# Physics MCQs for Class 12 with Answers Chapter 4 Moving Charges and Magnetism

Free PDF Download of CBSE Physics Multiple Choice Questions for Class 12 with Answers Chapter 4 Moving Charges and Magnetism. Physics MCQs for Class 12 Chapter Wise with Answers PDF Download was Prepared Based on Latest Exam Pattern. Students can solve NCERT Class 12 Physics Moving Charges and Magnetism MCQs Pdf with Answers to know their preparation level.

## Moving Charges and Magnetism Class 12 Physics MCQs Pdf

1. Two charged particles traverse identical helical paths in a completely opposite sense in a uniform magnetic field B = B0$$\hat{k}$$. [NCERT Exemplar]
(a) They have equal z-components of momenta.
(b) They must have equal charges.
(c) They necessarily represent a particle- antiparticle pair.
(d) The charge to mass ratio satisfy:
$$\left(\frac{e}{m}\right)_{1}+\left(\frac{e}{m}\right)_{2}=0$$

Explaination:
(d) Charged particles traverse identical helical paths in a completely opposite sense in a uniform magnetic field B.
Therefore
$$\left(\frac{e}{m}\right)_{1}+\left(\frac{e}{m}\right)_{2}=0$$

2. Biot-Savart law indicates that the moving electrons (velocity v) produce a magnetic field B such that [NCERT Exemplar]
(a) B ⊥ v.
(b) B || v.
(c) it obeys inverse cube law.
(d) it is along the line joining the electron and point of observation.

Explaination:
(a) Magnetic field is given by

Where n is the direction of $$\vec{B}$$ which is in the direction of cross product of $$\vec{v}$$ and $$\vec{r}$$. Or we can say that $$\vec{B}$$ ⊥ to both $$\vec{v}$$ and $$\vec{r}$$.

3. A current carrying circular loop of radius R is placed in the x-y plane with centre at the origin. Half of the loop with x > 0 is now bent so that it now lies in the y – z plane. [NCERT Exemplar]
(a) The magnitude of magnetic moment now diminishes.
(b) The magnetic moment does not change.
(c) The magnitude of B at (0.0.z), z» R increases.
(d) The magnitude of B at (0, 0, z), z » R is unchanged.

Explaination:
(a) Direction of magnetic moment (M= I A) of circular loop is perpendicular to the loop as per right hand thumb rule.
The magnitudes of magnetic moment of each semicircular loop of radius R lie in the x-y plane and y-z plane is M1 – M2= $$I \frac{\pi R^{2}}{2}$$ and the direction of magnetic moments are along z-direction and ^-direction respectively. Their resultant

4. An electron is projected with uniform velocity along the axis of a current carrying long solenoid. Which of the following is true? [NCERT Exemplar]
(a) The electron will be accelerated along the axis.
(b) The electron path will be circular about the axis.
(c) The electron will experience a force at 45° to the axis and hence execute a helical path.
(d) The electron will continue to move with uniform velocity along the axis of the solenoid.

Explaination:
(d) F = -evB sin 180° = 0 (i.e 0= 0°or 180° in both cases F = 0). The electron will continue to move with uniform velocity or will go undeflected along the axis of the solenoid.

5. In a cyclotron, a charged particle [NCERT Exemplar]
(a) undergoes acceleration all the time.
(b) speeds up between the dees because of the magnetic field.
(c) speeds up in a dee.
(d) slows down within a dee and speeds up between dees.

Explaination:
(a) It is based on the fact that the electric field accelerates a charged particle and the perpendicular magnetic field keeps it revolving in circular orbits of constant frequency.

6. A circular current loop of magnetic moment Mis in an arbitrary orientation in an external magnetic field B. The work done to rotate the loop by 30° about an axis perpendicular to its plane is [NCERT Exemplar]
(a) MB
(b) √3$$\frac{MB}{2}$$
(c) $$\frac{MB}{2}$$
(d) zero

Explaination:
(d) The rotation of the loop by 30° about an axis perpendicular to its plane makes no change in the angle made by axis of the loop with the direction of magnetic field, therefore, the work done to rotate the loop is zero.

