given by
τ = NIABsinθ

where θ is the angle between the normal to plane of coil and the magnetic field of strength B, N is the number of turns in a coil.

When the magnetic field is radial, as in the case of cylindrical pole pieces and soft iron core, then in every position of coil the plane of the coil, is parallel to the magnetic field lines, so that θ = 90° and sin 90 ° = 1. The coil experiences a uniform coupler.
Deflecting torque, τ = NIAB
If C is the torsional rigidity of the wire and θ is the twist of suspension strip, then restoring torque = C0. For equilibrium, deflecting torque = restoring torque
i.e., NIAB = Cθ
θ = \(\frac{N A B}{C}\)I ………… (1)
i.e., θ ∝ I
Deflection of coil is directly proportional to current flowing in the coil and hence we can construct a linear scale.

Importance (or Function) of Uniform Radial Magnetic Field
Torque as current carrying coil in a magnetic field is τ = NIAB sinθ In radial magnetic field sinθ = 1, so torque is τ = NIAB.
This makes the deflection (θ) proportional to current. In other words, the radial magnetic field makes the scale linear.

(b)
(i) The cylindrical, soft iron core makes the (1) field radial and (2) increases the strength of the magnetic field, i.e., the magnitude of the torque.

(ii) Sensitivity of Galvanometer
Current sensitivity: It is defined as the deflection of coil per unit current flowing in it.
Sensitivity,
I = (\(\frac{\theta}{I}\)) = \(\frac{N A B}{C}\)………… (1)
Voltage sensitivity: It is defined as the deflection of coil per unit potential difference across its ends.
i.e., SV = \(\frac{\theta}{V}\) = \(\frac{N A B}{R_{g} \cdot C}\) …………. (2)
where Rg is resistance of galvanometer.
Clearly for greater sensitivity number of turns N, area A and magnetic field strength B should be large and torsional rigidity C of suspension should be small.
Dividing eqs. (2) by (1)
\(\frac{S_{V}}{S_{I}}=\frac{1}{G}\) = 1 ⇒ SV = \(\frac{1}{G}\) SI
Clearly, the voltage sensitivity depends on current sensitivity and the resistance of galvanometer. If we increase current sensitivity and resistance G is larger, then it is not certain that voltage sensitivity will be increased. Thus, the increase of current sensitivity does not imply the increase of voltage sensitivity.

PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism

Punjab State Board PSEB 12th Class Physics Book Solutions Chapter 4 Moving Charges and Magnetism Textbook Exercise Questions and Answers.

PSEB Solutions for Class 12 Physics Chapter 4 Moving Charges and Magnetism

PSEB 12th Class Physics Guide Moving Charges and Magnetism Textbook Questions and Answers

Question 1.
A circular coil of wire consisting of 100 turns, each of radius
8.0 cm carries a current of 0.40 A. What is the magnitude of the magnetic field B at the centre of the coil?
Answer:
Number of turns on the circular coil, n = 100
Radius of each turn, r = 8.0 cm = 0.08 m
Current flowing in the coil, I = 0.4 A
Magnitude of the magnetic field at the centre of the coil is given by the relation,
\(|B|=\frac{\mu_{0}}{4 \pi} \frac{2 \pi n I}{r}\)
where, μ0 = 4π × 10-7 TmA-1
\(|B|\) = \(\frac{4 \pi \times 10^{-7}}{4 \pi}\) × \(\frac{2 \pi \times 100 \times 0.4}{0.08}\)
= 3.14 × 10-4T
Hence, the magnitude of the magnetic field is 3.14 × 10-4 T

Question 2.
A long straight wire carries a current of 35 A. What is the magnitude of the field B at a point 20 cm from the wire?
Answer:
Current in the wire, I = 35 A
Distance of the point from the wire, r = 20 cm = 0.2 m
Magnitude of the magnetic field at this point is given as
B = \(\frac{\mu_{0}}{4 \pi} \frac{2 I}{r}\)
B = \(\frac{4 \pi \times 10^{-7} \times 2 \times 35}{4 \pi \times 0.2}\)
= 3.5 × 10-5T
Hence, the magnitude of the magnetic field at a point 20 cm from the wire is 3.5 × 10-5 T.

PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism

Question 3.
A long straight wire in the horizontal plane carries a current of 50 A in north to south direction. Give the magnitude and direction of B at a point 2.5 m east of the wire.
Answer:
Current in the wire, I = 50 A
A point is 2.5 m away from the east of the wire.
∴ Magnitude of the distance of the point from the wire, r = 2.5 m
Moving Charges and Magnetism ini
Magnitude of the magnetic field at that point is given by the relation,
B = \(\frac{\mu_{0} 2 I}{4 \pi r}\)
= \(\frac{4 \pi \times 10^{-7} \times 2 \times 50}{4 \pi \times 2.5}\)
= 4 × 10-6 T
The point is located normal to the wire length at a distance of 2.5 m. The direction of the current in the wire is vertically downward. Hence, according to the Maxwell’s right hand thumb rule, the direction of the magnetic field at the given point is vertically upward.

Question 4.
A horizontal overhead power line carries a current of 90 A in east to west direction. What is the magnitude and direction of the magnetic field due to the current 1.5 m below the line?
Answer:
Current in the power line, I = 90 A
Point is located below the power line at distance, r = 1.5 m
Hence, magnetic field at that point is given by the relation,
B = \(\frac{\mu_{0} 2 I}{4 \pi r}\)
= \(\frac{4 \pi \times 10^{-7} \times 2 \times 90}{4 \pi \times 1.5}\) = 1.2 × 10-5T
The current is flowing from east to west. The point is below the power line. Hence, according to Maxwell’s right hand thumb rule, the direction of the magnetic field is towards the south.

Question 5.
What is the magnitude of magnetic force per unit length on a wire carrying a current of 8 A and making an angle of 30° with the direction of a uniform magnetic field of 0.15 T?
Answer:
Current in the wire, I = 8 A
Magnitude of the uniform magnetic field, B = 0.15 T
Angle between the wire and magnetic field, θ = 30°.
Magnetic force per unit length on the wire is given as,
F = BI sinθ
= 0.15 × 8 × sin30°
= 0.15 × 8 × \(\frac{1}{2}\)
= 0.15 × 4 = 0.6 Nm-1
Hence, the magnetic force per unit length on the wire is 0.6 Nm-1.

PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism

Question 6.
A 3.0 cm wire carrying a current of 10 A is placed inside a solenoid perpendicular to its axis. The magnetic field inside the solenoid is given to be 0.27 T. What is the magnetic force on the wire?
Answer:
Length of the wire, l = 3 cm = 0.03 m
Current flowing in the wire, I = 10 A
Magnetic field, B = 0.27 T
Angle between the current and magnetic field, θ = 90°
Magnetic force exerted on the wire is given as,
F = BIl sinθ
= 0.27 × 10 × 0.03 × sin 90°
= 8.1 × 10-2N
Hence, the magnetic force on the wire is 8.1 × 10-2 N.

Question 7.
Two long and parallel straight wires A and B carrying currents of
8.0 A and 5.0 A in the same direction are separated by a distance of 4.0 cm. Estimate the force on a 10 cm section of wire A.
Answer:
Here, let I1 and I2 be the currents flowing in the straight long and parallel wires A and B respectively.
∴ I1 = 8.0 A, I2 = 5.0 A flowing in the same direction
r = distance between A and B = 4.0 cm = 4 × 10-2 m
If F’ be the force per unit length on wire A, then using
F’ = \(\frac{\mu_{0}}{4 \pi} \cdot \frac{2 I_{1} I_{2}}{r}\), we get
F’ = 10-7 × \(\frac{2 \times 8 \times 5}{4 \times 10^{-2}}\) Nm-1
= 20 × 10-5 Nm-1

If F be the force on a section of length 10 cm of wire A, then
F = F’ × l (Here,l = 10 × 10-2m)
= 20 × 10-5 × 10 × 10-2N
= 2 × 10-5N

Question 8.
A closely wound solenoid 80 cm long has 5 layers of windings of 400 turns each. The diameter of the solenoid is 1.8 cm. If the current carried is 8.0 A, estimate the magnitude of B inside the solenoid near its centre.
Answer:
Length of the solenoid, l = 80 cm = 0.8 m
Number of turns in each layer = 400
Number of layers in the solenoid = 5
∴ Total number of turns on the solenoid, N = 5 × 400 = 2000
Diameter of the solenoid, D = 1.8 cm = 0.018 m
Current carried by the solenoid, I = 8.0 A
Magnitude of the magnetic field inside the solenoid near its centre is given by the relation,
g.hoM
B = \(=\frac{\mu_{0} N I}{l}\)
B = \(\frac{4 \pi \times 10^{-7} \times 2000 \times 8}{0.8}\)
= 8 π × 10-3 = 2.512 × 10-2 T
Hence, the magnitude of the magnetic field inside the solenoid near its centre is 2.512 × 10-2 T.

PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism

Question 9.
A square coil of side 10 cm consists of 20 turns and carries a current of 12 A. The coil is suspended vertically and the normal to the plane of the coil makes an angle of 30° with the direction of a uniform horizontal magnetic field of magnitude 0.80 T. What is the magnitude of torque experienced by the coil?
Answer:
Length of a side of the square coil, l = 10 cm = 0.1 m
Current flowing in the coil, I = 12 A
Number of turns on the coil, N = 20
Angle made by the plane of the coil with magnetic field, θ = 30°
Strength of magnetic field, B = 0.80 T
Magnitude of the torque experienced by the coil in the magnetic field is given by the relation,
τ = NBIAsinθ
where, A = Area of the square coil
⇒ l × l = 0.1 × 0.1 = 0.01 m2
∴ τ = 20 × 0.80 × 12 × 0.01 × sin30°
= 20 × 0.80 × 12 × 0.01 × \(\frac{1}{2}\)
= 0.96 N m
Hence, the magnitude of the torque experienced by the coil is 0.96 N m.

