# 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.
(a) $\frac {QBa}{2mb}$
(b) $\frac {QBa^2}{2mb}$
(c) $\frac {QBa}{2mb^2}$
(d) $\frac {QBa^2}{2mb^2}$