A closed current-carrying loop having a current I is having area A. Magnetic moment of this loop is defined as `vec(mu) = vec(IA)` where direction of area vector is towards the observer if current is flowing in anticlockwise direction with respect to the observer. If this loop is placed in a uniform magnetic field `vec(B)`, then torque acting on the loop is given by `vec(tau) = vec(mu) xx vec(B)`. Now answer the following questions:
A uniformly charged insulating ring is rotated in a uniform magnetic field about its own axis, then

Options

A. Ring will experience a magnetic force
B. Ring must experience a magnetic torque
C. Ring may experience a magnetic torque
D. None of the above

Answer

C

Explanation

when ring is rotated in the field, it will form a closed current loop. We know that net force on a closed current carrying loop in a uniform magnetic field is zero.
we can also explain like this: when ring is rotated, on the ring the pair of charge have equal and opposite velocities. so forces on them will be cancelled.
But torque may be acting on the ring depending on its orientation in the field.