A ring of mass m, radius R having charge q uniformly distributed over it and free to rotate about its own axis is placed in a region having a magnetic field B parallel to its axis. If the magnetic field is suddenly switched off, the angular velocity acquired by the ring is

Consider a shaded ring of thickness dr , having small charge dq. dq=qπR22πrdr Induced electric field is E(say) E2πr=πr2dBdt ⇒E=r2dBdt This electric field will apply force on charge dq, which will generate torque dτ. τ=∫dτ=∫rdF=∫rEdq=∫r×r2dBdt×qπR22πrdr=qR24△B△t We know,  Angular Impulse=△L=τ△t ⇒I△ω=qR24△B△t×△t ⇒mR22△ω=qR24△B ⇒△ω=qB2m