2003 AIPMT Moving Charges and MagnetismMotion of Charge in Magnetic Field Easy Question 80 of 89 in Moving Charges and Magnetism

A charged particle moves through a magnetic field in a direction perpendicular to it. Then the

A Speed of the particle remains unchanged
B Direction of the particle remains unchanged
C Acceleration remains unchanged
D Velocity remains unchanged

The answer is A.

Solution

Magnetic force on a moving charge: $\vec{F} = q(\vec{v} \times \vec{B})$. This force is always perpendicular to the velocity $\vec{v}$.
A force perpendicular to the velocity does no work, so the kinetic energy and hence the speed of the particle remain constant.
The force continuously changes the direction of motion, so the particle moves in a circle. Its velocity (a vector) and its acceleration (directed towards the centre, changing in direction) both change.
So the speed of the particle remains unchanged.

Concept Magnetic force does no work

Moving Charges and Magnetism in past papers

89 questions from this chapter have appeared across 27 exam years.