Choose the correct answer

A flat dielectric disc of radius R carries an excess charge on its surface. The surface charge density is . The disc rotates about an axis perpendicular to its plane passing through the centre with angular velocity . Find the magnitude of the torque on the disc if it is placed in a uniform magnetic field whose strength is B which is directed perpendicular to the axis of rotation

- (a) 1/4 BR

- (b) 1/4 BR2

- (c) 1/4 BR3

- (d) \1/4 BR4