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Investigatory Project on Eddy Current


INVESTIGATORY PROJECT ON EDDY CURRENT (also called Foucault's currents)

Electromagnetic induction takes place not only in circuits, but also in pieces of metals. Whenever the magnetic flux connected with a part of metal changes an induced current, and that current is called eddy current, will circulate inside it. Eddy current flows throughout the whole volume of the metal by forming different closed paths.

Since the resistance of the metal is small, eddy current is appreciable. It heats the metal and so is a waste of electrical energy. Eddy currents are undesirable in most of the electrical machineries like transformer, induction coil, choke coil etc.

Eddy current can be minimised by increasing the resistance of the metal. That is why laminated soft iron cores are used in transformers, electric generators, motors, chokes etc and cores made up of bundles of soft iron rods are used in induction coils. The direction of the eddy current is given by Lenz's rule.

Experiments to demonstrate the force exerted by eddy current

Experiment 1:

A light flat metallic disc is placed on the top of a cylindrical electromagnet. When the current is switched on, the flux linked with the disc changes from zero to maximum and, hence, eddy current is produced in the disc. If the upper end of the electro-magnet becomes north pole, the lower side of the disc acquires north polarity. The disc is seen to be thrown up because of the magnetic repulsive force, as the electromagnet is switched on.

Experiment 2: Waltenhofen's pendulum

The apparatus comprises of pendulum having a flat copper plate as its bob. It is set so that it suspends amid the poles of an electromagnet. So long as the magnetic flux linked with the pendulum does not change, the pendulum swings liberally. But as soon as the electro-magnet is switched on, the oscillations of the pendulum are closely damped and die away quickly. Since the pendulum swings up and down, the magnetic flux linked with it changes and consequently eddy currents are produced inside the copper plate, which in accordance with Lenz's law, oppose the motion of pendulum. Hence the pendulum comes to rest very quickly. If a number of slots are cut in the copper plate, eddy currents in the plate become much smaller compared to the former case as the length of the closed path of the current and hence the resistance of the path increases. So the damping is reduced and the pendulum comes to rest only slowly.

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