A magnetic field B is confined to a region r a ≤ and

The magnetic flux across the conducting ring reduces to zero from maximum when the magnetic field is reduced in ∆t.

Induced emf = E2πb

From Faraday’s law of emf,

The induced emf = rate of change of magnetic flux = Bπa2/∆t

Using the above equations, we get

E2πb = Bπa2/∆t

QE is the electric force experienced by the ring

Torque on the ring is Q.Ba2/2∆t

Change in angular momentum = torque × ∆t

The initial momentum is zero and the final momentum is Q.Ba2/2

ꞷ = Q.Ba2/2mb2