Eddy Current Brake

A spinning copper disc is slowed down when a magnet is brought near it. Note that copper is not a magnetic material. Eddy currents are induced in the disc due to the relative motion of the disc and the magnet. The magnetic fields associated with these currents have such a direction that they oppose the cause that created them in the first place (Lenz rule). As a result they oppose the relative motion between the disc and the magnet and hence the disc is slowed down.

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MIT 8.02 Electricity and Magnetism Lecture 18

MIT 8.02 Electricity and Magnetism, Spring 2002

Professor Walter Lewin

Displacement Current, Synchronous Motors, Induction Motors, Secret Top: How does it work?

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MIT Physics Demo -- Pendulum and Magnet

A solid copper pendulum is set into motion between the poles of an electromagnet. The magnets induce eddy currents in the copper which oppose the motion of the pendulum. The pendulum quickly slows to a stop.

When a copper pendulum with strips cut into it is swung between the same magnets, it is not slowed nearly as much as the solid pendulum. This is because the cuts in the copper prevent large eddy currents from forming.

Source:  MIT TechTV

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