Moving Charges and Magnetism

What you'll learn

A patient, from-scratch guide to how magnetic fields push moving charges and current-carrying wires, how currents create fields (Biot-Savart and Ampere's law), the solenoid, and the moving-coil galvanometer, with a fully worked numerical and clear SI units for CBSE Class 12 (and your JEE/NEET/CUET foundation).

This chapter has

Moving Charges and Magnetism — solved practice questions

8 Class 12 Physics questions with step-by-step solutions. Attempt each, then reveal the answer.

1. Q1easy

   A charged particle moving with velocity v enters a uniform magnetic field B. The magnetic force on it is zero when

   • Av is perpendicular to B
   • Bv makes 45 degrees with B
   • Cv is parallel to B
   • Dthe charge is negative
   Show answer & solution

   Correct answer: (C) v is parallel to B

   Magnetic force F = qvB sin(theta) is zero when theta = 0, i.e. when v is parallel (or antiparallel) to B.

2. Q2easy

   The SI unit of magnetic field (magnetic flux density) is

   • Atesla (T)
   • Bweber (Wb)
   • Cgauss (G)
   • Dhenry (H)
   Show answer & solution

   Correct answer: (A) tesla (T)

   The SI unit of magnetic flux density B is the tesla (T), where 1 T = 1 Wb m^-2.

3. Q3easy

   A straight conductor of length L carrying current I is placed in a uniform magnetic field B perpendicular to it. The force on the conductor is

   • ABIL^2
   • BBI/L
   • CB^2 IL
   • DBIL
   Show answer & solution

   Correct answer: (D) BIL

   Force on a current-carrying conductor is F = BIL sin(theta); for B perpendicular to the wire (theta = 90) F = BIL.

4. Q4easy

   The magnetic field at the centre of a circular current loop of radius R carrying current I is

   • Amu0 I/(4 pi R)
   • Bmu0 I/(2R)
   • Cmu0 I/(2 pi R)
   • Dmu0 I/R
   Show answer & solution

   Correct answer: (B) mu0 I/(2R)

   From the Biot-Savart law, the field at the centre of a circular loop is B = mu0 I/(2R).

5. Q5medium

   A proton moves with a speed of 2 x 10^6 m/s perpendicular to a magnetic field of 0.5 T. The magnitude of the magnetic force on it is (charge = 1.6 x 10^-19 C)

   • A3.2 x 10^-13 N
   • B0.8 x 10^-13 N
   • C1.6 x 10^-13 N
   • D1.6 x 10^-19 N
   Show answer & solution

   Correct answer: (C) 1.6 x 10^-13 N

   F = qvB = (1.6 x 10^-19)(2 x 10^6)(0.5) = 1.6 x 10^-13 N.

6. Q6medium

   A charged particle of mass m and charge q moves in a circle of radius r in a magnetic field B perpendicular to its velocity v. The radius r is given by

   • Amv/(qB)
   • BqB/(mv)
   • CmvB/q
   • DqvB/m
   Show answer & solution

   Correct answer: (A) mv/(qB)

   The magnetic force provides the centripetal force: qvB = mv^2/r, giving r = mv/(qB).

7. Q7medium

   The frequency of revolution (cyclotron frequency) of a charged particle in a uniform magnetic field is independent of

   • Athe charge of the particle
   • Bthe mass of the particle
   • Cthe magnetic field strength
   • Dthe speed of the particle
   Show answer & solution

   Correct answer: (D) the speed of the particle

   Cyclotron frequency f = qB/(2 pi m) depends only on charge, mass and field; it is independent of the particle's speed and radius.

8. Q8medium

   The magnetic field at a distance of 0.1 m from a long straight wire carrying a current of 5 A is (mu0/4 pi = 10^-7 T m A^-1)

   • A2 x 10^-5 T
   • B1 x 10^-5 T
   • C5 x 10^-6 T
   • D1 x 10^-6 T
   Show answer & solution

   Correct answer: (B) 1 x 10^-5 T

   B = mu0 I/(2 pi r) = (2 x 10^-7)(5)/(0.1) = 1 x 10^-5 T.

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