Question 10.
Two moving coil meters, M1 and M2 have the following particulars:
R1 = 10 Ω, N1 = 30,
A1 = 3.6 × 10-3 m2, B1 = 0.25T
R2 = 14Ω, N2 = 42,
A2 = 1.8 × 10-3 m2, B2 = 0.50 T
(The spring constants are identical for the two meters). Determine the ratio of (a) current sensitivity and (b) voltage sensitivity of M2 and M1.
Answer:
Here,R1 = 10 n, N1 = 30, A1 = 3.6 x 10-3 m2,B1 = 0.25T for coil M1
R2 = 14 Q, N2 = 42, A2 = 1.8 x 10-3 m2,B2 = 0.50T for coil M2.
We know that current sensitivity and voltage sensitivity are given by the formulae
Current sensitivity = \(\frac{N B A}{k}\)
and Voltage sensitivity = \(\frac{N B A}{k R}\)
Here, k1 = k2 for the two coils = k (say)
∴ Current sensitivity for M1 is given by = N1B1A1/ k and for M2 = N2B2A2 / k

(a) Current sensitivity ratio for M2 and M1 is given by
= \(\frac{\frac{N_{2} B_{2} A_{2}}{k}}{\frac{N_{1} B_{1} A_{1}}{k}}\)
PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism 1

Question 11.
In a chamber, a uniform magnetic field of 6.5 G (1 G = 10-4 T) is maintained. An electron is shot into the field with a speed of 4.8 × 106 ms-1 normal to the field. Explain why the path of the electron is a circle. Determine the radius of the circuit orbit.
e,= 1.6 × 10-19 (me = 9.1 × 10 -31 kg
Answer:
Magnetic field strength, B = 6.5 G = 6.5 × 10-4 T
Speed of the electron, y = 4.8 × 106 m/s
Charge on the electron, e,= 1.6 × 10-19 C
Mass of the electron, me 9.1 × 10-31 kg
Angle between the electron and magnetic field, θ = 90°
Magnetic force exerted on the electron in the magnetic field is given as :
F = evBsinθ
This force provides centripetal force to the moving electron. Hence, the electron starts moving in a circular path of radius r.
Hence, centripetal force exerted on the electron,
Fe = \(\frac{m v^{2}}{r}\)
In equilibrium, the centripetal force exerted on the electron is equal to the magnetic force t.e.,
Fe = F
\(\frac{m v^{2}}{r}\) = evBsinθ
r = \(\frac{m v}{B e \sin \theta}\)
= \(\frac{9.1 \times 10^{-31} \times 4.8 \times 10^{6}}{6.5 \times 10^{-4} \times 1.6 \times 10^{-19} \times \sin 90^{\circ}}\)
= 4.2 × 10-2 m = 4.2 cm
Hence, the radius of the circular orbit of the electron is 4.2 cm.

PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism

Question 12.
In Exercise 4.11 obtain the frequency of revolution of the electron in its circular orbit. Does the answer depend on the speed of the electron? Explain.
Answer:
Magnetic field strength, B = 6.5 × 10-4 T
Charge on the electron, e = 1.6 × 10-19 C
Mass of the electron, me = 9.1 × 10-31 kg
Velocity of the electron, v = 4.8 × 106 m/s
Radius of the orbit, r = 4.2 cm = 0.042 m
Frequency of revolution of the electron = v
Angular frequency of the electron = ω = 2πv
Velocity of the electron is related to the angular frequency as :
v = rω
In the circular orbit, the magnetic force on the electron is balanced by the centripetal force. Hence, we can write :
evB = \(\frac{m v^{2}}{r}\)
eB = \(\frac{m}{r}\) (rω) = \(\frac{m}{r}\) (r2πv)
v = \(\frac{B e}{2 \pi m}\)

This expression for frequency is independent of the speed of the electron. On substituting the known values in this expression, we get the frequency as:
V = \(=\frac{6.5 \times 10^{-4} \times 1.6 \times 10^{-19}}{2 \times 3.14 \times 9.1 \times 10^{-31}}\)
= 18.2 × 106 Hz ≈ 18 MHz
Hence, the frequency of the electron is around 18 MHz and is independent of the speed of the electron.

Question 13.
(a) A circular coil of 30 turns and radius 8.0 cm carrying a current of 6.0 A is suspended vertically in a uniform horizontal magnetic field of magnitude 1.0 T. The field lines make an angle of 60° with the normal of the coil. Calculate the magnitude of the counter torque that must be applied to prevent the coil from turning.

(b) Would your answer change, if the circular coil in (a) were replaced by a planar coil of some irregular shape that encloses, the same area? (All other particulars are also unaltered.)
Answer:
(a) Number of turns on the circular coil, N = 30
Radius of the coil, r = 8.0 cm = 0.08 m
Area of the coil = πr2 = π(0.08)2 = 0.0201 m2
Current flowing in the coil, I = 6.0 A
Magnetic field strength, B = 1.0 T
Angle between the field lines and normal with the coil surface,
θ = 60°
The coil experiences a torque in the magnetic field. Hence, it turns. The. counter torque applied to prevent the coil from turning is given by the relation,
τ = N IBAsinθ …………… (1)
= 30 × 6 × 1 × 0.0201 × sin60°
= 180 × 0.0201 × \(\frac{\sqrt{3}}{2}\)
= 3.133 Nm

(b) It can be inferred from relation (1) that the magnitude of the applied torque is not dependent on the shape of the coil. It depends on the area of the coil. Hence, the answer would not change if the circular coil in the above case is replaced by a planar coil of some irregular shape that encloses the same area.

PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism

Question 14.
Two concentric circular coils X and Y of radii 16 cm and 10 cm, respectively, lie in the same vertical plane containing the north to south direction. Coil X has 20 turns and carries a current of 16 A; coil Y has 25 turns and carries a current of 18 A. The sense of the current in X is anticlockwise, and clockwise in Y, for an observer looking at the coils facing west. Give the magnitude and direction of the net magnetic field due to the coils at their centre.
Answer:
Radius of coil X, r1 = 16 cm = 0.16 m
Radius of coil Y, r2 = 10 cm = 0.10 m
Number of turns on coil X, n1 = 20
Number of turns on coil Y, n2 = 25
Current in coil X,I1 =16 A
Current in coil Y, I2 = 18 A
Magnetic field due to coil X at their centre is given by the relation,
B1 = \(\frac{\mu_{0} n_{1} I_{1}}{2 r_{1}}\)
∴ B1 = \(\frac{4 \pi \times 10^{-7} \times 20 \times 16}{2 \times 0.16}\)
= 4π × 10-4 T (towards East)
Magnetic field due to coil Y at their centre is given by the relation,
B2 = \(\frac{\mu_{0} n_{2} I_{2}}{2 r_{2}}\)
\(\frac{4 \pi \times 10^{-7} \times 25 \times 18}{2 \times 0.10}\)
= 9π × 10-4 T (towards West)

Hence, net magnetic field can be obtained as:
B = B2 – B1
= 9π × 10-4 – 4π × 10-4
= 5π × 10 T
= 1.57 × 10-3 T (towards West)

Question 15.
A magnetic field of 100 G (1 G = 10-4 T) is required which is uniform in a region of linear dimension about 10 cm and area of cross-section about 10-3 m2. The maximum current-carrying capacity of a given coil of wire is 15 A and the number of turns per unit length that can be wound round a core is at most 1000 turns m-1 . Suggest some appropriate design particulars of a solenoid for the required purpose. Assume the core is not ferromagnetic.
Answer:
Here, B = magnetic field = 100 G = 100 × 10-4 = 10-2 T,
Imax = maximum current carried by the coil = 15 A
n = number of turns per unit length = 1000 turns m-1 = 10 tums/cm
l = length of linear region = 10 cm
A = area of cross-section = 10-3 m2.

To produce a magnetic field in the above mentioned region, a solenoid can be made so that well within the solenoid, the magnetic field is uniform. To do so, we may take the length L of the solenoid 5 times the length of the region and area of the solenoid 5 times the area of region.

∴ L = 5l = 5 × 10 = 50 cm = 0.5m
and A = 5 × 10-3 m2
∴ If r be the radius of the solenoid, then
πr2 = A = 5 × 10-3
or r = \(\sqrt{\frac{5 \times 10^{-3}}{3.14}}\) = 0.04 m = 4 cm
Also let us wind 500 turns on the coil so that the number of turns per m is
n = \(\frac{500}{0.5}\) = 1000 turns m-1
∴ Using formula, μ0nI = B, we get
I = \(\frac{B}{\mu_{0} n}\) = \(\frac{10^{-2}}{4 \pi \times 10^{-7} \times 1000}\) = 7.96 A ≈ 8A

So, a current of 8 A can be passed through it to produce a uniform magnetic field of 100 G in the region. But this is not a unique way. If we wind 300 turns on the solenoid, then number of turns is
n = \(\frac{300}{0.5}\) = 600 per m.
∴ I= \(\frac{B}{\mu_{0} n}\) = \(\frac{10^{-2}}{4 \pi \times 10^{-7} \times 600}\) = 13.3 A
i. e., a current of 13.3 A can be passed through it to produce the magnetic field of loo G.
Similarly, if no. of turns = 400,
then, n = \(\frac{400}{0.5}\) = 800 per m.
∴ I = \(\frac{B}{\mu_{0} n}\) = \(\frac{10^{-2}}{4 \pi \times 10^{-7} \times 800}\) = 9.95 A
i. e., a current of 10 A can be passed ≈ 10 A
Through it to produce B = 100 G
Thus we may achieve the result in a number of ways.

PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism

Question 16.
For a circular coil of radius R and N turns carrying current J, the magnitude of the magnetic field at a point on its axis at a distance x from its centre is given by,
B = \(\frac{\mu_{0} I R^{2} N}{2\left(x^{2}+R^{2}\right)^{3 / 2}}\)
(a) Show that this reduces to the familiar result for field at the centre of the coil.
(b) Consider two parallel co-axial circular coils of equal radius R, and number of turns N, carrying equal currents in the same direction, and separated by a distance R. Show that the field on the axis around the mid-point between the coils is uniform over a distance that is small as compared to JR, and is given by,
B = 0.72 \(\frac{\mu_{0} \boldsymbol{N I}}{\boldsymbol{R}}\), approximately.
[Such an arrangement to produce a nearly uniform magnetic field over a small region is known as Helmholtz coils.]
Answer:
Radius of circular coil = R
Number of turns on the coil = N
Current in the coil = I
Magnetic field at a point on its axis at distance x from its centre is given by the relation,
B = \(\frac{\mu_{0} I R^{2} N}{2\left(x^{2}+R^{2}\right)^{3 / 2}}\)

(a) If the magnetic field at the centre of the coil is considered, then x = 0
∴ B = \(\frac{\mu_{0} I R^{2} N}{2 R^{3}}=\frac{\mu_{0} I N}{2 R}\)
This is the familiar result for magnetic field at the centre of the coil,

(b) Radii of two parallel co-axial circular coils = R
Number of turns on each coil = N
Current in both coils = I
Distance between both the coils = R
Let us consider point Q at distance d from the centre.
Then, one coil is at a distance of \(\frac{R}{2}\) + d from point Q.
PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism 2
PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism 3
Hence, it is proved that the field on the axis around the mid-point between the coils is uniform.

