Eddy Currents

When large pieces of conductors are placed in changing magnetic flux, small circulating currents are induced in them. These currents are called eddy currents because they swirl around in the conductor like eddies in water. This effect was discovered by physicist Foucault (1819-1868).

Magnetic flux linked with the plate keeps on changing as the plate moves into and out of the magnetic field. The eddy currents are induces in the plate due to flux change. Directions of eddy currents are opposite when the plate swings into the region between the poles and when it swings out of the region. See figure 11.

These currents are due to the motion of the free charges that are caused to move by the changing magnetic field.

Eddy currents are usually undesirable since they cause the conductor to heat up if they are excessive and dissipate electrical energy in the form of heat. Eddy currents are minimized by using laminated strips of metal, separated by insulating layers like lacquer. This prevents eddy currents from travelling across the laminations. The plane of the laminations is kept parallel to the magnetic field, so that they cut across the eddy current paths.

Advantages of eddy currents :

1. Electromagnetic damping : When a metallic frame or plate oscillates in a magnetic field (e.g. in a dead-beat galvanometer), the eddy currents generated in the frame or plate oppose the motion and bring the frame to rest as the oscillations die out quickly. This is called electromagnetic damping.


2. Magnetic braking : This is provided in electric trains by placing strong electromagnets above the rails. During braking, the metal wheels are exposed to a magnetic field from the electromagnets, generating eddy currents in the wheels. The magnetic interaction between the applied field and the eddy currents acts to slow the wheels down. The faster the wheels are spinning, the stronger the effect, meaning that as the train slows the braking force is reduced, producing a smooth stopping motion.


3. Electric power meters : The analogue electric power meter (presently most of the electric meters are digital) has a shiny metal disc. This disc rotates due to the eddy currents which are induced in the disc by varying magnetic fields produced by sinusoidally varying currents in a coil.


4. Induction furnace : In an induction furnace, high frequency alternating current is passed through a coil which surrounds the metals to be melted. The eddy currents generated in the metals produce high temperatures sufficient to melt it.

CBSE Electromagnetic Induction ( With Hint / Solution)
Class XII (By Mr. Ashis Kumar Satapathy)
email - [email protected]

Electromagnetic Induction