Eddy currents are induced currents that flow in closed, circular paths inside the body of a solid conductor (a metal plate, disc or core) when the magnetic flux through it changes. This happens when the conductor moves in a magnetic field, or when the magnetic field through it changes with time. They are also called Foucault currents.
- They swirl like eddies (whirlpools) in water, which gives them their name.
- By Lenz's law, they always flow in a direction that opposes the change in flux, so they oppose the motion of the conductor and produce heat in it.
Two applications:
- Electromagnetic brakes: in electric trains, a strong magnetic field is applied to a metal drum on the wheel axle. The eddy currents produced oppose the rotation and slow the train down smoothly.
- Induction furnace: a high-frequency alternating current in a coil produces large eddy currents in the metal kept inside it. The heat produced is enough to melt the metal, for example when making alloys.
Other applications you can also write: dead-beat (electromagnetic) damping in moving-coil galvanometers, induction cooktops, and speedometers of vehicles.
Note: eddy currents are unwanted in transformers and motors because they waste energy as heat. This is reduced by using a laminated core (thin insulated sheets instead of one solid block).