7. A rectangular loop carrying a current i is situated near a long straight wire such that the wire is parallel to the one of the sides of the loop and is in the plane of the loop. If a steady current I is established in wire as shown in figure, the loop will

(a) rotate about an axis parallel to the wire.
(b) move away from the wire or towards right.
(c) move towards the wire.
(d) remain stationary.

8. A circular coil of radius 4 cm and of 20 turns carries a current of 3 amperes. It is placed in a magnetic field of intensity of 0.5 weber/m². The magnetic dipole moment of the coil is
(a) 0.15 ampere-m²
(b) 0.3 ampere-m²
(c) 0.45 ampere-m²
(d) 0.6 ampere-m²

9. A cubical region of space is filled with some uniform electric and magnetic fields. An electron enters the cube across one of its faces with velocity v and a positron enters via opposite face with velocity -v. At this instant,
(a) the electric forces on both the particles cause identical accelerations.
(b) the magnetic forces on both the particles cause equal accelerations.
(c) Only electron gains or looses energy.
(d) the motion of the centre of mass (CM) is determined by E alone.

10. Consider a wire carrying a steady current, I placed in a uniform magnetic field B perpendicular to its length. Consider the charges inside the wire. It is known that magnetic forces do not work. This implies that,
(a) motion of charges inside the conductor is unaffected by B, since they do not absorb energy.
(b) Some charges inside the wire move to the surface as a result of B.
(c) if the wire moves under the influence of B, no work is done by the force.
(d) If the wire moves under the influence of B, no work is done by the electric force on the ions, assumed fixed within the wire.

11. Two identical current carrying coaxial loops, carry current I in an opposite sense. A simple amperian loop passes through both of them once. Calling the loop as C,
(a) $$\oint_{C}$$B.dl = ± 2µ0I.
(b) the value of $$\oint_{C}$$B.dl is independent of sense of C. c
(c) there may be a point on C where, B and dl are parallel.
(d) B vanishes everywhere on C.

Explaination:
(b) Ampere’s law gives another method to calculate the magnetic field due to a given current distribution.
Applying the Ampere’s circuital law, we have
$$\oint_{C}$$B.dl = i0(I – I) = 0 (because current is in opposite sense).
Also, there may be a point on C where B and dl are perpendicular and hence $$\oint_{C}$$B.dl = 0

12. The strength of magnetic field at the centre of circular coil is

Explaination:
(c) B = Field to circular portion
– Field due to straight portion

13. If a charged particle moves through a magnetic field perpendicular to it
(a) both momentum and energy of particle change.
(b) momentum as well as energy are constant.
(c) energy is constant but momentum changes.
(d) momentum is constant but energy changes.

Explaination:
(c) Since the direction of velocity of a particle varies so momentum changes but direction of magnetic force is always perpendicular to direction of charged particle. So no work is done, i.e. energy remains the same.

14. A current carrying closed loop of an irregular shape lying in more than one plane when placed in uniform magnetic field, the force acting on it
(a) will be more in the plane where its larger position is covered.
(b) is zero.
(c) is infinite.
(d) may or may not be zero.

Explaination:
(b) A current carrying closed loop of any shape when placed in a uniform magnetic field does not experience any force.

15. The maximum current that can be measured by a galvanometer of resistance 40 Ω is 10 mA. It is converted into voltmeter that can read upto 50 V. The resistance to be connected in the series with the galvanometer is
(a) 2010 Ω
(b) 4050 Ω
(c) 5040 Ω
(d) 4960 Ω

Explaination:

16. A current loop placed in a non-uniform magnetic field experiences
(a) a force of repulsion.
(b) a force of attraction.
(c) a torque but not force.
(d) a force and a torque.

Explaination:
(d) In non-uniform magnetic field, current loop experiences an unequal opposite force which forms torque.

