Eddy current
A circulating current induced inside a bulk conductor by a changing magnetic flux. Eddy currents dissipate energy as heat, and Lenz's law ensures the force on the conductor always opposes the relative motion of source and conductor.
Definition
Eddy currents are the circulating induced currents that appear inside a solid piece of conductor whenever it is exposed to a changing magnetic flux. Unlike the currents in a wire-loop circuit, which flow along the one path the wire defines, eddy currents flow in whatever closed loops the bulk conductor's geometry allows — swirling patterns shaped by the field gradient, the conductor's shape, and the skin depth at the operating frequency. The name comes from the resemblance of these circulating patterns to the eddies in a stream.
By Lenz's law, the induced eddy currents always flow in whichever direction makes their own magnetic field oppose the flux change driving them. This turns eddy currents into the electromagnetic equivalent of viscous drag. Drop a strong magnet down a vertical copper pipe: as the magnet falls, it changes the flux through each horizontal slice of copper, inducing eddy currents in those slices; those currents' fields push back on the magnet, slowing its descent to a slow terminal velocity. Swing a copper plate between the poles of a strong electromagnet, and the plate comes to rest in seconds, its kinetic energy dissipated as resistive heating from the eddies. Roller-coaster brake fins exploit exactly this: no mechanical contact, no wear, no brake fade.
The same physics is a nuisance in transformers and motors. The iron cores of these machines sit in rapidly alternating fluxes; eddy currents induced in the bulk iron dissipate energy as heat, reducing efficiency and causing the core to overheat. Two standard fixes — used since the early 1900s — cut the eddy-current paths: either laminate the core (stack it from thin, insulated sheets) so that eddy-current loops can't extend perpendicular to the lamination plane, or use a high-resistivity ferrite rather than metallic iron, making even tiny eddy loops highly dissipative. In induction cooktops, the same effect is turned to a productive end: 20–50 kHz magnetic fields couple into iron-containing pans and dump 1–3 kW of eddy-current heating directly into the food.