JEE Advanced 2020 Full Test 4 Paper 2

A uniform disc of radius R having charge Q distributed uniformly all over its surface is placed on a smooth horizontal surface. A magnetic field, B=kxt2, where k is a constant, x is the distance (in metre) from the centre of the disc and t is the time (in second), is switched on perpendicular to the plane of the disc. Find the torque (in N−m ) acting on the disc after 15 sec. (Take 4kQ=2.50S.I. unit and R=1m ).