Energy and angular momentum storage in a rotating magnet

Abstract

We consider a cylindrical metallic magnet that is set into rotation about a horizontal axis by a falling mass. In such a system the magnetic field causes a radial current which is non-solenoidal. The current leads to charge accumulation and a partial attenuation of the radial current. The magnetic field acting on the radially flowing current slows down the acceleration. We evaluate the angular velocity and displacement using Newtonian dynamics. We show that the electromagnetic angular momentum must be taken into account to calculate the change in the angular momentum due to the external torque on the system. The loss in the potential energy of the falling mass can be explained only by taking into consideration the electrostatic energy and the Joule loss.