Find the ratio of magnetic dipole moment and magnetic field at the centre of a disc. Radius of disc is `R` and it is rotating at constant angular speed o about its axis. The disc is insulating and uniformly charged

Answer

B, C

Explanation

Let surface charge density is sigma

`d=[(2pir)dr]sigma`
Equivalent current,
`i=(dq)f`
`dM=iA=[(dq)f][pir^2]`
`:. M=int_0^RdM`
`dB=(mu_0i)/(2r)=(mu_0(dq)f)/(2r)`
`B=int_0^R dB`
now, we can find the ratio `M/B`.