Past exam of the mathematics course of the University of Cambridge / 2024 / ib / Paper 1 / 15C / ii / Solution

An annulus of radius $s$ and width $ds$ has charge $dQ=2\pi s\sigma ds$, currentand dipole moment $dm=\pi s^{2}dI\hat{n}$. Therefore the magnetic dipole moment of a rigidly rotating charge distribution is