A plastic disc of radius `'R'` has a charge `'q'` uniformly distributed over its surface. If the disc is rotated with a frequency `'f'` about its axis, then the magnetic induction at the centre of the disc is given by

Explanation

`dB = (mu_0di)/(2x). Dq = (q)/(pi R^2) (2pi x) dx`
` di = (dq) f = (2qxdx)/(R^2)f`
` dB = (mu_0 2qxdx)/(2xR^2) rArr B = int_0^R (mu_0 2qdx)/(2R^2) (f)`
` B = (mu_0 qf)/(R^2) ( R ) rArr B = (mu_0 qf)/(R ) `