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_(0)di)/(2x),dq=q/(piR^(2))(2pix)dx`
`di=(dq)f=(2qxdx)/(R^(2))f` `dB=(mu_(0)2qxdx)/(2xR^(2))rArrB=int_(0)^R(mu_(0)2q.dx)/(2R^(2))(f)`
`B=(mu_(0)qf)/(R^(2))(R)rArrB=(mu_(0)qf)/R`