Question 17.
A toroid has a core (non-ferromagnetic) of inner radius 25 cm and outer radius 26 cm, around which 3500 turns of a wire are wound. If the current in the wire is 11 A, what is the magnetic field (a) outside the toroid, (b) inside the core of the toroid, and (c) in the empty space surrounded by the toroid.
Answer:
Here, I = 11 A,
Total number of turns = 3500
Mean radius of toroid, r = \(\frac{25+26}{2}\)
r = 25.5cm = 25.5 × 10-2 m
Total length of the toroid = 2πr = 2π × 25.5 × 10-2
= 51π × 10-2m
Therefore, number of turns per unit length,
n = \(\frac{3500}{51 \pi \times 10^{-2}}\)

(a) The field is non-zero only inside the core surrounded by the windings of the toroid. Therefore, the field outside the toroid is zero.

(b) The field inside the core of the toroid
B = μ0nI
B = 4π × 10-7 × \(\frac{3500}{51 \pi \times 10^{-2}}\) × 11
B = 3.02 × 10-2 T

(c) For the reason given in (a), the field in the empty space surrounded by toroid is also zero.

PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism

Question 18.
Answer the following questions:
(a) A magnetic field that varies in magnitude from point to point but has a constant direction (east to west) is set up in a chamber. A charged particle enters the chamber and travels undeflected along a straight path with constant speed. What can you say about the initial velocity of the particle?

(b) A charged particle enters an environment of a strong and non-uniform magnetic field varying from point to point both in magnitude and direction and comes out of it following a complicated trajectory. Would its final speed equal the initial speed if it suffered no collisions with the environment?

(c) An electron travelling west to east enters a chamber having a uniform electrostatic field in north to south direction. Specify the direction in which a uniform magnetic field should be set up to prevent the electron from deflecting from its straight line path.
Answer:
(a) The initial velocity of the particle is either parallel or anti-parallel to the magnetic field. Hence, it travels along a straight path without suffering any deflection in the field.

(b) Yes, the final speed’ of the charged particle will be equal to its initial speed. This is because magnetic force can change the direction of velocity, but not its magnitude.

(c) An electron travelling from west to east enters a chamber having a uniform electrostatic field in the north-south direction. This moving electron can remain undeflected if the electric force acting on it is equal and opposite of magnetic field. Magnetic force is directed towards the south. According to Fleming’s left hand rule, magnetic field should be applied in a vertically downward direction.

Question 19.
An electron emitted by a heated cathode and accelerated through a potential difference of 2.0 kV, enters a region with uniform magnetic field of 0.15 T. Determine the trajectory of the electron if the field (a) is transverse to its initial velocity, (b) makes an angle of 30° with the initial velocity.
Answer:
Magnetic field strength, B = 0.15 T
Charge on the electron, e = 1.6 × 10-19C
Mass of the electron, m = 9.1 × 10-31 kg
Potential difference, V = 2.0 kV = 2 × 103 V
Thus, kinetic energy of the electron = eV
⇒ eV = \(\frac{1}{2}\)mv2
v = \(\sqrt{\frac{2 e V}{m}}\) ……………. (1)
where, v = Velocity of the electron
Magnetic force on the electron provides the required centripetal force of the electron. Hence, the electron traces a circular path of radius r.

(a) When the magnetic field is transverse to the initial velocity. The force on the electron due to transverse magnetic field = Bev
PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism 4
= 100.55 × 10-5
= 1.01 × 10-3 m = 1 mm
Hence, the electron has a circular trajectory of radius 1.0 mm normal to the magnetic field.

(b) When the magnetic field makes an angle θ of 30° with initial velocity, the initial velocity will be,
v1 = vsinθ
From equation (2), we can write the expression for new radius as :
r1 = \(\frac{m v_{1}}{B e}\)
= \(\frac{m v \sin \theta}{B e}\)
PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism 5
= 0.5 × 10-3 m = 0.5mm
Hence, the electron has a helical trajectory of radius 0.5 mm along the magnetic field direction.

PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism

Question 20.
A magnetic field set up using Helmholtz coils (described in Exercise 4.16) is uniform in a small region and has a magnitude of 0.75 T. In the same region, a uniform electrostatic field is maintained in a direction normal to the common axis of the coils. A narrow beam of (single species) charged particles all accelerated through 15 kV enters this region in a direction perpendicular to both the axis of the coils and the electrostatic Held. If the beam remains undeflected when the electrostatic field is 9.0 × 10-5 V m-1, make a simple guess as to what the beam contains. Why is the answer not unique?
Answer:
Magnetic field, B = 0.75 T
Accelerating voltage, V = 15 kV = 15 × 103 V
Electrostatic field, E = 9 × 10-5 Vm-1
Mass of the electron = m
Charge on the electron = e
Velocity of the electron = v
Kinetic energy of the electron = eV
⇒ \(\frac{1}{2}\)mv2 = eV
∴ \(\frac{e}{m}=\frac{v^{2}}{2 V}\) ……………. (1)
Since the particle remains undeflected by electric and magnetic fields, we can infer that the electric field is balancing the magnetic field.
∴ eE = evB
v = \(\frac{E}{B}\) …………. (2)
Putting equation (2) in equation (1), we get
PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism 6
= 4.8 × 107 C/kg
Also, we know that \(\frac{e}{m}\) for proton is 9.6 × 10-7 C kg-1 . It follows that the charged particle under reference has the value of \(\frac{e}{m}\) half of that for the
proton, so its mass is clearly double the mass of proton. Thus the beam may be of deutrons.
The answer is not unique as the ratio of charge to mass i. e.,
4.8 × 107 C kg-1 may be satisfied by many other charged particles, surch as
He++(\(\frac{2 e}{2 m}\)) and Li3+ (\(\frac{3 e}{3 m}\))
which have the same value of \(\frac{e}{m}\).

Question 21.
A straight horizontal conducting rod of length 0.45 m and mass 60 g is suspended by two vertical wires at its ends. A current of 5.0 A is set up in the rod through the wires.
(a) What magnetic field should be set up normal to the conductor in order that the tension in the wires is zero?
(b) What will be the total tension in the wires if the direction of current is reversed keeping the magnetic field same as before? (Ignore the mass of the wires.) g = 9.8 ms-2.
Answer:
Length of the rod, l = 0.45 m
Mass of the rod, m = 60 g = 60 × 10-3 kg
Acceleration due to gravity, g = 9.8 m/s2
Current in the rod flowing through the wire, I = 5 A

(a) Magnetic field (B) is equal and opposite to the weight of the rod i.e.,
BIl = mg
∴ B = \(\frac{m g}{I l}\) = \(\frac{60 \times 10^{-3} \times 9.8}{5 \times 0.45}\) = 0.26T
A horizontal magnetic field of 0.26 T normal to the length of the conductor should be set up in order to get zero tension in the wire. The magnetic field should be such that Fleming’s left hand rule gives an upward magnetic force.

(b) If the direction of the current is reversed, then the force due to magnetic field and the weight of the rod acts in a vertically downward direction.
∴ Total tension in the wire = BIl + mg
= 0.26 × 5 × 0.45 + (60 × 10-3) × 9.8
= 1.176 N

PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism

Question 22.
The wires which connect the battery of an automobile to its starting motor carry a current of 300 A (for a short time). What is the force per unit length between the wires if they are 70 cm long and 1.5 cm apart? Is the force attractive or repulsive?
Answer:
Current in both wires, I = 300 A
Distance between the wires, r = 1.5 cm = 0.015 m
Length of the both wires, l = 70 cm = 0.7 m
Force between the two wires is given by the relation,
F = \(\frac{\mu_{0} I^{2}}{2 \pi r}\)
∴ F = \(\frac{4 \pi \times 10^{-7} \times(300)^{2}}{2 \pi \times 0.015}\) = 1.2 N/M
Since the direction of the current in the wires is opposite, a repulsive force exists between them.

Question 23.
A uniform magnetic field of 1.5 T exists in a cylindrical region of radius 10.0 cm, its direction parallel to the axis along east to west. A wire carrying current of 7.0 A in the north to south direction passes through this region. What is the magnitude and direction of the force on the wire if,
(a) the wire intersects the axis,
(b) the wire is turned from N-S to northeast-northwest direction,
(c) the wire in the N-S direction is lowered from the axis by a distance of 6.0 cm?
Answer:
Magnetic field strength, B = 1.5 T
Radius of the cylindrical region, r = 10cm = 0.1m
Current in the wire passing through the cylindrical region, I = 7 A

(a) If the wire intersects the axis, then the length of the wire is the diameter of the cylindrical region.
Thus, l = 2r = 2 × 0.1 = 0.2 m
Angle between magnetic field and current, θ = 90°
Magnetic force acting on the wire is given by the relation,
F = BIl sinθ
= 1.5 × 7 × 0.2 × sin90° = 2.1N
Hence, a force of 2.1 N acts on the wire in a vertically downward direction.

(b) New length of the wire after turning it to the northeast-northwest direction can be given as:
l1 = \(\frac{l}{\sin \theta}\)
Angle between magnetic field and current, θ = 45°
Force on the wire,
F1 = BIl1 sinθ
\(\frac{B I l}{\sin \theta}\) = sinθ
= BIl = 1.5 × 7 × 0.2 = 2.1 N
Hence, a force of 2.1 N acts vertically downward on the wire. This is independent of angle because Z sinG is fixed.

(c) The wire is lowered from the axis by distance, d = 6.0 cm
Let l2 be the new length of the wire.
∴ (\(\frac{l_{2}}{2}\))2 = 4(d + r)
= 4 (10 + 6) = 4(16)
∴ l2 = 8 × 2 = 16 cm = 0.16 m
Magnetic force exerted on the wire,
F2 = BIl2
= 1.5 × 7 × 0.16 = 1.68 N
Hence, a force of 1.68 N acts in a vertically downward direction on the wire.

PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism

Question 24.
A uniform magnetic field of 3000 G is established along the positive z-direction. A rectangular loop of sides 10 cm and 5 cm carries a current of 12 A. What is the torque on the loop in the different cases shown in Fig. 4.28? What is the force on each case? Which case corresponds to stable equilibrium?
PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism 7
Answer:
Here,
B = uniform magnetic field
= 3000 gauss along z-axis
= 3000 × 10-4T = 0.3 T
l = length of rectangular loop
= 10 cm = 0.1 m
b = breath of rectangular loop
= 5 cm = 0.05 m
∴ A = area of rectangular loop
= l × b = 10 × 5 = 50cm2 = 50 × 10-4 m2
Torque on the loop is given by
\(\vec{\tau}\) = (I\(\vec{A}\)) × \(\vec{B}\)
IA = 50 × 10-4 × 12 = 0.06 Am+2

(a) Here, I\(\vec{A}\) = 0.06î Am2, \(\vec{B}\) = 0.3k̂T
∴ \(\vec{\tau}\) = 0.06î × 0.3k̂= -1.8 × 10-2 Nm ĵ
i.e., τ = 1.8 × 10-2 Nm and acts along negative y-axis.
PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism 8
PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism 9
PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism 10
Net force on a planar loop in a magnetic field is always zero, so force is
zero in each case.
Case (e) corresponds to stable equilibrium as 7 A is aligned with B while (f) corresponds to unstable equilibrium as 7 A is antiparallel to B.

PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism

Question 25.
A circular coil of 20 turns and radius 10 cm is placed in a uniform magnetic field of 0.10 T normal to the plane of the coil. If the current in the coil is 5.0 A, what is the
(a) total torque on the coil,
(b) total force on the coil,
(c) average force on each electron in the coil due’ to the magnetic field?
(The coil is made of copper wire of cross-sectional area 10-5 m2, and the free electron density in copper is given to be about 1029 m-3.)
Answer:
Number of turns on the circular coil, n = 20
Radius of the coil, r = 10cm = 0.1m
Magnetic field strength, B = 0.10 T
Current in the coil, I = 5.0 A
(a) The total torque on the coil is zero because the field is uniform.
(b) The total force on the coil is zero because the field is uniform.
(c) Cross-sectional area of copper coil, A = 10-5 m2
Number of free electrons per cubic meter in copper, N = 1029 / m3
Charge on the electron, e = 1.6 × 10-19C
Magnetic force, F = Bevd
Where, vd = \(\frac{I}{N e A}\)
∴ F = \(\frac{B e I}{N e A}=\frac{B I}{N A}\) = \(\frac{0.10 \times 5.0}{10^{29} \times 10^{-5}}\) 5 × 10-25N
Hence, the average force on each electron is 5 × 10-25 N.

Question 26.
A solenoid 60 cm long and of radius 4.0 cm has 3 layers of windings of 300 turns each. A 2.0 cm long wire of mass 2.5 g lies inside the solenoid (near its centre) normal to its axis; both the wire and the axis of the solenoid are in the horizontal plane. The wire is connected through two leads parallel to the axis of the solenoid to an external battery which supplies a current of 6.0 A in the wire. What value of current (with appropriate sense of circulation) in the windings of the solenoid can support the weight of the wire? g = 9.8 ms-2.
Length of the solenoid, L = 60 cm = 0.6 m
Radius of the solenoid, r = 4.0 cm = 0.04 m
It is given that there are 3 layers of windings of 300 turns each.
∴ Total number of turns, n = 3 × 300 = 900
Length of the wire, l = 2 cm = 0.02 m
Mass of the wire, m = 2.5 g = 2.5 × 10 -3 kg
Current flowing through the wire, i = 6 A
Acceleration due to gravity, g=9.8m/s2
Magnetic field produced inside the solenoid, B = \(\frac{\mu_{0} n I}{L}\)
where, μ0 = 4π × 10-7 TmA-1
I = Current flowing through the windings of the solenoid Magnetic force is given by the relation,
F = Bil = \(\frac{\mu_{0} n i I}{L}\)l
Also, the force on the wire is equal to the weight of the wire.
∴ mg = \(\frac{\mu_{0} n \text { Iil }}{L}\)
I = \(\frac{m g L}{\mu_{0} \text { nil }}\)
= \(\frac{2.5 \times 10^{-3} \times 9.8 \times 0.6}{4 \pi \times 10^{-7} \times 900 \times 0.02 \times 6}\) = 108A
Hence, the current flowing through the solenoid is 108 A.

PSEB 12th Class Physics Solutions Chapter 4 Moving Charges and Magnetism

Question 27.
A galvanometer coil has a resistance of 12 Ω and the metre shows full scale deflection for a current of 3 mA. How will you convert the metre into a voltmeter of range 0 to 18 V?
Answer:
Resistance of the galvanometer coil, G = 12 Ω
Current for which there is full scale deflection, Ig = 3 mA = 3 × 10-3 A
Range of the voltmeter is 0, which needs to be converted to 18 V.
∴ V = 18 V
Let a resistor of resistance R be connected in series with the galvanometer to convert it into a voltmeter. This resistance is given as
R = \(\frac{V}{I_{g}}\) – G
= \(\frac{18}{3 \times 10^{-3}}\) – 12 = 6000 – 12 = 5988 Ω
Hence, a resistor of resistance 5988 Ω is to be connected in series with the galvanometer.

Question 28.
A galvanometer coil has a resistance of 15 Ω and the metre shows full scale deflection for a current of 4 mA. How will you convert the metre into an ammeter of range 0 to 6 A?
Answer:
Resistance of the galvanometer coil, G = 15 Ω
Current for which the galvanometer shows full scale deflection,
Ig = 4 mA = 4 × 10-3A
Range of the ammeter is 0, which needs to be converted to 6 A.
∴ Current, I = 6 A
A shunt resistor of resistance S is to be connected in parallel with the galvanometer to convert it into an ammeter. The value of S is given as :
S = \(\frac{I_{g} G}{I-I_{g}}\) = \(\frac{4 \times 10^{-3} \times 15}{6-4 \times 10^{-3}}\)
S = \(\frac{6 \times 10^{-2}}{6-0.004}=\frac{0.06}{5.996}\) ≈ 0.01Ω = 10mΩ
Hence, a 10 mΩ shunt resistor is to be connected in parallel with the galvanometer.

PSEB 12th Class Physics Important Questions Chapter 12 Atoms

Punjab State Board PSEB 12th Class Physics Important Questions Chapter 12 Atoms Important Questions and Answers.

PSEB 12th Class Physics Important Questions Chapter 12 Atoms

Very short answer type questions

Question 1.
Why is the classical (Rutherford) model for an atom of electron orbiting around the nucleus not able to explain the atomic structure?
Answer:
The classical method could not explain the atomic structure as the electron revolving around the nucleus are accelerated and emit energy as the result, the radius of the circular paths goes on decreasing. Ultimately electrons fall into the nucleus, which is not in practice.

Question 2.
Consider two different hydrogen atoms. The electron in each atom is in an excited state. Is it possible for the electrons to have different energies but the same orbital angular momentum according to the Bohr model? (NCERT Exemplar)
Answer:
According to Bohr model electrons having different energies belong to different levels having different values of n. So, their angular momenta will be different, as.
L = \(\frac{n h}{2 \pi} \text { or } L \propto n\)

Question 3.
State Bohr’s quantization condition for defining stationary orbits.
Answer:
According to Bohr’s quantization condition, electrons are permitted to revolve in only those orbits in which the angular momentum of electron is an integral multiple of \(\frac{h}{2 \pi}\) i.e.,
mvr = \(\frac{n h}{2 \pi}\) ,Where n = 1,2,3, ………………
m, y, rare mass, speed, and radius of electron respectively and h being Planck’s constant.

PSEB 12th Class Physics Important Questions Chapter 12 Atoms

Question 4.
Define ionization energy. What Is Its value for a hydrogen atom?
Answer:
Ionisation Energy: The minimum amount of energy required to remove an electron from the ground state of the atom is known as ionization energy.
Ionisation energy for hydrogen atom =E – E1 = – (-13.6 eV) = 13.6 eV

Question 5.
When an electron falls from a higher energy to a lower energy level, the difference in the energies appears in the form of electromagnetic radiation. Why cannot it be emitted as other forms of energy? (NCERT Exemplar)
Answer:
The transition of an electron from a higher energy to a lower energy level can appear in the form of electromagnetic radiation because electrons interact only electromagnetically.

Question 6.
Where is H a -line of the Balmer series in the emission spectrum of hydrogen atom obtained?
Answer:
Hα -line of the Balmer series in the emission spectrum of hydrogen atom is obtained in visible region.

Question 7.
Imagine removing one electron from He4 and He3. Their energy levels, as worked out on the basis of Bohr model will be very close. Explain why. (NCERT Exemplar)
Answer:
This is because both the nuclei are very heavy as compared to electron mass.

Question 8.
The mass of H-atom is less than the sum of the masses of a proton and electron. Why is this so? (NCERT Exemplar)
Answer:
Einstein’s mass-energy equivalence gives E – mc2.
Thus the mass of an H-atom is mp + me – \(\frac{B}{C^{2}}\)
where B ≈ 13.6 eV is the binding energy. It is less than the sum of masses of a proton and an electron.

Question 9.
Assume that there is no repulsive force between the electrons in an atom but the force between positive and negative charges is given by Coulomb’s law as usual. Under such circumstances, calculate the ground state energy of a He-atom. (NCERT Exemplar)
Answer:
For a He-nucleus with charge 2 e and electrons of charge -e, the energy level in ground state is -En = Z2\(\frac{-13.6 \mathrm{eV}}{n^{2}}=2^{2} \frac{-13.6 \mathrm{eV}}{1^{2}}\)= -54.4eV
Thus, the ground state will have two electrons each of energy E and the total ground state energy would be -(4 x 13.6) eV = -54.4 eV.

Question 10.
Would the Bohr formula for the H-atom remain unchanged if proton had a charge (+ 4/3)e and electron a charge (-3/4)e, where e = 1.6 x 10-19 C ? Give reasons for your answer. (NCERT Exemplar)
Answer:
Yes, since the Bohr formula involves only the product of the charges.

Short answer type questions

Question 1.
In an experiment of α-particle scattering by a thin foil of gold, draw a plot showing the number of particles scattered versus the scattering angle θ. Why is it that a very small fraction of the particles are scattered at θ > 90°?
PSEB 12th Class Physics Important Questions Chapter 12 Atoms 1
Answer:
A small fraction of the alpha particles scattered at angle θ > 90° is due to the reason. That if impact parameter ‘b’ reduces to zero, coulomb force increases, hence alpha particles are scattered at angle θ>9O°, and only one alpha particle is scattered at angle 180°.

