Eddy Currents

The induced circulating currents produced in a metal itself due to change in magnetic flux linked with the metal are called eddy currents.

When a metallic piece is placed in a changing magnetic field, the induced currents are set-up in the metal piece which is eddy currents. The direction of eddy currents is given by Lenz's law.

Drawbacks of Eddy Currents

1. The production of eddy currents in a metallic block leads to the loss of electrical energy in the form of heat.
2. The heat produced due to eddy currents break the insulation used in the electrical machine.
3. Eddy currents may cause unwanted dampening effect.

Application of Eddy Current

1. Induction Furnace
   It is based on the heating effect of eddy currents. The metallic block to be melt is placed in a high frequency changing magnetic field. Strong eddy current is produced in a block. A large amount of heat is produced in the block due to high resistance of the metal. The metallic block melts due to heat. So the induction furnace is used to separate metals from their ores and to make some alloys.
2. Diathermy
   Eddy currents are used for the localized heating of tissues in the human body which is called diathermy.
3. Speedometer
   Speedometer is a device used to measure the instantaneous speed of vehicle. A small magnet is attached to the axle of wheel of the speedometer. Due to the rotation of the magnet magnetic flux linked with the aluminium drum changes and hence eddy currents are produced in it. A pointer attached to the drum is deflected in the direction of the rotation of the drum. This speed is measured which corresponds to the deflection of the pointer.
4. Eddy current damping or dead beat galvanometer
   According to Lenz's law, eddy currents always flow in such a direction as to oppose the motion which has produced them. So they can reduce the oscillations of a vibrating system. The coil of galvanometer is wound on a metal frame. As the coil swings in the magnetic field of the instrument, eddy currents are induced in the frame. These eddy currents oppose the motion of the coil and hence the pointer attached to it. The pointer quickly attains the final position without overshooting or oscillating violently. Thus eddy currents reduce the oscillations of the pointer.
5. Electromagnetic brakes
   Eddy current breaking can be used to control the speed of electric trains. In order to reduce the speed of train, an electromagnet is turned on that applies its field to the wheels. Large eddy currents are set up which produce the retarding effect.
6. Energy meters
   Energy meters uses the concept of eddy current to record the consumption of electricity.

Reference

Manu Kumar Khatry, Manoj Kumar Thapa etal.Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.