Problem 3.290

A metal disc of radius $a=25 \mathrm{~cm}$ rotates with a constant angular velocity $\omega=130 \mathrm{rad} / \mathrm{s}$ about its axis. Find the potential difference between the centre and the rim of the disc if (a) the external magnetic field is absent; (b) the external uniform magnetic field of induction $B=5.0 \mathrm{mT}$ is directed perpendicular to the disc.