Question 2.
(i) State Bohr postulate of hydrogen atom that gives the relationship for the frequency of emitted photon in a transition,
(ii) An electron jumps from fourth to first orbit in an atom. How many maximum number of spectral lines can be emitted by the atom? To which series these lines correspond?
Answer:
(i) Bohr’s Third Postulate: It states that an electron might make a transition from one of its specified non-radiating orbits to another of lower energy. When it does so, a photon is emitted having energy equal to the energy difference between the initial and final states. The frequency of the emitted photon is given by
hv = Ei – Ef
Where Ei and Ef are the energies of the initial and final states and Ei > Ef.
(ii) Electron jumps from fourth to first orbit in an atom
PSEB 12th Class Physics Important Questions Chapter 12 Atoms 2
∴ Maximum number of spectral lines can be
4c2 = \(\frac{4 !}{2 ! 2 !}=\frac{4 \times 3}{2}\) = 6
The line responds to Lyman series (e jumps to 1st orbit), Balmer series (e jumps to 2nd orbit), Paschen series (e jumps to 3rd orbit).

PSEB 12th Class Physics Important Questions Chapter 12 Atoms

Question 3.
Using de Broglie’s hypothesis, explain with the help of a suitable diagram, Bohr’s second postulate of quantization of energy levels in a hydrogen atom.
Answer:
According to de Broglie’s hypothesis.
λ = \(\frac{h}{m v}\) ……………………….. (i)
According to de Broglie’s condition of stationary orbits, the stationary orbits are those which contain complete de Broglie wavelength.
2πr = nλ ………………………….. (2)

Substituting value of λ from eq. (2) in eq. (1), we get
2πr = n \(\frac{h}{m v}\)
⇒ mvr = n \(\frac{h}{2 \pi}\) ………………………… (3)
This is Bohr’s postulate of quantisation of energy levels.
PSEB 12th Class Physics Important Questions Chapter 12 Atoms 3

Question 4.
In the study of Geiger-Marsden experiment on scattering of α-particles by a thin foil of gold, draw the trajectory of a-particles in the coulomb field of target nucleus. Explain briefly how one gets the information on the size of the nucleus from this study. From the relation R = R0 A1/3, where, R0 is constant and A is the mass number of the nucleus, show that nuclear matter density is independent of A.
Answer:
PSEB 12th Class Physics Important Questions Chapter 12 Atoms 4
From this experiment, the following is observed :
1. Most of the α-particles pass straight through the gold foil. It means that they do not suffer any collision with gold atoms.
2. About one α-particle in every 8000 α-particles deflects by more than 90°. As most of the a-particles gounder flected and only a- few get deflected, this shows that most of the space in an atom is empty and at the center of the atom, there exists a nucleus.

By the number of a-particles get deflected, the information regarding size of the nucleus can be known.
If m is the average mass of the nucleus and R is the nuclear radius, then mass of nucleus = mA, where A is the mass number of the element. Volume of the nucleus,
PSEB 12th Class Physics Important Questions Chapter 12 Atoms 5
This shows that the nuclear density is independent of A.

Question 5.
Show that the first few frequencies of light that is emitted when electrons fall to nth level from levels higher than n, are approximate harmonics (L e., in the ratio 1: 2: 3,…) when n>> 1. (NCERTExempiar)
Answer:
The frequency of any line in a series in the spectrum of hydrogen-like atoms corresponding to the transition of electrons from (n + p) level to nth level can be expressed as a difference of two terms:
Vmn = \(c R Z^{2}\left[\frac{1}{(n+p)^{2}}-\frac{1}{n^{2}}\right] \)
where, m=n+p,(p=1,2,3,…………………………..)
and R is Rydberg constant.
For p << n

Vmn = \(c R Z^{2}\left[\frac{1}{n^{2}}\left(1+\frac{p}{n}\right)^{-2}-\frac{1}{n^{2}}\right]\)
PSEB 12th Class Physics Important Questions Chapter 12 Atoms 6
Thus, the first few frequencies of light that is emitted when electrons fall to the nth level from levels higher than n, are approximate harmonic (i. e., in the ratio 1:2:3,…) when n>>1.

PSEB 12th Class Physics Important Questions Chapter 12 Atoms

Long answer type questions

Question 1.
Using the postulates of Bohr’s model of hydrogen atom, obtain an expression for the frequency of radiation emitted when atom make a transition from the higher energy state with quantum number n1 to the lower energy state with quantum number nf (nf < ni).
Or
Using Bohr’s postulates, obtain the expression for the total energy of the electron in the stationary states of the hydrogen atom. Hence draw the energy level diagram showing how the line spectra corresponding to Balmer series occur due to transition between energy levels.
Or
Using Rutherford model of the atom, derive the expression for the total energy of the electron in hydrogen atom. What is the significance of total negative energy possessed by the electron?
Answer:
Suppose m be the mass of an electron and v be its speed in nth orbit of radius r. The centripetal force for revolution is produced by electrostatic attraction between electron and nucleus.
\(\frac{m v^{2}}{r}=\frac{1}{4 \pi \varepsilon_{0}} \frac{(Z e)(e)}{r^{2}}\) …………………… (1) [Form rutherford Model]
or mv2 = \(\frac{1}{4 \pi \varepsilon_{0}} \frac{Z e^{2}}{r}\)
So, Kinetic energy Ek = \(\frac{1}{2} m v^{2}\)
Ek = \(\frac{1}{4 \pi \varepsilon_{0}} \frac{Z e^{2}}{2 r}\)
Potential energy (PE) = \(\frac{1}{4 \pi \varepsilon_{0}} \frac{(Z e)(-e)}{r}=-\frac{1}{4 \pi \varepsilon_{0}} \frac{Z e^{2}}{r}\)
Total energy E = \(E_{K}+P E=\frac{1}{4 \pi \varepsilon_{0}} \frac{Z e^{2}}{2 r}+\left(-\frac{1}{4 \pi \varepsilon_{0}} \frac{Z e^{2}}{r}\right)\)
= \(-\frac{1}{4 \pi \varepsilon_{0}} \frac{Z e^{2}}{2 r}\)

For nth orbit, E can be written as En
so,En = \(-\frac{1}{4 \pi \varepsilon_{0}} \frac{Z e^{2}}{2 r_{n}}\) …………………. (2)
Negative sign indicates that the electron remains bound with the nucleus (or electron-nucleus form an attractive system) From Bohr’s postulate for quantisation of angular momentum.
mvr = \(\frac{n h}{2 \pi}\)
⇒ v = \(\frac{n h}{2 \pi m r} \)
Substituting this value of v in equation (1), we get
PSEB 12th Class Physics Important Questions Chapter 12 Atoms 7

For Bohr’s radius, n = 1
Substituting value of rn in equation (2), we get
En = \(\frac{1}{4 \pi \varepsilon_{0}} \frac{Z e^{2}}{2\left(\frac{\varepsilon_{0} h^{2} n^{2}}{\pi m Z e^{2}}\right)}=-\frac{m Z^{2} e^{4}}{8 \varepsilon_{0} h^{2} n^{2}}\)
R is called Rydberg constant.

PSEB 12th Class Physics Important Questions Chapter 12 Atoms

For hydrogen atom Z =1, En = \(\frac{-R c h}{n^{2}}\)
If ni and nf are the quantum numbers of initial and final states and Ei and
Ef are energies of electrons in H-atoms in initial and final state, we have
PSEB 12th Class Physics Important Questions Chapter 12 Atoms 8
For Balmer series, nf=2, while ni =3, 4, 5, …… ∞.

PSEB 12th Class Physics Solutions Chapter 12 Atoms

Punjab State Board PSEB 12th Class Physics Book Solutions Chapter 12 Atoms Textbook Exercise Questions and Answers.

PSEB Solutions for Class 12 Physics Chapter 12 Atoms

PSEB 12th Class Physics Guide Atoms Textbook Questions and Answers

Question 1.
Choose the correct alternative from the clues given at the end of the each statement:
(a) The size of the atom in Thomson’s model is ………………….. the atomic size in Rutherford’s model, (much greater than/no different from/much less than.)
(b) In the ground state of ………………………………… electrons are in stable equilibrium, while in …………………….. electrons always experience a net force. (Thomson’s model/Rutherford’s model.)
(c) A classical atom based on ……………………………. is doomed to collapse. (Thomson’s model/Rutherford’s model.)
(d) An atom has a nearly continuous mass distribution in a ………………………… but has a highly non-uniform mass distribution in …………………….. (Thomson’s model/Rutherford’s model.)
(e) The positively charged part of the atom possesses most of ………………………. the mass in ………………….. (Rutherford’s model/both the models.)
Answer:
(a) The size of the atom in Thomson’s model is no different from the atomic size in Rutherford’s model.
(b) In the ground state of Thomson’s model, electrons are in stable equilibrium while, in Rutherford’s model, electrons always experience a net force.
(c) A classical atom based on Rutherford’s model is doomed to collapse.
(d) An atom has a nearly continuous mass distribution in Thomson’s model but has a highly non-uniform mass distribution in Rutherford’s model.
(e) The positively charged part of the atom possesses most of the mass in both the models.

Question 2.
Suppose you are given a chance to repeat the alpha-particle scattering experiment using a thin sheet of solid hydrogen in place of the gold foil. (Hydrogen is a solid at temperatures below 14 K.) What results do you expect?
Answer:
The basic purpose of scattering experiment is not completed because solid hydrogen will be a much lighter target as compared to the alpha particle acting as a projectile. By using the conditions of elastic collisions, the hydrogen will move much faster as compared to alpha after the collision. We cannot determine the size of hydrogen nucleus.