17. What is the net force on the rectangular coil?

(a) 25 × 10-7 N towards wire.
(b) 25 × 10-7 N away from wire.
(c) 35 × 10-7 N towards wire.
(d) 35 × 10-7 N away from wire.

Explaination:

18. If the beams of electrons and protons move parallel to each other in the same direction, then they
(a) attract each other.
(b) repel each other.
(c) no relation.
(d) neither attract nor repel.

Explaination:
(b) As current carried by electrons and protons are in opposite direction.

19. A conducting circular loop of radius r carries a constant current i. It is placed in a uniform magnetic field B, such that B is perpendicular to the plane of the loop. The magnetic force acting on the loop is
(a) irB.
(b) 2πriB
(c) zero
(d) πriB

Explaination:
(c) Net force on a current carrying closed loop is always zero, if it is placed in an uniform magnetic field.

20. The gyro-magnetic ratio of an electron in an H-atom, according to Bohr model, is
(a) independent of which orbit it is in.
(b) neutral
(c) positive
(d) increases with the quantum number n.

Explaination:
(a) The gyro-magnetic ratio is given by
$$\mu_{l}=\frac{-e}{2 m} L, \mathrm{L}$$ – Angular momentum of electron

21. An electron is projected along the axis of a circular conductor carrying the same current. Electron will experience
(а) a force along the axis.
(б) a force perpendicular to the axis.
(c) a force at an angle of 4° with axis.
(d) no force experienced.

Explaination:
(d) Since electron is moving parallel to direction of magnetic field of the conductor
Force (F) = qvB sin 0 = 0

22. Three long, straight parallel wires, carrying current are arranged as shown in the figure. The force experienced by a 25 cm length of wire C is

(a) 10-3 N
(b) 2.5 × 10-3 N
(c) zero
(d) 1.5 × 3 N

Explaination:
(c) Force of repulsion by wire D and G on wire C is equal and opposite.

23. In a circular coil of radius r, the magnetic field at the centre is proportional to
(a) r²
(b) r
(c) $$\frac{1}{r}$$
(d) $$\frac{1}{r²}$$

Explaination:

24. A positive charge enters in a magnetic field and travels parallel to but opposite the field. If experiences
(a) an upward force.
(b) a downward force.
(c) an accelerated force.
(d) no force.

Explaination: (c) Force of repulsion by wire D and G on wire C is equal and opposite.

25. When a magnetic compass needle is carried nearby to a straight wire carrying current, then
(I) the straight wire cause a noticeable deflection in the compass needle.
(II) the alignment of the needle is tangential to an imaginary circle with straight wire as its centre and has a plane perpendicular to the wire
(a) (I) is correct
(b) (II) is correct
(c) both (I) and (II) are correct
(d) neither (I) nor (II) is correct

26. A strong magnetic field is applied on a stationary electron. Then the electron
(a) moves in the direction of the field.
(b) remained stationary.
(c) moves perpendicular to the direction of the field.
(d) moves opposite to the direction of the field.

27. In an inertial frame of reference, the magnetic force on a moving charged particle is $$\vec{F}$$ Its value in another inertial frame of reference will be
(a) remained same
(b) changed due to change in the amount of charge
(c) changed due to change in velocity of charged particle
(d) changed due to change in field direction

28. Which one of the following is correct statement about magnetic forces?
(a) Magnetic forces always obey Newton’s third law.
(b) Magnetic forces do not obey Newton’s third law.
(c) For very high current, magnetic forces obey Newton’s third law.
(d) Inside low magnetic field, magnetic forces obey Newton’s third law.

29. A charged particle is moving on circular path with velocity v in a uniform magnetic field B, if the velocity of the charged particle is doubled and strength of magnetic field is halved, then radius becomes
(a) 8 times
(b) 4 times
(c) 2 times
(d) 16 times

30. Two a-particles have the ratio of their velocities as 3 : 2 on entering the field. If they move in different circular paths, then the ratio of the radii of their paths is
(a) 2 : 3
(b) 3 : 2
(c) 9 : 4
(d) 4 : 9

31. A charged particle is moving in a cyclotron, what effect on the radius of path of this charged particle will occur when the frequency of the ratio frequency field is doubled?
(a) It will also be doubled.
(b) It will be halved.
(c) It will be increased by four times.
(d) It will remain unchanged.

