Does a Superconducting Disc Spin When Exposed to a Spinning Magnet?

[Gusbob]

Problem Statement

In a recent exam, I was asked the following question:
"A superconducting disc is suspended on strings (horizontally, flat side facing ground) and cooled below it's critical temperature. A bar magnet is placed on a spinning axle under the disc. What happens to the disc once the magnets begin to spin?"

Attempt at Solution

What the answer key indicated was that the disc will spin in the same direction as the magnet because a current will be induced due to Lenz's law. However, my teacher said that explanation does not take into account of the meissner effect, and that the disc will not spin at all because the magnetic field is expelled. Can someone clear this up for me?

 

[jbsteele]

ExplanationThe answer provided by your teacher is correct. When a superconducting disc is cooled below its critical temperature, it becomes a type-II superconductor and exhibits the Meissner effect. This effect states that the superconductor will expel any external magnetic field, resulting in no current being induced in the disc and thus no motion. Therefore, the disc will not spin when the magnet begins to spin.

 

[tomcue]

I would like to clarify the answer to this question. The behavior of the superconducting disc in this scenario is a result of both Lenz's law and the Meissner effect.

Lenz's law states that when a magnetic field is applied to a superconductor, a current will be induced in the superconductor that creates a magnetic field that opposes the applied field. This is what causes the disc to spin in the same direction as the magnet.

However, the Meissner effect also plays a role in this scenario. The Meissner effect is the complete expulsion of magnetic fields from the interior of a superconductor when it is cooled below its critical temperature. This means that the magnetic field from the spinning magnet will not penetrate the superconductor and therefore, the disc will not spin.

In this scenario, the Meissner effect will dominate and the disc will not spin. However, if the temperature of the superconductor is raised above its critical temperature, the Meissner effect will no longer be present and the disc will spin in the same direction as the magnet due to Lenz's law.

I hope this clarifies the role of both Lenz's law and the Meissner effect in the behavior of a superconducting disc in this scenario.