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 the plane of the ring is parallel to the surface. A vertical magnetic field B=B0t2T is switched on. After 2s from switching on the magnetic field, the ring is just about to rotate about the vertical axis through its centre.
(1) Find the coefficient of friction μ between the ring and the surface.
(2) If the magnetic field is switched off after 4s, find the angle rotated by the ring before coming to rest after switching off the magnetic field.