PSEB 12th Class Physics Solutions Chapter 12 Atoms

Question 3.
What is the shortest wavelength present in the Paschen series of spectral lines?
Answer:
Rydberg’s formula is given as
\(\frac{h c}{\lambda}\) = \(21.76 \times 10^{-19}\left[\frac{1}{n_{1}^{2}}-\frac{1}{n_{2}^{2}}\right]\)
where, h = Planck’s constant = 6.63 x 10-34 Js
c=Speed oflight=3 x 108 m/s (n1 and n2 are integers)
The shortest wavelength present in the Paschen series of the spectral lines
is given for values n1 = 3 and n2 = ∞
PSEB 12th Class Physics Solutions Chapter 12 Atoms 1
= 822.65 nm

Question 4.
A difference of 2.3 eV separates two energy levels in an atom. What is the frequency of radiation emitted when the atom makes a transition from the upper level to the lower level?
Answer:
According to Bohr’s postulate
E2 – E1 = hv
∴ Frequency of emitted radiation
PSEB 12th Class Physics Solutions Chapter 12 Atoms 2

Question 5.
The ground state energy of hydrogen atom is -13.6 eV. What are the kinetic and potential energies of the electron in this state?
Answer:
Given, the ground state energy of hydrogen atom
E=-13.6eV
We know that,
Kinetic Energy, EK = -E = 13.6 eV
Potential Energy Ep = -2KE =-2 x 13.6 = -27.2eV

Question 6.
A hydrogen atom initially In the ground level absorbs a photon, which excites it to the n = 4 level. Determine the wavelength and frequency of photons.
Answer:
The energy levels of H-atom are given by
En = \(-\frac{R h c}{n^{2}}\)
For given transition n1 =1, n2 = 4
∴ E1 = \(-\frac{R h c}{1^{2}}\) ,E2= \(-\frac{R h c}{4^{2}}\)
∴ Energy of absorbed photon
ΔE=E2 -E1 =Rhc \(\left(\frac{1}{1^{2}}-\frac{1}{4^{2}}\right)\)
or
ΔE = \(\frac{15}{16}\) Rhc ………………………….. (1)
∴ The wavelength of absorbed photon λ is given by
PSEB 12th Class Physics Solutions Chapter 12 Atoms 3

Question 7.
(a) Using the Bohr’s model, calculate the speed of the electron in a hydrogen atom in the n =1, 2, and 3 levels.
(b) Calculate the orbital period in each of these levels.
Answer:
(a) Let y1 be the orbital speed of the electron in a hydrogen atom in the ground state level, n1 =1.
For charge (e) of an electron, v1 is given by the relation,
v1 = \(\frac{e^{2}}{n_{1} 4 \pi \varepsilon_{0}\left(\frac{h}{2 \pi}\right)}=\frac{e^{2}}{2 \varepsilon_{0} h} \)
where, e=1.6 x 10-19 C
\(\varepsilon_{0}\) = Permittivity of free space = 8.85 x 10-12 N-1 C2m2
h = Planck’s constant = 6.63 x 10-34 Js
∴ v1 = \(\frac{\left(1.6 \times 10^{-19}\right)^{2}}{2 \times 8.85 \times 10^{-12} \times 6.63 \times 10^{-34}}\)
= 0.0218 x 108 =2.18 x 106 m/s

For level n2 =2, we can write the relation for the corresponding orbital speed as
v2 = \(\frac{e^{2}}{n_{2} 2 \varepsilon_{0} h}\) = \(\frac{\left(1.6 \times 10^{-19}\right)^{2}}{2 \times 2 \times 8.85 \times 10^{-12} \times 6.63 \times 10^{-34}}\) = 1.09 x 106 m/s
And, for n3 =3, we can write the relation for the corresponding orbital speed as
PSEB 12th Class Physics Solutions Chapter 12 Atoms 4
PSEB 12th Class Physics Solutions Chapter 12 Atoms 5
Hence, the speed of the electron in a hydrogen atom in n = 1, n = 2 and n = 3 is 2.18 x 106 m/s,
1.09 x 106 m/s, 7.27 x 105 m/s respectively.

(b) Orbital period of electron is given by
T = \(\frac{2 \pi r}{v}\)
Radius of nth orbit
rn = \(\frac{n^{2} h^{2}}{4 \pi^{2} K m e^{2}}\)
∴ r1 = \(\frac{(1)^{2} \times\left(6.63 \times 10^{-34}\right)^{2}}{4 \times 9.87 \times\left(9 \times 10^{9}\right) \times 9 \times 10^{-31} \times\left(1.6 \times 10^{-19}\right)}\)
= 0.53 x 10-10 m
For n=1, T1 = \(\frac{2 \pi r_{1}}{v_{1}}\)
= \(\frac{2 \times 3.14 \times 0.53 \times 10^{-10}}{2.19 \times 10^{6}}\) = 1.52 x 10-16s

For n = 2, radius rn = n2r1
∴ r2 =’22.r1 =4 x0.53 x 10-10
and velocity vn, = \(\frac{v_{1}}{n}\)
∴ v2 = \(\frac{v_{1}}{2}=\frac{2.19 \times 10^{6}}{2}\)
Time period T2 = \(\frac{2 \times 3.14 \times 4 \times 0.53 \times 10^{-10} \times 2}{2.19 \times 10^{6}}\)
=1216 x 10-15 s
For n=3,radius r3 =32,r1 =9r1 =9 x 0.53 x 10-10m and velocity v3 = \(\frac{v_{1}}{3}=\frac{2.19 \times 10^{6}}{3}\) m/s
Time period T3 = \(\frac{2 \pi r_{3}}{v_{3}}=\frac{2 \times 3.14 \times 9 \times 0.53 \times 10^{-10} \times 3}{2.19 \times 10^{6}}\) = 4.1 x 10-15 s

PSEB 12th Class Physics Solutions Chapter 12 Atoms

Question 8.
The radius of the innermost electron orbit of a hydrogen atom is 5.3 x 10-11 m. What are the radii of the n = 2 and n = 3 orbits?
Answer:
The radius of the innermost electron orbit of a hydrogen atom, r1 = 5.3 x 10-11 m.
Let r2 be the radius of the orbit at n = 2.
It is related to the radius of the innermost orbit as r2 = (n)2r1 = (2)2 x 5.3 x 10-11
= 4 x 5.3 x 10-11 = 2.12 x 10-10m
For n = 3, we can write the corresponding electron radius as
r3 =(n)2r1 = (3)2 x 5.3 x 10-11
n = 9 x 5.3 x 10-11 = 4.77 x 10-10m
Hence, the radii of an electron for n = 2 and n = 3 orbits are 2.12 x 10-10 m and 4.77 x 10-10 m respectively.

Question 9.
A 12.5 eV electron beam is used to bombard gaseous hydrogen at room temperature. What series of wavelengths will be emitted?
Answer:
It is given that the energy of the electron beam used to bombard gaseous hydrogen at room temperature is 12.5 eV. Also, the energy of the gaseous hydrogen in its ground state at room temperature is -13.6 eV. When gaseous hydrogen is bombarded with an electron beam, the energy of the gaseous hydrogen becomes -13.6 + 12.5 eV i. e., -1.1 eV.

Orbital energy is related to orbit level (n) as
E = \(\frac{-13.6}{(n)^{2}}\)eV
For n=3, E = \(\frac{-13.6}{(3)^{2}}=\frac{-13.6}{9}\) = -1.5 eV
This energy is approximately equal to the energy of gaseous hydrogen. it can be concluded that the electron has jumped from n I to n = 3 level.

During its de-excitation, the electrons can jump from n = 3 to n = 1 directly, which forms a line of the Lyman series of the hydrogen spectrum.
We have the relation for wave number for Lyman series as
\(\frac{1}{\lambda}=R_{y}\left(\frac{1}{1^{2}}-\frac{1}{n^{2}}\right)\)
where, Ry =Rydberg constant = 1.097 x 107 m-1,
λ = Wavelength of radiation emitted by the transition of the electron for
n =3,
We can obtain λ as
\(\frac{1}{\lambda}\) = 1.097 x 107\(\left(\frac{1}{1^{2}}-\frac{1}{3^{2}}\right)\)
= 1.097 x 107 \(\left(1-\frac{1}{9}\right)\) = 1.097 x 107x \(\frac{8}{9}\)

λ = \(\frac{9}{8 \times 1.097 \times 10^{7}}\) = 102.55nm
If the electron jumps from n = 2 to n = 1, then the wavelength of the radiation is given as
\(\frac{1}{\lambda}\) = 1.097 x 107 \(\left(\frac{1}{1^{2}}-\frac{1}{2^{2}}\right)\)
= 1.097 x 107\(\left(1-\frac{1}{4}\right)\) = 1.097 x 107x \(\frac{3}{4}\)
λ = \(\frac{4}{1.097 \times 10^{7} \times 3}\) = 121.54 nm

If the transition takes place from n = 3 to n = 2, then the wavelength of the radiation is given as
PSEB 12th Class Physics Solutions Chapter 12 Atoms 6
This radiation corresponds to the Balmer series of the hydrogen spectrum. Hence, in Lyman series, two wavelengths i. e., 102.54 nm, and 121.55 nm are emitted. And in the Balmer series, one wavelength i. e., 656.33 nm is emitted.

Question 10.
In accordance with the Bohr’s model, find the quantum number that characterizes the earth’s revolution around the sun in an orbit of radius 1.5 x 1011 m with orbital speed 3 x 104 m/s. (Mass of earth = 6.0 x 1024 kg.)
Answer:
Radius of the orbit of the Earth around the Sun, r = 1.5 x 1011 m
Orbital speed of the Earth, v = 3 x 104 m/s
Mass of the Earth, m = 6.0 x 1024 kg
According to Bohr’s model, angular momentum is quantized and given as
mvr = \(\frac{n h}{2 \pi}\)

where, h = Planck’s constant = 6.63 x 10-34 Js
n = Quantum number
∴ n = \(\frac{m v r 2 \pi}{h}\)
= \(\frac{2 \pi \times 6 \times 10^{24} \times 3 \times 10^{4} \times 1.5 \times 10^{11}}{6.63 \times 10^{-34}} \) = 25.61 x 1073 = 2.6 x 1074
Hence, the quanta number that characterizes the Earth’s revolution is 2.6 x 1074.

PSEB 12th Class Physics Solutions Chapter 12 Atoms

Additional Exercises

Question 11.
Answer the following questions, which help you to understand the difference between Thomson’s model and Rutherford’s model better.
(a) Is the average angle of deflection of α-particles by a thin gold foil predicted by Thomson’s model much less, about the same, or much greater than that predicted by Rutherford’s model?

(b) Is the probability of backward scattering (i. e., scattering of α-particles at angles greater than 90°) predicted by Thomson’s model much less, about the same, or much greater than that predicted by Rutherford’s model?

(c) Keeping other factors fixed, it is found experimentally that for small thickness t, the number of α-particles scattered at moderate angles is proportional to t. What clue does this linear dependence on t provide?

(d) In which model is it completely wrong to ignore multiple scattering for the calculation of average angle of scattering of α-particles by. a thin foil?
Answer:
(a) The average angle of deflection of α-particles by a thin gold foil predicted by Thomson’s model is about the same size as predicted by Rutherford’s model. This is because the average angle was taken in both models.

(b) The probability of scattering of α-particles at angles greater than 90° predicted by Thomson’s model is much less than that predicted by Rutherford’s model. This is because there is no such massive central core called the nucleus in Rutherford’s model.

(c) Scattering is mainly due to single collisions. The chances of a single collision increase linearly with the number of target atoms. Since the number of target atoms increases with an increase in thickness, the collision probability depends linearly on the thickness of the target.

