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 
Tesla is switched on. After t second from switching on the magnetic field the ring is just about to rotate about vertical axis through its centre. Find the friction coefficient between the ring and the surface.
Solution (.wmv download)
Tesla is switched on. After t second from switching on the magnetic field the ring is just about to rotate about vertical axis through its centre. Find the friction coefficient between the ring and the surface.Solution (.wmv download)