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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Eddy Current Brake from Horatiu Pop on Vimeo.