(d) It is wrong to ignore multiple scattering in Thomson’s model for the calculation of average angle of scattering of α-particles by a thin foil. This is because a single collision causes very little deflection in this model. Hence, the observed average scattering angle can be explained only by considering multiple scattering.

Question 12.
The gravitational attraction between electron and proton in a hydrogen atom is weaker than the Coulomb attraction by a factor of about 10-40. An alternative way of looking at this fact is to estimate the radius of the first Bohr orbit of a hydrogen atom if the electron and proton were bound by gravitational attraction. You will find the answer interesting.
Answer:
Radius of the first Bohr orbit is given by the relation,
r1 = \(\frac{4 \pi \varepsilon_{0}\left(\frac{h}{2 \pi}\right)^{2}}{m_{e} e^{2}}\) ……………….. (i)
where, ε0 = Permittivity of free space
h = Planck’s constant = 6.63 x 10-34 Js
me = Mass of an electron = 9.1 x 10-31 kg
e = Charge of an electron = 1.9x 10-19C
mp = Mass of a proton = 1.67 x 10-27 kg
r = Distance between the electron and the proton Coulomb attraction between an electron and a proton is given as
FC = \(\frac{e^{2}}{4 \pi \varepsilon_{0} r^{2}} \) ………………………….. (2)

Gravitational force of attraction between an electron and a proton is
FG = \(\frac{G m_{p} m_{e}}{r^{2}}\) ……………………………………. (3)
where, G = Gravitational constant = 6.67 x 10-11 N m2/kg2
If the electrostatic (Coulomb) force and the gravitational force between an electron and a proton are equal, then we can write
∴ FG = FC
\(\frac{G m_{p} m_{e}}{r^{2}}\) = \(\frac{e^{2}}{4 \pi \varepsilon_{0} r^{2}}\)
\(\frac{e^{2}}{4 \pi \varepsilon_{0} r^{2}}\) = Gmpme …………………………. (4)
Putting the value of equation (4) in equation (1), we get
PSEB 12th Class Physics Solutions Chapter 12 Atoms 7

Question 13.
Obtain an expression for the frequency of radiation emitted when a hydrogen atom de-excites from level n to level (n -1). For large n, show that this frequency equals the classical frequency of revolution of the electron in the orbit.
Answer:
It is given that a hydrogen atom de-excites from an upper level (n) to a lower level (n —1). We have the relation for energy (E1) of radiation at level n as
E1 = hv1 = \(\frac{h m e^{4}}{(4 \pi)^{3} \varepsilon_{0}^{2}\left(\frac{h}{2 \pi}\right)^{3}} \times\left(\frac{1}{n^{2}}\right)\)
where, v1 = Frequency of radiation at level n
h = Planck’s constant
m = Mass of hydrogen atom
e = Charge on an electron
εo = Permittivity of free space

Now, the relation for energy (E2) of radiation at level (n -1) is given as
E2 = hv2 = \(\frac{h m e^{4}}{(4 \pi)^{3} \varepsilon_{0}^{2}\left(\frac{h}{2 \pi}\right)^{3}} \times \frac{1}{(n-1)^{2}}\) ………………………… (2)
where, v2 = Frequency of radiation at level (n -1)
Energy (E) released as a result of de-excitation
E = E2 – E1 hv= E2 – E 1 ………………….. (3)
where, v = Frequency of radiation emitted
Putting values from equations (1) and (2) in equation (3), we get
PSEB 12th Class Physics Solutions Chapter 12 Atoms 8
For large n, we can write (2 n -1) ≈ 2 n and (n-1) ≈ n.
V = \(\frac{m e^{4}}{32 \pi^{3} \varepsilon_{0}^{2}\left(\frac{h}{2 \pi}\right)^{3} n^{3}} \)
∵ v = \(\frac{m e^{4}}{32 \pi^{3} \varepsilon_{0}^{2}\left(\frac{h}{2 \pi}\right)^{3} n^{3}}\) ………………….. (4)
Classical relation of frequency of revolution of an electron is given as
Vc = \(\frac{v}{2 \pi r}\) ……………………………….. (5)
where, velocity of the electron in the nth orbit is given as
v = \(\frac{e^{2}}{4 \pi \varepsilon_{0}\left(\frac{h}{2 \pi}\right) n}\) ……………………………… (5)
And, radius of the nth orbit is given as
v = \(\frac{e^{2}}{4 \pi \varepsilon_{0}\left(\frac{h}{2 \pi}\right) n}\) ………………………………(6)
Putting the values of equations (6) and (7) in equation (5), we get
Vc = \( \frac{m e^{4}}{32 \pi^{3} \varepsilon_{0}^{2}\left(\frac{h}{2 \pi}\right)^{3} n^{3}}\)
Hence, the frequency of radiation emitted by the hydrogen atom is equal to its classical orbital frequency.

Question 14.
Classically, an electron can be in any orbit around the nucleus of an atom. Then what determines the typical atomic size? Why is an atom not, say, a thousand times bigger than its typical size? The question had greatly puzzled Bohr before he arrived at his famous model of the atom that you have learnt in the text.

To simulate what he might well have done before his discovery, let us play as follows with the basic constants of nature and see if we can get a quantity with the dimensions of length that is roughly equal to the known size of an atom (~10-10 m).

(a) Construct a quantity with the dimensions of length from the fundamental constants e, me and c. Determine its numerical value.

(b) You will find that the length obtained in (a) is many orders of magnitude smaller than the atomic dimensions. Further, it involves c. But energies of atoms are mostly in non-relativistic domain where c is not expected to play any role. This is what may have suggested Bohr to discard c and look for ‘something else’ to get the right atomic size. Now, the Planck’s constant h had already made its appearance elsewhere. Bohr’s great insight lay in recognizing that h, me, and e will yield the right atomic size.

Construct a quantity with the dimension of length from h, me, and e and confirm that its numerical value has indeed the correct order of magnitude.
Answer:
(a) Charge on an electron, e = 1.6 x 10-19 C
Mass of an electron, me = 9.1 x 10-31 kg
Speed of light, c = 3 x 108 m/s
Let us take a quantity involving the given quantities as \(\left(\frac{e^{2}}{4 \pi \varepsilon_{0} m_{e} c^{2}}\right)\)
where, ε0 = Permittivity of free space and, \(\frac{1}{4 \pi \varepsilon_{0}}\) = 9 x 109 Nm2C-2 .
The numerical value of the taken quantity will be
PSEB 12th Class Physics Solutions Chapter 12 Atoms 9
Hence, the numerical value of the taken quantity is much smaller than the typical size of an atom.

(b) Charge on an electron, e = 1.6 x 10-19 C
Mass of an electron, me = 9.1 x 10-31 kg
Planck’s constant, h = 6.63 x 10-34 Js
Let us take a quantity involving the given quantities as \(\frac{4 \pi \varepsilon_{0}\left(\frac{h}{2 \pi}\right)^{2}}{m_{e} e^{2}}\)
where, ε0 = Permittivity of free space
and, \(\frac{1}{4 \pi \varepsilon_{0}}\) = 9 x 109Nm2C-2

The numerical value of the taken quantity will be
\(\frac{1}{4 \pi \varepsilon_{0}} \times \frac{\left(\frac{h}{2 \pi}\right)^{2}}{m_{e} e^{2}}=9 \times 10^{9} \times \frac{\left(\frac{6.63 \times 10^{-34}}{2 \times 3.14}\right)^{2}}{9.1 \times 10^{-31} \times\left(1.6 \times 10^{-19}\right)^{2}} \)
= 0.53 x 10-10 m
Hence, the value of the quantity taken is of the order of the atomic size.

PSEB 12th Class Physics Solutions Chapter 12 Atoms

Question 15.
The total energy of an electron in the first excited state of the hydrogen atom is about -3.4 eV.
(a) What is the kinetic energy of the electron in this state?
(b) What is the potential energy of the electron in this state?
(c) Which of the answers above would change if the choice of the zero of potential energy is changed?
Answer:
(a) Total energy of the electron, E = -3.4 eV ’
Kinetic energy of the electron is equal to the negative of the total energy.
⇒ K = -E
= -(-3.4) = + 3.4 eV
Hence, the kinetic energy of the electron in the given state is + 3.4 eV.

(b) Potential energy (JJ) of the electron is equal to the negative of twice of its kinetic energy.
⇒ U = -2 K
= -2 x 3.4 = -6.8 eV
Hence, the potential energy of the electron in the given state is -6.8 eV.

(c) The potential energy of a system depends on the reference point taken. Here, the potential energy of the reference point is taken as zero. If the reference point is changed, then the value of the potential energy of the system also changes. Since total energy is the sum of kinetic and potential energies, total energy of the system will also change.

Question 16.
If Bohr’s quantization postulate (angular momentum = nh/2π) is a basic law of nature, it should be equally valid for the case of planetary motion also. Why then do we never speak of quantization of orbits of planets around the sun?
Answer:
We never speak of quantization of orbits of planets around the Sun because the angular momentum associated with planetary motion is largely relative to the value of Planck’s constant (h).
The angular momentum of the Earth in its orbit is of the order of 1070h. This leads to a very high value of quantum levels n of the order of 1070.
For large values of n, successive energies and angular momenta are relatively very small. Hence, the quantum levels for planetary motion are considered continuous.

Question 17.
Obtain the first Bohr’s radius and the ground state energy of a muonic hydrogen atom an atom in which a negatively
charged muon (μ ) of mass about 207 me orbits around a proton.
Answer:
Muonic hydrogen is the atom in which a negatively charged muon of mass about 207 me revolves around a proton.
In Bohr’s atom model, r ∝ \(\frac{1}{m}\)
∵ \(\frac{r_{\text {muon }}}{r_{\text {electron }}}=\frac{m_{e}}{m_{\mu}}=\frac{m_{e}}{207 m_{e}}=\frac{1}{207}\) [ ∵mμ = 207 me]
Here, re is radius of orbit of electron in hydrogen atom is 0.53 Å.

PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity

Punjab State Board PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity Important Questions and Answers.

PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity

Very short answer type questions

Question 1.
Two identical cells, each of emf E, having negligible internal resistance, are connected in parallel with each other across an external resistance R. What is the current through this resistance?
Answer:
PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity 1
⇒ I = \(\frac{E_{\text {eq. }}}{R+r_{\text {eq. }}}\)
Given,internal resistance, r = 0
∴ I = \(\frac{E_{\mathrm{eq} .}}{R}\)

Question 2.
Define mobility of a charge carrier. What is its relation with relaxation time?
Answer:
It is defined as how fast electron moves from one place to another.
It is also defined as drift velocity per unit electric field. The SI unit of mobility is m2/V-sec and it is denoted as μ.
μ = \(\frac{\left|v_{d}\right|}{E}=\frac{e E \tau}{m E}=\frac{e \tau}{m}\)
⇒ μ ∝ τ

PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity

Question 3.
Define the term ‘relaxation time’ in a conductor.
Answer:
The average time between successive collisions of electrons conductor is known as relaxation time.

