Eddy Currents

What an eddy current is. Faraday's law does not care whether the changing flux threads through a neat, thin wire loop or through a solid block of conducting metal -- either way, changing flux induces an emf, and if there is conducting material available for current to flow through, a current results. Inside a solid conductor (an iron core, a copper plate, a metal sheet) exposed to a changing magnetic field, this induced current does not follow one single well-defined path the way current in a wire does; instead it circulates in large, roughly circular swirling loops throughout the bulk of the metal, called eddy currents (or Foucault currents), named after the eddies, or swirls, seen where water flows past an obstacle in a river.

Direction and effect. By Lenz's law, eddy currents always flow in whatever pattern opposes the change that induced them -- opposing relative motion between the conductor and the field (producing a retarding, braking force) or opposing a changing field in a stationary conductor (producing a field that resists the change). Because they flow through the comparatively low resistance of the bulk metal rather than a long thin wire, eddy currents can be very large in magnitude, and the $I^2R$ heating they produce can be substantial -- sometimes a useful effect (the table for this section lists several deliberate applications), and sometimes a wasteful one that engineers must actively minimise.

Minimising unwanted eddy currents: lamination. In devices such as transformers and the armatures of motors and generators, the iron core is deliberately exposed to a rapidly changing magnetic flux as part of its normal operation, and a SOLID iron core would develop large eddy currents, wasting energy as unwanted heat and reducing the device's efficiency. The standard remedy is LAMINATION: the core is built up not as one solid block but as a stack of thin sheets (laminae), each electrically insulated from its neighbours by a thin varnish or oxide coating. Confining each eddy current loop to the thin cross-section of a single lamina -- rather than letting it circulate freely through the full thickness of a solid block -- drastically increases the effective resistance the eddy current sees, cutting the eddy-current loss (which scales with the square of the lamina thickness) to a small fraction of what a solid core would waste, while barely disturbing the core's useful magnetic properties at all. …