question_answer 44)

  A magnetic field B is confined to a region r  a and points out of the paper (the z-axis), r = 0 being the centre of the circular region. A charged ring (charge = Q) of radius b, b > a and mass m lies in the x-y plane with its centre at the origin. The ring is free to rotate and is at rest. The magnetic field is brought to zero in time . Find the angular velocity  of the ring after the field vanishes.

Answer:

                  If E is the electric field generated around the charged ring of radius b, then as   or           ???..(i) Torque acting on the ring using (i),  If  is change in angular momentum of the charged ring, then as As initial angular momentum,                 This is the angular velocity of the ring after the field vanishes.