A ring of mass M and radius r rests on a rough horizontal plane with coefficient of friction µ. A vertical magnetic field through the ring varies as B=Kt². Charge Q is uniformly distributed over the ring. The time after which the ring starts rotating is:
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Question

A ring of mass M and radius r rests on a rough horizontal plane with coefficient of friction µ. A vertical magnetic field through the ring varies as B=Kt². Charge Q is uniformly distributed over the ring. The time after which the ring starts rotating is:

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G1 Physics · Accepted Answer

The changing field produces E=(r/2)dB/dt=Krt. Torque on total charge Q is QEr=QKr²t. Maximum resisting friction torque is μMgr. Equating gives t=μMg/(QKr). Hence A.