Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Electromagnetic Damping

Apparatus

An aluminium plate, a magnet, support to hang the magnet

Procedure

Hang the magnet from the given support. Do not put the aluminium plate right now. Pull the magnet to one side and release. It oscillates like a pendulum. Watch it for about 10 oscillations. See if there is a significant damping of amplitude. Stop the magnet. Put the aluminium plate below the hanging magnet. The magnet should not touch the plate and the separation should be small, say 1~mm. Now pull the magnet to one side and release. Look at the oscillations. Can you watch it for 10 oscillations? Oscillations are very quickly damped.

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Discussion

As the magnet moves over the plate, magnetic field over the parts of the plate change in time. This changing magnetic field creates induced currents on the surface of the plate. These currents opposes the cause of the relative motion between the plate and the magnet. Thus the magnet slows down very quickly and almost stop in 2 or 3 oscillations.