32. Which of the following is not correct about cyclotron?
(a) It is a machine to accelerate charged particles or ions to high energies.
(b) Cyclotron uses both electric and magnetic fields in combination to increase the energy of charged particles.
(c) The operation of the cyclotron is based on the fact that the time for one revolution of an ion is independent of its speed or radius of its orbit.
(d) The charged particles and ions in cyclotron can move on any arbitrary path.

33. If an electron is moving with velocity $$\vec{ν}$$ produces a magnetic field $$\vec{B}$$, then
(a) the direction of field $$\vec{B}$$ will be same as the direction of velocity $$\vec{ν}$$ .
(b) the direction of field $$\vec{B}$$ will be opposite to the direction of velocity $$\vec{ν}$$ .
(c) the direction of field $$\vec{B}$$ will be perpendicular to the direction of velocity $$\vec{ν}$$ .
(d) the direction of field $$\vec{B}$$ does not depend upon the direction of velocity $$\vec{ν}$$ .

34. Current flows through uniform, square frames as shown in the figure. In which case is the magnetic field at the centre of the frame not zero?

35. Ampere’s circuital law is given by

36. Two identical current carrying coaxial loops, carry current I in opposite sense. A simple amperian loop passes through both of them once. Calling the loop as C, then which statement is correct?

(c) there may be a point on C where B and dl are
parallel.
(d) none of these

37. The correct plot of the magnitude of magnetic field $$\vec{B}$$ vs distance r from centre of the wire is, if the radius of wire is R

38. The nature of parallel and anti-parallel currents are
(a) parallel currents repel and antiparallel cur¬rents attract.
(b) parallel currents attract and antiparallel cur-rents repel.
(c) both currents attract. ’
(d) both currents repel.

39. The magnetic moment of a current I carrying circular coil of radius r and number of turns N varies as
(a) $$\frac{1}{r²}$$
(b) $$\frac{1}{r}$$
(c) r
(d) r²

40. A short bar magnet has a magnetic moment of 0. 65 J T-1, then the magnitude and direction of the magnetic field produced by the magnet at a distance 8 cm from the centre of magnet on the axis is
(a) 2.5 × 10-4 T, along NS direction
(b) 2.5 × 10-4 T along SN direction
(c) 4.5 × 10-4 T, along NS direction
(d) 4.5 × 10-4 T, along SN direction

41. A current carrying loop is placed in a uniform magnetic field. The torqe acting on it does not depend upon
(a) area of loop
(b) value of current
(c) magnetic field
(d) None of these

42. In a moving coil galvanometer the deflection (Φ) on the scale by a pointer attached to the spring is

43. A moving coil galvanometer can be converted into an ammeter by
(a) introducing a shunt resistance of large value in series.
(b) introducing a shunt resistance of small value in parallel.
(c) introducing a resistance of small value in series.
(d) introducing a resistance of large value in parallel.

44. The conversion of a moving coil galvanometer into a voltmeter is done by
(a) introducing a resistance of large value in series.
(b) introducing a resistance of small value in parallel.
(c) introducing a resistance of large value in parallel.
(d) introducing a resistance of small value in series.

45. Deflection produced in a galvanometer when a unit current flows through it is known as _________ .

Explaination: current sensitivity.

46. A moving coil galvanometer can be converted into voltmeter by connecting a large resistance R in _________ with it.

Explaination: series

47. Maximum torque acts on a current carrying coil when it is suspended in magnetic field such that its plane is _________ to magnetic field.

Explaination: parallel

48. An ammeter is _________ resistance galvanometer.

Explaination: low

49. The magnetic field due to a straight current carrying conductor of infinite length at a perpendicular distance a is equal to _________ .

