Question

A charged particle moves in a gravity-free space without change in velocity. Which of the following is/are possible?
(a) E = 0, B = 0
(b) E = 0, B ≠ 0
(c) E ≠ 0, B = 0
(d) E ≠ 0, B ≠ 0

Answer 1

(a) E = 0, B = 0
(b) E = 0, B ≠ 0
(d) E ≠ 0, B ≠ 0

A charged particle can move in a gravity-free space without any change in velocity in the following three ways:

(1) E = 0, B = 0, i.e. no force is acting on the particle and hence, it moves with a constant velocity.

(2) E = 0, B ≠​ 0. If magnetic field is along the direction of the velocity v, then the force acting on the charged particle will be zero, as F = q v × B = 0. Hence, the particle will not accelerate.

(3) If the force due to magnetic field and the force due to electric field counterbalance each other, then the net force acting on the particle will be zero and hence, the particle will move with a constant velocity.