A magnetic field B is confined to a region $r \leq a$ and points out 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 xy 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 $\bigtriangleup t$. Find the angular velocity $\omega$ of the ring after the field vanister.