Question

A charge ‘q’ is uniformly distributed on a non-conducting disc, of radius R, it is rotated with angular speed 'ω' about an axis passing through the centre of mass of the disc and perpendicular to its plane. The magnetic moment of the disc will be

(1) 1/4 ωqR²

(2) 1/2 ωqR²

(3) ωqR²

(4) 1/8 ωqR²

Answer 1

(1) 1/4 ωqR²

Consider the disc to be made up of large number of elementary concentric rings, consider one such ring of radius ‘x’ and thickness ‘dx’.