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EDDY CURRENTS

After reading this section you will be able to do the following:

• Explain what an Eddy Current is.
• Discuss the one requirement necessary for a current to be induced into an object.

In the discussion on the previous page you learned about electromagnetic induction. You learned that anytime a conductor was placed in a changing magnetic field that electrical current was generated in the conductor. We talked about the conductor being a piece of wire that is often wrapped into a coil, but the conductor does not need to be in the shape of a coil and does not even need to be wire. It could be a piece of flat steel, aluminum plate, or any other conductive object. The only requirement is that the object must be able to conduct electrical current.

When current is induced in a conductor such as the square piece of metal shown above, the induced current often flows in small circles that are strongest at the surface and penetrate a short distance into the material. These current flow patterns are thought to resemble eddies in a stream, which are the tornado looking swirls of the water that we sometimes see. Because of this presumed resemblance, the electrical currents were named eddy currents.

Uses of eddy currents

Just like in our transformer experiment, these induced eddy currents generate their own magnetic field. After all, this is an actual electrical current and any current flowing in a conductor produces a magnetic field, right? The detection and measurements of the strength of the magnetic fields produced by the eddy currents makes it possible for us to learn things about conductive materials without even contacting them. For example, the electrical conductivity of a material can be determined by the strength of the eddy currents that form. Also since cracks and other breaks in the surface of a material will prevent eddy currents from forming in that region of the surface, eddy currents can be used to detect cracks in materials. This is referred to as eddy current testing in the field of nondestructive testing (NDT). NDT technicians and engineers use eddy current testing to find cracks and other flaws in part of airplanes and other systems where bad things can happen if the part breaks. On the next page you will learn more about eddy current testing and be able to try an inspection yourself.

Review

1. Any electrically conductive object will conduct an induced current if it is placed in a changing magnetic field.
2. Eddy currents are circular induced currents.
3. Eddy currents generate their own magnetic fields.