Explaination: $$B=\frac{\mu_{0} I}{2 \pi a}$$

50. Relation between S.I. unit and C.G.S unit magnetic field is _________ .

Explaination: 1 T=104 G

51. According to ampere circuital law, the line integral of the magnetic field $$\vec{B}$$ around any closed path enclosing current 7, is equal to _________ .

Explaination: µ0I

52. Force on a charge q moving in a magnetic field B with velocity v at angle 0 is equal to _________ .

Explaination: F = Bqv sin θ

53. Force on a current carrying conductor in a magnetic field is _________ .

Explaination: F = BIl sin θ

54. The magnetic field of a straight solenoid carrying current l and having n turns per unit length is _________ .

Explaination: B = µ0nI

55. Why does a moving charge experience a force when placed in a magnetic field? [HOTS]

Explaination:
A moving charge produces a magnetic field. This magnetic field interacts with another magnetic field of a magnet and hence, it experiences force.

56. Write the expression, in a vector form, for the Lorentz magnetic force $$\vec{F}$$ due to a charge moving with velocity $$\vec{v}$$ in a magnetic field $$\vec{B}$$. What is the direction of the magnetic force? [Delhi 2014]

Explaination:
Lorentz magnetic force ($$\vec{F}_{m}$$) = q($$\vec{v}$$ × $$\vec{B}$$). The direction of magnetic force is perpendicular to the plane containing velocity and magnetic field vectors.

57. Define one tesla using the expression for the magnetic force acting on a particle of charge q moving with velocity $$\vec{v}$$ in a magnetic field $$\vec{B}$$. [Foreign 2014]

Explaination:

If Fmax = 1N, q = +1C and v = 1 m/s and
θ = 90°, then B = IT
Hence, one tesla is the magnetic field in which a normally entering + 1C charge, moving at 1 ms-1 experiences a maximum force of 1 N.

58. A beam of a-particles projected along +x-axis, experiences a force due to a magnetic field along the +y-axis. What is the direction of the magnetic field? [AI2010]

Explaination:

Therefore, the direction of magnetic field is towards the negative direction of z-axis.

59. A long straight wire carries a steady current l along the positive y-axis in a coordinate system. A particle of charge +Q is moving with a velocity $$\vec{v}$$ along the x-axis. In which direction will the particle experience a force? [Foreign 2013]

Explaination:
By the right-hand thumb rule, the direction of magnetic field due to current / acts normally into the plane of paper. So, $$\vec{B}=-B \hat{k}$$, i.e. along negative z-axis. The magnetic Lorentz force is given by

Thus, the force on charge +Q is along +y axis.

60. In a certain region of space, electric field $$\vec{E}$$ and magnetic field $$\vec{B}$$ are perpendicular to each other. An electron enters in the region perpendicular to the directions of both $$\vec{B}$$ and $$\vec{E}$$ and moves undeflected. Find the velocity of the electron. [Foreign 2013]

Explaination:
∵ qvB = qE ⇒ v = $$\frac{E}{B}$$

61. An electron and a proton moving with the same speed enter the same magnetic field region at right angles to the direction of the field. For which of the two particles will the radius of circular path be smaller? [HOTS]

Explaination:
As r = $$\frac{mv}{qB}$$ for same v and B r ∝ $$\frac{m}{q}$$.
Since $$\frac{m}{q}$$ is smaller for an electron, the radius of the circular path followed by the electron will be smaller.

62. A proton and an electron travelling along parallel paths enter a region of uniform magnetic field, acting perpendicular to their paths. Which of them will move in a circular path with higher frequency? [CBSE 2018]

Explaination:
Electron
Reason:

Thus electron will move in circular path with higher frequency.

63. Write the expression for the magnetic moment of a circular coil of area A carrying a current I, in a vector form. [Foreign 2014]

Explaination:
M$$\vec{M}=\overrightarrow{I A}$$, as vector $$\vec{A}$$ is perpendicular to the surface, the magnetic moment M will also be perpendicular to the plane of circular coil.

