30.8: Eddy Currents
Since eddy currents occur only in conductors, magnets can separate metals from other materials. For example, in a recycling center, trash is dumped in batches down a ramp, beneath which lies a powerful magnet. Conductors in the trash are slowed by eddy currents, while nonmetals in the trash move on, separating from the metals. This works for all metals, not just ferromagnetic ones.
Other major applications of eddy currents appear in metal detectors and the braking systems of trains and roller coasters. Portable metal detectors consist of a primary coil carrying an alternating current and a secondary coil in which a current is induced. An eddy current is induced in a piece of metal close to the detector, causing a change in the induced current within the secondary coil. This can trigger some sort of signal, such as a shrill noise.
Applying brakes using eddy currents is safer because factors such as rain do not affect the braking, and the braking is also smoother. However, eddy currents cannot bring the motion to a complete stop since the braking force produced decreases as speed is reduced. As a result, speed can be reduced, but another form of braking is needed to stop the vehicle completely. Generally, powerful rare-earth magnets, such as neodymium magnets, are used in roller coasters. Roller coasters have metal fins, normally containing copper, that pass through the magnetic field, slowing the vehicle down in much the same way as with a pendulum bob.
In addition, induction cooktops have electromagnets under their surface. The magnetic field varies rapidly, producing eddy currents in the pot's base, causing the pot and its contents to increase in temperature. Induction cooktops have high efficiencies and good response times, but the pot's base must be a conductor, such as iron or steel, for induction to work.
