A non-conducting ring of mass m and radius R has a charge Q uniformly distributed over its circumference. The ring is placed on a rough horizontal surface such that plane of the ring is parallel to the surface. A vertical magnetic field B=B0t2 tesla is switched on. After 2 s from switching on the magnetic field the ring is just about to rotate about vertical axis through its centre.
a. - Find friction coefficient μ between the ring and the surface.
b. - If magnetic field is switched off after 4 s, then find the angle rotated by the ring before coming to stop after switching off the magnetic field.