Question

A thin non-conducting ring of mass m carrying a charge q can freely rotate about its axis. At the initial moment the ring was at rest and no magnetic field was present. Then a practically uniform magnetic field was switched on, which was perpendicular to the plane of the ring and increased with time according to a certain law B (t). Find the angular velocity ω of the ring as a function of the induction B (t).

Answer 1

Go to the rotating frame with an instantaneous angular velocity  In this frame, a Coriolis force, 

acts which must be balanced by the magnetic force, 

(It is assumed that  small and varies slowly, so ω² and ω can be neglected.)