# Eddy Current Demo

### Lab Demonstration #30

* When the switch is turned on, the disk will begin to rotate
* A magnet will be moved toward the disk when a suitably impressive speed has been reached (this thing will go 3000 rpm if you are careful!)
* The rotation will be rapidly reduced toward zero.

Further information on Theory, Apparatus, Procedure, and Helpful Hints is available. The equipment for this demo is stored in Rockefeller room 302.

The purpose of this experiment is to observe the resisting force of the eddy current that appears in the disk to oppose the motion.

### Theory

When metal moves through a spatially varying magnetic field, or is located in a changing magnetic field, induced currents begin to circulate through the metal. These currents are called eddy currents because of their similarity to eddies in a flowing stream. In the case of the eddy current brake, a rotating disk has a magnetic field passing through it perpendicularly, but it is only strong in the area where the magnet is. The currents in that area experience a side thrust, which opposes the rotation of the disk. This interaction of field and current results in the "braking" of the disk, and thus the name "eddy current brake." The return currents close via parts of the disk where the field is weak, so there is a drag force only in the "generating" region.

It can be pointed out to advanced students that : (A) this effect is very hard to calculate for common magnetic field and disc geometries; (B) if the disc is slitted radially over the region covered by the magnet, the calculation becomes much easier, since the currents then must close through the rim and hub, whose resistance can be estimated; and (C) if the magnetic field is uniform everywhere in the disc, you have an open-circuited Faraday generator, and an electrostatic field builds up to cancel the v X b force, so no current flows, and there would be no braking effect.

### Apparatus

The following equipment is needed for this demonstration:
* an aluminum-disc eddy current brake + motor -- available in Rockefeller room 302, cabinet D
* a large, strong magnet (three suitable magnets are also available in room 302, cabinet D)

### Procedure

After turning on the eddy current brake, allow the disk to spin for a while to build up to a fair velocity. THen bring up a magnet to the disk. The disk's spinning will begin to decelerate, and will eventually come to a halt because of the induction of eddy currents by the magnetic field. If the magnet is not strong enough, the eddy currents that appear in opposition to the motion of the disk will not have an effect on its motion. The magnetron magnets with 4 cm pole pieces are quite satisfactory, and the "monster magnet" in Cabinet D gives spectacular results if you are astute enough (or strong enough) to get the keeper off.