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Moving Charges and Magnetism MCQs

Class 12 Physics — questions with answers and worked explanations.

20 free questions Class 12 Physics Answers + explanations No sign-up

These questions are drawn from the Pariksha Sutra question bank for Moving Charges and Magnetism, part of the Class 12 Physics syllabus. Each one shows the correct answer and, where a method helps, the working behind it.

Read the question, decide your answer before looking, then check the explanation — that is what turns practice into marks. If a question catches you out, the explanation is the part worth re-reading.

Questions

Question 1
A charge q moving with velocity v enters a magnetic field B parallel to the direction of B. The magnetic force on the charge is:
  1. qvB
  2. qv/B
  3. zero
  4. qvB sin(45 degrees)
Answer: zero
Question 2
According to the Biot-Savart law, the magnetic field due to a current element is inversely proportional to:
  1. the square of the distance
  2. the cube of the distance
  3. the current
  4. the distance
Answer: the square of the distance
Question 3
The magnetic field inside a long solenoid having n turns per unit length carrying current I is:
  1. B = mu_0 I/n
  2. B = mu_0 n I/2
  3. B = mu_0 n I/(2(pi))
  4. B = mu_0 n I
Answer: B = mu_0 n I
Question 4
Two parallel wires carry currents of 3 A and 4 A in opposite directions and are separated by 0.06 m. The force per unit length is (mu_0 = 4(pi)x10^-7 T m/A):
  1. 4x10^-5 N/m (repulsive)
  2. 2x10^-5 N/m (repulsive)
  3. 8x10^-5 N/m (repulsive)
  4. 4x10^-5 N/m (attractive)
Answer: 4x10^-5 N/m (repulsive)
Question 5
A wire carrying current 5 A and length 2 m experiences a maximum force of 3 N in a magnetic field. The magnetic field strength is:
  1. 3.0 T
  2. 1.5 T
  3. 0.3 T
  4. 0.6 T
Answer: 0.3 T
Question 6
Inside a long straight current-carrying wire of uniform current density, at distance r from the axis (r less than radius R), the magnetic field varies as:
  1. proportional to r^2
  2. proportional to r
  3. proportional to 1/r
  4. independent of r
Answer: proportional to r
Question 7
The definition of one ampere is based on the force between two parallel wires. One ampere produces a force per metre of 2x10^-7 N/m when the wires are separated by:
  1. 1 metre
  2. 0.5 metre
  3. 0.1 metre
  4. 2 metre
Answer: 1 metre
Question 8
A radial magnetic field is used in a moving coil galvanometer so that:
  1. the field is stronger
  2. the scale is linear and torque is maximum for all positions
  3. the coil moves faster
  4. the current is reduced
Answer: the scale is linear and torque is maximum for all positions
Question 9
A proton of charge 1.6x10^-19 C moves with speed 2x10^6 m/s perpendicular to a magnetic field of 0.5 T. The magnitude of the magnetic force on it is:
  1. 8.0x10^-14 N
  2. 1.6x10^-13 N
  3. 1.6x10^-19 N
  4. 3.2x10^-13 N
Answer: 1.6x10^-13 N
Question 10
The magnetic field at the centre of a circular coil of radius R carrying current I with N turns is:
  1. B = mu_0 N I/R
  2. B = mu_0 I/(2(pi)R)
  3. B = mu_0 N I/(2(pi)R)
  4. B = mu_0 N I/(2R)
Answer: B = mu_0 N I/(2R)
Question 11
A solenoid of length 0.5 m has 1000 turns and carries a current of 2 A. The magnetic field inside is (mu_0 = 4(pi)x10^-7 T m/A):
  1. 1.0x10^-2 T
  2. 5.03x10^-3 T
  3. 2.51x10^-3 T
  4. 1.26x10^-3 T
Answer: 5.03x10^-3 T
Question 12
The torque on a rectangular current loop of area A carrying current I in a magnetic field B is maximum when the plane of the loop is:
  1. at 45 degrees to B
  2. perpendicular to B
  3. along B and rotating
  4. parallel to B
Answer: parallel to B
Question 13
The work done by the magnetic force on a charged particle moving in a magnetic field is:
  1. always positive
  2. equal to change in potential energy
  3. always negative
  4. always zero
Answer: always zero
Question 14
A circular coil of radius 0.1 m carries a current of 2 A. The magnetic field at its centre is (mu_0 = 4(pi)x10^-7 T m/A):
  1. 4.0x10^-5 T
  2. 1.26x10^-5 T
  3. 6.28x10^-6 T
  4. 2.51x10^-5 T
Answer: 1.26x10^-5 T
Question 15
The magnetic field at one end of a long solenoid, compared with the field at its centre, is:
  1. half the field at the centre
  2. equal to the field at the centre
  3. twice the field at the centre
  4. zero
Answer: half the field at the centre
Question 16
The torque on a current loop of N turns, area A, current I in a field B, with the loop normal making angle theta with B, is:
  1. tau = N I A B cos(theta)
  2. tau = N I A B tan(theta)
  3. tau = N I A B sin(theta)
  4. tau = N I A B
Answer: tau = N I A B sin(theta)
Question 17
A charged particle moves in a circular path of radius r in a uniform magnetic field B. The radius of the path is given by:
  1. r = qB/(mv)
  2. r = qvB/m
  3. r = mv/(qB)
  4. r = mB/(qv)
Answer: r = mv/(qB)
Question 18
The magnetic field due to a straight current element is zero at points lying:
  1. perpendicular to the element
  2. at unit distance from the element
  3. along the axis of the element
  4. at 45 degrees to the element
Answer: along the axis of the element
Question 19
The magnetic field in the region outside a toroid (in the empty interior hole or outside) is:
  1. mu_0 n I
  2. proportional to distance
  3. zero
  4. maximum
Answer: zero
Question 20
A circular coil of 50 turns and area 4x10^-3 m^2 carries a current of 2 A in a field of 0.5 T. The maximum torque is:
  1. 0.05 N m
  2. 0.1 N m
  3. 0.2 N m
  4. 0.4 N m
Answer: 0.2 N m

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