Eddy
Currents
Faraday’s Law implies that a changing magnetic flux produces an induced electric field even in empty space.
If
a metal plate is inserted into this empty space the induced electric field
produces an electric currents in the metal. These induced currents are called
eddy currents.
If the induced currents are created by a changing magnetic field then the Eddy currents will be perpendicular to the magnetic field and flowing in circles if the B-field is uniform.
These
induced electric fields are very different from electrostatic electric fields
of point charges. For one thing they do not began and end on charges but circle
around on them self in loops. Secondy, and more importantly practically, the
induced electric field is nonconservative, i.e. the work done by generated
electric field can not be recovered except as heat.
A light bulb in a microwave QTMovie (1.3M) is an example of eddy current heating. All electromagnetic waves are oscillating electric and magnetic fields which are moving in space.
Eddy
currents are useful to dissipate unwanted energy like the swinging arm of
mechanical blance. Magnetics in the end of the balance support sets up eddy
currents in a metal plate attached to the end of the arm.
Eddy
currents can also be used as a effective braking force in rapid transit train
cars. Elecrtomagnets on the train near the train’s rails are turned on
creating eddy currents the rails. As an added side effect, the magnitude of the
dissipation of energy is a function of the speed of the train – the
change in flux – so that as the trains slows so does the retarding force, producing a smooth stop. Another advantage is that there are no pads to were out.
Eddy currents are undesirable in
transformers. A metal core is used in the transformer to increase the flux. Unfortunately, the eddy currents produced in the metal core increase the energy lost. By constructing the metal core of alternating layers of conducting and nonconducting materials, the size of the induced loops are reduced thereby reducing the energy lost. The buzz you often hear when you are near a transformer substation is the due to alternating layers vibrating against each other.
Transformer
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Yet another interesting use of the Eddy current drag force effect is found in the electric meter that measures the amount of electricity being used in your house. In the lower portion of each meter is a thin aluminum disk that is always revolving. That disk is moving in a magnetic field so that there is always an eddy current trying to slow the disk down. This makes the meter more accurate.
Eddy currents can kill you during a lighting storm. You do not have to be struck directly to be killed. If you actually are directly struck by a lighting bolt, you would be obliterated. Most people who die are usually only near the lighting strike. If you are standing next to, but not touching a metal pole that is struck by lighing the induced eddy currents in your body which is mostly salty water - good conductor - can also fry you as the sudden flow of charge (on the order of 10,000 Amps) rushes down the metal pole creates large induced magnetic field around it that is changing with time. It is this changing field which induces the eddy currents in your body.