Question 4.
Write the expression for the drift velocity of charge carriers in a conductor of length ‘L’ across which a potential difference ‘V’ is applied.
Answer:
vd = \(\frac{e V}{m L}\)τ

Question 5.
For household electrical wiring, one uses Cu wires or Al wires. What considerations are kept in mind? (NCERT Exemplar)
Answer:
Two considerations are required: (i) cost of metal, and (ii) good conductivity of metal. Cost factor inhibits silver. Cu and Al are the next best conductors.

Question 6.
Define the current sensitivity of a galvanometer. Write its SI unit.
Answer:
Ratio of deflection produced in the galvanometer and the current flowing through it is called current sensitivity.
Current sensitivity Si = \(\frac{\theta}{I}\)
SI unit of current sensitivity Si is division/ampere or radian/ampere.

PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity

Question 7.
Nichrome and Copper wires of same length and same radius are connected in series. Current I is passed through them. Which wire gets heated up more? Justify your answer.
Answer:
Nichrome, since its resistance is high.

Question 8.
Why are alloys used for making standard resistance coils?
(NCERT Exemplar)
Answer:
Alloys have:

  • low value of temperature coefficient and the resistance of the alloy does not vary much with rise in temperature.
  • high resistivity, so even a smaller length of the material is sufficient to design high standard resistance.

Question 9.
What is the advantage of using thick metallic strips to join wires in a potentiometer? (NCERT Exemplar)
Answer:
The metal strips have low resistance and need not be counted in the potentiometer length l of the null point. One measures only their lengths along the straight segments (of length 1 metre each). This is easily done with the help of centimeter rulings or meter ruler and leads to accurate measurements.

Question 10.
Is the motion of a charge across junction momentum conserving? Why or why not? (NCERT Exemplar)
Answer:
When an electron approaches a junction, in addition to the uniform electric field E facing it normally, it keep the drift velocity fixed as drift velocity depend on E by the relation
Vd = \(\frac{e E \tau}{m}\)
This result into accumulation of charges on the surface of wires at the junction. These produce additional field. These fields change the direction of momentum.
Thus, the motion of a charge across junction is not momentum conserving.

PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity

Short answer type questions

Question 1.
Sketch a graph showing the variation of resistivity of carbon with temperature.
Or Plot a graph showing temperature dependence of resistivity for a typical semiconductor. How is this behaviour explained?
Answer:
The resistivity of a typical semiconductor (carbon) decreases with increase of temperature. The graph is shown in figure.
PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity 2
Explanation: In semiconductor the number density of free electrons (n) increases with increase in temperature (T) and consequently the relaxation period decreases. But the effect of increase in n has higher impact than decrease of τ. So, resistivity decreases with increase in temperature.

Question 2.
Two cells of emf ε1 and ε2 having internal resistances r1 and r2 respectively are connected in parallel as shown. Deduce the expressions for the equivalent emf and equivalent internal resistance of a cell which can replace the combination between the points B1 and B2.
PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity 3
Answer:
Consider a parallel combination of the cells. I1 and I2 are the currents leaving the positive electrodes of the cells. At point B1, I1 and I2 flow in whereas current I flows out. Therefore, we have
I = I1 + I2 …………….. (1)
Let V(B1) and V(B2) be the potentials at B1 and B2 respectively.
Then, considering the first cell, the potential difference across its terminals is V(B1) – V(B2). Hence, from equation V = E – Ir we have
V = V(B1) – V(B2) = E1 – I1r1 …………… (2)
Points B1 and B2 are connected exactly Similarly to the second cell. Hence, considering the second cell, we also have
V = V(B1) – V(B2)
= E2 – I2r2 …………… (3)
Combining equations (1), (2) and (3), we have
PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity 4
If we want to replace the combination by a single cell, between Bl and B2, of emf Eeq and internal resistance req, we would have
V = Eeq – Ireq
PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity 5

PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity

Question 3.
State Kirchhoff s rules of current distribution in an electrical network.
Or State KirchhofPs rules. Explain briefly how these rules are justified.
Answer:
Junction Rule: In an electric circuit, the algebraic sum of currents at any junction is zero.
At any junction, the sum of the currents entering the junction is equal to the sum of currents leaving the junction.
ΣI = 0
Justification: This rule is based on the law of conservation of charge.
Loop Rule: The algebraic sum of changes in potential around any closed loop involving resistors and cells in the loop must be zero.
ΣΔV = 0
or The algebraic sum of emf s in any loop of a circuit is equal to the
sum of products of currents and resistances in it.
ΣΔE = ΣIR
Justification: This rule is based on the law of conservation of energy,

Question 4.
Define the term current density of a metallic conductor. Deduce the relation connecting current density (J) and the conductivity σ of the conductor, when an electric field E, is applied to it.
Answer:
Current density at a point in a conductor is defined as the amount of current flowing per unit area of the conductor around that point provided the area is held in a direction normal to the current,
J = \(\frac{I}{A}\)
Current density is a vector quantity. Its direction is the direction of motion of positive charge. The unit of current density is ampere/metre2 or [Am-2].
Relation between J, σ and E
PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity 6

PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity

Question 5.
What is Wheatstone bridge? Deduce the condition for which Wheatstone bridge is balanced.
Or The given figure shows a network of resistances R1, R2, R3 and R4.
PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity 7
Using Kirchhoffs laws, establish the balance condition for the network.
Or Use Kirchhoffs law to obtain the balance Wheatstone’s bridge.
Answer:
PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity 8
The Wheatstone bridge is an arrangement of four resistances. In this bridge, four resistances are connected on four arms of quadrilateral. In one diagonal, a battery and a key are connected. In second diagonal a galvanometer is connected as shown in fig. Consider P,Q,R and S four resistances are connected on the sidesAB,BC, AD and DC of the quadrilateral respectively.

Galvanometer G is connected between points B and D and a battery E is connected between A and C. Now in balance condition, when the deflection in a galvanometer is zero in closed mesh ABDA, then by applying Kirchhoffs law,
I1p – IR = 0
or I1P = I2R ………….. (1)
In closed mesh CBDC,
I1Q = I2S ……………… (2)
Dividing (1) by (2) \(\frac{P}{Q}=\frac{R}{S}[latex]
This is the balanced condition of the Wheatstone bridge.

Question 6.
First a set of n equal resistors of R each are connected in series to a battery of emf E and internal resistance R. A current I is observed to flow. Then the n resistors are connected in parallel to the same battery. It is observed that the current is increased 10 times. What is n ? (NCERT Exemplar)
Answer:
When n resistors are in series, I = [latex]\frac{E}{R+n R}\) ;
When n resistors are in parallel, \(\frac{E}{R+\frac{R}{n}}\) 10I
\(\frac{1+n}{1+\frac{1}{n}}\) = 10 ⇒ \(\frac{1+n}{n+1}\) n = 10
∴ n = 10

PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity

Question 7.
Two cells of same emf E but internal resistance r and r1 and r2 are connected in series to an external resistor R (Fig.). What should be the value of R so that the potential difference across the terminals of the first cell becomes zero. NCERT Exemplar)
Answer:
I = \(\frac{E+E}{R+r_{1}+r_{2}}\)
V1 = E – Ir1 = E – \(\frac{2 E}{r_{1}+r_{2}+R}\) r1 = 0
PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity 9

Long answer type questions

Question 1.
(i) Find the magnitude and direction of current in 1Ω resistor in given circuit.
PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity 10

(ii)Two students X and Y perform an experiment on potentiometer separately using the circuit diagram shown below.
Keeping other things unchanged (a) X increases the value of resistance R, (b) Y decreases the value of resistance S in the set up. How will these changes affect the position of null point in each case and why?
PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity 11
Answer:
(i) For the mesh APQBA
-6 -1 (I2 – I1) + 3I1 = 0
or -I2 + 4I1 = 6 …………… (1)
PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity 12
For the mesh PCDQP
2I2 – 9 + 3I2 + 1(I2 – I1) = 0
or 6I2 – I1 = 9 …………… (2)
Solving eqs. (1) and (2), we get
I = \(\frac{45}{23}\) A
I= \(\frac{42}{23}\) A
∴ Current through the 1Ω resistor = (I2 – I1) = \(\) A

(ii) (a) By increasing resistance R, the current in main circuit decreases, so potential gradient decreases. Hence, a greater length of wire would be needed for balancing the same potential difference. So, the null point would shift towards right (i.e., towards B).
PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity 13
(b) By decreasing resistance S, the terminal potential difference V = \(\frac{E}{1+\frac{r}{S}}\) across cell decreases, so balance is obtained at small length i.e., point will be obtained at smaller length. So, the null point would shift towards left (i.e., towards A).

PSEB 12th Class Physics Important Questions Chapter 3 Current Electricity

Question 2.
A room has AC run for 5 hours a day at a voltage of 220 V. The wiring of the room consists of Cu of 1 mm radius and a length of 10m. Power consumption per day is 10 commercial units. What fraction of it goes in the joule heating in wires? What would happen if the wiring is made of aluminium of the same dimensions? [ρCu = 1.7 × 10-8Ωm, ρAL = 2.7 × 10-8Ωm] (NCERT Exemplar)
Answer:
Power consumption in a day i.e., in 5 hours = 10 units
Or power consumption per hour = 2 units
Or power consumption = 2 units = 2 kW = 2000 W
Also, we know that power consumption in resistor,
P = V × I
⇒ 2000 W = 220 V × I
or I = 9 A
Now, the resistance of wire is given by R = ρ \(\frac{l}{A}\)
where, A is cross-sectional area of conductor. Power consumption in first current carrying wire is given by
P = I2 . R
ρ \(\frac{l}{A}\) I2 = 1.7 × 10-8 × \(\frac{10}{\pi \times 10^{-6}}\) × 81 = 4J/s A
The fractional loss due to the joule heating in first wire
= \(\frac{4}{2000}\) × 100 = 0.2%
Power loss in Al wire = 4\(\frac{\rho_{A l}}{\rho_{C u}}\) = 1.6 × 4 = 6.4 J/s
The fractional loss due to the joule heating in second wire
= \(\frac{6.4}{2000}\) × 100 =0.32%