Eddy current: induced circulating currents in conductors and their effects

How they form and their characteristics

When a conductor experiences a time-varying magnetic field, an electric field is generated that drives charges in closed paths inside the material. The size and strength of eddy currents depend on factors including the conductor's electrical conductivity, thickness, magnetic permeability, and the rate (frequency) of change of the magnetic field. At high frequencies the currents concentrate near the surface of the conductor (skin effect), while slower changes produce broader loops. The magnetic field produced by these induced currents always acts to oppose the original change in magnetic flux.

History and underlying principles

The phenomenon follows from nineteenth-century work on electromagnetic induction by Michael Faraday and the direction rule given by Heinrich Lenz. Faraday's experiments demonstrated that changing magnetic environments generate electromotive forces in nearby conductors; Lenz's principle explains the opposing sense of the induced currents. The descriptive name "eddy" evokes swirling water eddies that circulate in response to an obstacle in a flow.

Common examples and practical uses

Simple demonstrations include dropping a magnet through a copper or aluminium tube: the magnet descends much more slowly than in air because eddy currents in the tube create a magnetic field that opposes the magnet's motion. In laboratory setups a solenoid with a soft iron core driven by alternating current (AC) will induce eddy currents in nearby metal objects, producing repulsive forces and heating.

• Useful applications: induction heating, eddy-current brakes and magnetic damping, metal detectors, and non-destructive testing of metal parts.
• Unwanted effects: energy losses in transformer and motor cores (eddy‑current losses) and heating in conducting structures exposed to changing fields.

Mitigation and engineering considerations

Engineers limit harmful eddy currents by interrupting the paths available for circulating currents. Common approaches include laminating magnetic cores (stacking thin insulated sheets), using high‑resistivity or ferrite materials, adding slots, or working at frequencies where skin depth is small. In contrast, systems that exploit eddy currents are designed to maximize conductive paths and field changes to achieve heating or braking as desired.

Notable distinctions: eddy currents are distinct from direct currents and from displacement currents; they are always induced by changing magnetic conditions and are constrained within the conducting body. Their dual role—as both a useful tool and a source of loss—makes understanding and controlling them important across electrical engineering, materials testing, and applied physics.