MIT Physics Demo -- Pendulum and Magnet
A solid copper pendulum is mounted between the poles of an electromagnet (solenoid). The pendulum is set into motion, and then the magnets are turned on. The magnets induce eddy currents in the copper which oppose the motion of the pendulum. The pendulum quickly slows to a stop, demonstrating an effect called eddy current braking. Eddy current brakes are widely used in trains and roller coasters.
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. Only eddy currents smaller than the strips of copper can be formed.
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