eddycurrent

Eddy currents are loops of electrical current induced within conductive materials by a changing magnetic field, as described by Faraday's law of induction. When a conductor experiences a time-varying magnetic flux, circulating currents are produced in planes perpendicular to the field. The currents generate magnetic fields that oppose the original change in flux (Lenz's law). Their magnitude depends on the rate of flux change, the material's conductivity, and the geometry of the conductor.

These currents decay as they dissipate energy through the material's resistance, converting some of the magnetic energy into heat. In magnetic devices such as transformers and electric machines, eddy currents cause unwanted power losses and heating, referred to as eddy current losses. To reduce them, engineers use laminated cores, thin sheets of high-resistivity insulation between layers, or materials with higher resistivity to confine currents to smaller paths or suppress them.

Applications include nondestructive testing (eddy current testing), where coils induced eddy currents on a surface and changes in impedance reveal cracks or flaws. Induction heating and induction cooking use eddy currents to heat metal directly. Eddy current braking provides contactless electromagnetic damping in trains and machinery. Wireless power transfer and metal detectors also rely on eddy currents in conductive targets or receiver coils.

Key parameters include skin depth, where the current concentrates near the surface at higher frequencies; delta = sqrt(2/(omega mu sigma)) approximates the depth at which current density falls to 1/e of its value. Eddy currents are influenced by frequency, conductivity, and magnetic permeability.