A plastic disc of radius `'R'` has a charge `'q'` uniformly distributed over its surface. If the dis 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_(0)di)/(2x),dq=(q)/(piR^(2))(2pix)dx`
`di=(dq)f=(2qxdx)/(R^(2))f`
`dB=(mu_(0)2qxdx)/(2xR^(2))impliesB=underset(0)overset(R)int(mu_(0)2q.dx)/(2R^(2))(f)`
`B=(mu_(0)qf)/(R^(2))(R)impliesB=(mu_(0)qf)/(R)`