# Do the Electrons Rotate with a Superconducting Disk?

In a superconductor the electrons can move with respect to the material they are part of without resistance, in a sense without friction. If I have a superconducting disk and I set it to rotating, do the electrons participate in the rotational motion? If so, how is the force transmitted to them?

• This answer is also relevant. Dec 22, 2017 at 15:56

The key to solving this problem is recognizing that the amount of current produced by even a small rotating disk where the free electrons aren't moving would be astounding. Suppose there was only one free superconducting electron per $100$ nucleons. In a superconductor with $0.01\operatorname{moles}$ of atoms in it, there will be about $6\times 10^{21}$ free electrons, giving a disk with about $10^5 \operatorname{C}$ of charge. Even a tiny rate of rotation would produce a staggering current.
Importantly, the process of accelerating the positively charged disk by an infinitesimal amount would produce a magnetic field of a rotating charged disk. That infinitesimal change in the magnetic field will produce an electric field of similar magnitude by one of Maxwell's equations: $$\nabla \times \mathbf{E} = - \frac{\partial \mathbf{B}}{\partial t}.$$ Examining the direction of the electric field reveals that it points in precisely the direction needed to exert a torque on the free electrons, spinning them up with the disk.