How can you quickly slow down a spinning wheel without touching it? With a magnet of course! A changing magnetic field generates temporary electric current loops in conducting metals. These current loops generate their own magnetic field, even if the metal itself is not magnetic. The magnetic fields from these induced eddy currents alternately repel and attract the external magnet in such a way as to slow down the spinning wheel-as if by magic! Once the wheel stops spinning, it is no longer magnetic. The principle is easy to demonstrate—simply spin the metal disk like a top and then hold the magnet over the rim. The disk will instantly stop rotating. Alternatively, a Bracken's Demonstration Spinner, available separately, may be used to show the continuous stopping power of the eddy currents. Detailed instructions, background information, and student worksheet are provided.
Concepts: Electromagnetism, Eddy currents, Lenz's law.
Time Required: 15 minutes
Materials Provided: Metal disk, plastic lid, neodymium magnet, screw. All the materials are completely reusable! The Bracken's Demonstration Spinner is a recommended accessory but is not required for the demonstration.