A magnetic field B is confined to a region r ≤aand points out of the paper (the z- axis) , r = 0 being the centre of 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 time ∆t. Find the angular velocity ω of the ring after the field vanishes.

Q: 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 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 time ∆t. Find the angular velocity ω of the ring after the field vanishes.