A changing magnetic flux through a bulk conductor can drive circulating currents inside the material even when there is no deliberately wound wire loop. These closed induced currents are called eddy currents.

The word eddy emphasizes that the current circulates locally through the conducting material, much like vortices circulating in a fluid.

How eddy currents arise

Imagine a conducting plate entering a region of magnetic field. As parts of the plate move into the field, the magnetic flux through possible closed paths inside the plate changes. Faraday's law therefore produces induced electromotive forces around those paths, and currents circulate through the conductor.

Lenz's law determines their direction: the induced currents create magnetic effects that oppose the change in flux that produced them.

If a conductor moves into a magnetic-field region, the induced currents oppose the increase of flux. If it moves out, their direction reverses so as to oppose the decrease.

Magnetic braking

The induced currents themselves experience magnetic forces. For a plate moving through a field, the net magnetic force typically opposes the plate's motion.

This creates magnetic braking without mechanical contact.

The faster the conductor moves, the more rapidly the flux changes and the stronger the induced currents tend to be. The braking force therefore often increases with speed over an appropriate operating range.

Energy accounting

Suppose a metal plate is allowed to swing like a pendulum through a strong magnetic field. Its motion decays much more rapidly when the plate crosses the field region.

The mechanical energy has not disappeared. Eddy currents flow through material with electrical resistance and dissipate energy as heat:

$$P=I^2R.$$

Thus the energy chain is

$$\text{mechanical energy} \longrightarrow \text{induced electrical current} \longrightarrow \text{thermal energy}.$$

Lenz's law is consistent with this energy transfer. If the induced force accelerated the conductor instead of opposing the flux-changing motion, the system could create mechanical energy without an energy source.

Reducing eddy currents

In devices such as transformer cores, eddy-current heating can be an unwanted loss. A common strategy is to build the magnetic core from thin electrically insulated laminations rather than one solid block.

The laminations interrupt large circulating current paths and increase their electrical resistance, reducing the magnitude of the eddy currents while still allowing magnetic flux through the core.

Using eddy currents deliberately

The same effect is useful when heating or drag is desired.

• Induction heating uses a time-varying magnetic field to generate currents that heat conducting material.
• Eddy-current brakes produce contactless damping in machinery and vehicles.
• Metal detection can sense how a conducting object modifies an applied alternating magnetic field through the currents induced in it.

Eddy currents are therefore not a new electromagnetic law. They are a spatially distributed consequence of Faraday induction, Lenz's law and electrical resistance inside conducting matter.