64. Magnetic field lines can be entirely confined within the core of a toroid, but not within a straight solenoid. Why?

Explaination:
At the edges of a solenoid, the field lines get diverged due to other fields or non-availability of dipole loops, while in a toroid, the dipoles (in loops) orient continuosly.

65. Using the concept of force between two infinitely long parallel current carrying conductors, define one ampere of current. [A1 2014]

Explaination:
One ampere is that amount of current which when flows through two thin infinitely long straight conductors kept parallel to each other at 1 m distance produces a force per unit length of magnitude 2 × 107 N/m.

66. Which has greater resistance
(a) milliammeter or ammeter
(b) milliammeter or voltmeter?

Explaination:
(a) Milliammeter
(b) Voltmeter

67. A voltmeter, an ammeter and a resistance are connected in series with a battery. The voltmeter gives same deflection but the deflection of ammeter is almost zero. Explain why?

Explaination:
When a voltmeter is connected in series, the current in the circuit decreases because the resistance of voltmeter is high. Therefore, it will show some deflection. But in an ammeter, the majority of this current will pass through the shunt and a very small fraction will pass through the galvanometer.

68. State the law used to determine the direction of magnetic field at the centre of current carrying circular coil.

Explaination:
The right-hand thumb rule gives the direction of magnetic field which is stated as under:
Curl the palm of your right hand around a circular wire with the fingers, pointing in the direction of the current and the right hand thumb gives the direction of magnetic field.

69. 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 paths described by them? [Similar Delhi 2019, Foreign 2011]

Explaination:

70. Write two properties of a material used as a suspension wire in a moving coil galvanometer.

Explaination:
(i) Low value of k (torsional constant), (ii) High conductivity.

71. A charged particle enters into a uniform magnetic field and experiences an upward force as indicated in the figure. What is the charge sign on the particle?

Explaination:

72. How does the magnetic moment of an electron in a circular orbit of radius r and moving with a speed v change, when the frequency of revolution is doubled?

Explaination:
As M ∝ v, the magnetic moment also gets doubled, when the frequency of revolution is doubled.

73. A current carrying loop is free to turn in a uniform magnetic field B. Under what conditions, will the torque acting on it be
(i) minimum and
(ii) maximum?

Explaination:
T = MB sin θ
(i) Torque is minimum when the area vector of the loop and the magnetic field vector are in the same direction, i.e. A\\B.
(ii) Torque is maximum when ALB.

74. Write two factors by which voltage sensitivity of a galvanometer can be increased.

Explaination:
As we know that $$V_{s}=\left(\frac{N A B}{k}\right) \frac{1}{R}$$
Thus, (i) Resistance should be less.
(ii) Torsional constant should be less.

75. An ammeter and a milliammeter are converted from the same galvanometer. Out of the two, which current measuring instrument has higher resistance? [HOTS]

Explaination:
The higher is the range, the lower will be the value of shunt, so a milliammeter will be having higher resistance.

76. What is the advantage of using radial magnetic field in a moving coil galvanometer? [HOTS] [Delhi 2019]

Explaination:
(i) Maximum torque is experienced.
(ii) Current is directly proportional to the deflection.
(iii) The plane of the coil is parallel to the direction of magnetic field.

77. Why is it necessary for voltmeter to have a high resistance?

Explaination:
Since voltmeter is to be connected across two ends of a conductor in parallel, if it has high resistance, then only a very small part of current will pass through, and it will not affect the actual potential difference to be measured.

78. What is figure of merit of a galvanometer? [DoE]

Explaination:
Figure of merit is defined as the amount of current which produces unit deflection in the galvanometer.

79. Define gyromagnetic ratio.

Explaination:
It is the ratio of the magnetic dipole moment to the angular momentum of the electron revolving round the nucleus.

80. Can we decrease the range of an ammeter?