Define Eddy Currents
Eddy currents are the currents induced in the bulk pieces of conductors when the amount of magnetic flux linked with the conductor changes.
However their flow patterns resemble swirling eddies in water. That is why they are called eddy currents. These were discovered by Foucault in the year 1895 and hence they are also called Foucault currents.
For example, when we move a metal plate out of magnetic field the conductor again induces a current in the conductor. The conduction electrons making up the induced current whirl about within the late as if they were caught in an eddy (or whirlpool) of water. This is called the eddy current.
The magnitude of eddy current is
I = induced e. m. f. / resistance = e / R
But e = - d ∅ / dt
∴ I = d ∅/ dt / R
The direction of eddy currents is given by Lenz's law, or Fleming's right hand rule.
Note that eddy currents are basically the currents induced in the body of a conductor due to change in magnetic flux linked with the conductor.
Experimental Demonstration
Experiment 1. Hold a light metallic disc D at top of the cross-section of an electromagnet connected to a source of a. c. when a.c. is switched on the disc is thrown up into the air. This is due to eddy current developed in the disc.
As current through the solenoid increases, the magnetic flux along the axis of the solenoid increases. Therefore magnetic flux linked with the disc increases. Induced currents or eddy currents develop in the disc and magnetize it. If upper end of solenoid initially acquires north polarity, the lower face of disc D also acquires north polarity in accordance with Lenz's law. The force of repulsion between the two throes the disc up in the air.
Experiment 2. Suspend a flat metallic plate between pole pieces N and S of an electromagnet.
When the magnetic field is off the metallic plate disturbed once form its equilibrium position the electromagnet is switched on the vibrations of the plate are damped and the plate stops vibrating sooner. This is because of eddy currents developed in the vibrating plate.
In the normal position of rest of the plate, magnetic flux linked with the plate is maximum. Then it is displaced towards any one extreme position, area of plate in the field decreases. Therefore magnetic flux lined with the plate which according to Lenz's law, oppose the decrease in flux and hence the motion of the plate towards extreme position. Similarly, when plate returns form extreme position to mean position, area of plate in the field increases magnetic flux lined with the plate increases. Eddy currents are developed which oppose the increases in flux and hence thaw motion of the plate towards the mean potion.
In either case, vibrations of the plate are opposed and hence damped.
When such a plate is made to oscillate in the magnetic feels the damping effect is there, but it is much smaller compared to the case when no slots were cut. This means eddy currents are reduced. This is because magnetic moments of the induced currents (which oppose the motion) depend upon the area enclosed by the currents (M = iA).
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