Select the correct answer in each of the following options.

(1) A moving charge is surrounded by

(a) 2 fields                          (b) 3 fields                          (c) 4 fields                           (d) none

Explanation: A moving charge is surrounded by two fields; one the electric field due to the charge and the other the magnetic field due to the motion of charge.

(2) A photon while moving through a magnetic field is deflected towards

(a) North pole                    (b) South pole                    (c) is ionized                       (d) none

Explanation: A photon has no charge on it and is not affected by any of the poles of the magnetic field or ionized.

(3) Magnetism is related to

(a) Stationary charge       (b) moving charge           (c) stationary and moving charge               (d) law of motion

Note that magnetism is the phenomenon associated with moving charges.

(4) When a charge particle enters perpendicular to a magnetic field, the path followed by it is

(a) A helix                           (b) a circle                          (c) straight line                  (d) ellipse

Explanation: The magnetic force acting on the particle is perpendicular to the velocity of the particle and provides the necessary centripetal force to move the particle in a circle.

(5) The torque acting on a coil can be increased by increasing

(a) number of turns          (b) current and magnetic field      (c) area of the coil            (d) all the above

Explanation:  Τ  = NIAB. Therefore, torque can be increased by increasing the number of turns N, current I, magnetic field B and area of the coil A. For more details see comprehensive question 2.

(6) The magnetic flux will be maximum for an angle of

(a) 0°                                   (b) 60°                                                (c) 90°                   (d) 180°

Explanation: Since magnetic flux = Φ = BA cosθ and cosθ is maximum for 0⁰ and so do the magnetic flux.

(7) The Weber is a unit of measure of

(a) Conductance               (b) electric current            (c) magnetic flux              (d) electric flux

(8) One Weber is equal to

(a) N A²/m                          (b) N m²/A                          (c) N A/m                            (d) N m/A

Explanation: Since B = F/IL, therefore, unit of magnetic induction B is N/A m = N A-1 m-1. Similarly, magnetic flux ϕ = BA = N A-1 m-1 m2 = N m A-1.

(9) An electron moves at 2 * 10² m/s and enters perpendicular to the magnetic field of 2 T. What is the magnitude of magnetic force?

(a) 1 * 10^-6 N                      (b) 6.4 * 10^-17 N                (c) 3.6 * 10^-24 N                 (d) 4 * 10⁶ N

Solution:  Force on the electron in a magnetic field is given by F = e(v x B) = evBsinθ . Put the values,

F = 1.6 * 10^-19 (2 * 10² * 2 * 1) = 1.6 * 2 * 2 * 10^-19+2 = 6.4 * 10^-17 N.

Here 1.6 * 10^-19 is charge on the electron and sin90⁰ = 1.

For details see comprehensive question 3.

(10) The force on a charged particle moving parallel to the magnetic field is

(a) Maximum                     (b) minimum                      (c) zero                               (d) none

Explanation: In a magnetic field B, the force on a charged particle q moving with velocity v is

F = q (v x B) = qvBsinθ

Put θ = 0⁰, then sinθ = 0 (the case when v and B are parallel) and the force will also be zero.

(11) Ampere law is applicable to

(a) Circular path                (b) rectangular path         (c) to any closed path    (d) none of these

(Do you know the difference between a circular path, a rectangular path and an any closed path?)

(12) The unit of permeability of free space is

(a)T.m/a                            (b) T.m²/A                           (c) T.m/A²                           (d) None