HSC Physics/Core/Motors and Generators/Generating electricity/Notes/Eddy currents and applications

Applications of eddy currents

Induction cooktops

An induction cooktop has a coil wound on ferromagnetic material under it. A high frequency alternating current in this coil produces a rapidly changing magnetic field. Placing a magnetic-based leads to a change in magnetic flux, because the magnetic field B varies. From Faraday's Law, ${\mathcal {E}}={\frac {\Delta \phi }{\Delta t}},$ an emf (electromagnetic field) is induced. From Lenz's Law, eddy currents flow in the pan. The resistance in the pan to the oscillating AC current results in heat being produced directly in the base of the pan. Unlike gas cooking, the ceramic cooktop is not heated directly. This makes induction cookers much more energy efficient than traditional cookers.

The direction of the eddy current is determined by Lenz's Law. In addition to efficiency, other benefits include safety (because the cooktop is not heated directly) and speed of cooking.

Electromagnetic braking

When a rotating metal disc passes through a magnetic field, there is a change in magnetic flux. From Faraday's Law, an emf is induced. From Lenz's Law, this induces eddy currents whose magnetic field interacts with the external magnetic field in a way that causes in magnetic flux in the disk. That is, the currents inside the magnetic field experience a force that acts in the opposite direction to the motion of the disc. Therefore, the speed of the disc's motion slows down.

One benefit of electromagnetic braking is that the braking is smooth. This is because the strength of the eddy currents is proportional to the speed of the disc. This means as the disc slows, eddy currents reduce. There is no physical contact between the disc and brake. Another benefit is that it is more effiecient at high speeds because the opposing magnetic forces is larger at higher speeds. These useful properties of electromagnetic brakes serve them well in many applications, such as amusement park ride brakes and brakes in some types of trains.