Introduction :

There are many experiments demonstrating Faraday's law of Electromagnetic Induction. whenever a conductor is placed in a varying magnetic foiled or it moves under a magnetic field, emf is induced. If there are conducting paths available, currents start in the conductor which we call Eddy current. This experiment is one nice way to demonstrate eddy currents. 

Keywords :

EM induction, eddy currents, motional emf.

Equipment :

An aluminum disc with an attachment to fit in the spindle of a tape motor, a magnet, a tape motor, an adapter and a power source.

Procedure:

Mount the disc on the spindle of the tape motor. Connect the motor to the power source through an adaptor. Switch on the power so that the tape motor along with the connected aluminum disc starts rotating. Soon it will pick up a good speed. Now bring a magnet very close to the rotating disc. A pole should face the disc surface. The disc gradually slows down to almost a halt. Take the magnet a bit away. The disc again picks up speed.

Discussion :

The free electrons of the disc also move with the disc. When the magnet is kept near the rotating aluminum disc, the free electrons of the aluminum disc below the magnet experience magnetic force causing a motional emf in the conductor. This produces eddy currents in the disc. Energy is consumed in these currents putting more load on the motor. So the disc slows down.