physics//electromagnetism//electromagnetic induction//eddy current
An eddy current is a loop of current induced inside a solid piece of conductor by a magnetic field that changes through it, and engineers either use it or fight it: it heats the pan on an induction hob, brakes trains and roller coasters without contact and finds coins under the ground, and it is the wasted heat that every transformer and motor core is built to avoid. Foucault currents is the same thing under the name of the physicist who studied them.
An eddy current is a loop of current induced inside a solid piece of conductor by a magnetic field that changes through it, and engineers either use it or fight it: it heats the pan on an induction hob, brakes trains and roller coasters without contact and finds coins under the ground, and it is the wasted heat that every transformer and motor core is built to avoid. Foucault currents is the same thing under the name of the physicist who studied them.
The mechanism is electromagnetic induction applied to a block instead of a wire. Faraday's law holds for any closed path, and a sheet of metal is full of closed paths. When the flux through the metal changes (a magnet sweeping past it, an alternating current in a coil beside it), a voltage appears around each of those paths, and since the metal conducts, current flows in whirls inside it. Lenz's law gives their direction: their own magnetic field opposes the change that made them. Two things follow at once. The current flows through the metal's resistance, so its energy becomes heat (Joule heating); and its field pushes back on whatever caused the change.
An eddy current turns a change of field into heat and a force against the change.
Every application wants one of those two effects, and every machine that does not want them pays for them in losses.
The force is a brake that needs motion. A magnet dropped down a copper pipe falls slowly although copper is not magnetic: each ring of the pipe sees the flux change as the magnet passes and pushes back on it. Magnetic brakes on trains and roller coasters move a conducting fin or rail past magnets the same way, with no contact and no wear. The force grows with speed and vanishes at standstill, so such a brake slows a vehicle but cannot hold it still; a mechanical brake does the last part.
The heat is the induction hob. A coil under the glass carries alternating current at about 25 to 50 kHz, and the base of the pan is the conductor where the eddy currents flow, so the pan heats itself and the glass only warms from the pan. Iron and steel pans heat well because, at that frequency, the current crowds into a thin layer of a metal that already resists (the skin effect, strongest in magnetic metals); in aluminium or copper the current spreads deeper through a far better conductor and makes little heat, which is why those pans need a steel plate in the base.
Cores are laminated to stop them. The alternating flux in a solid iron core would drive large eddy currents across its whole cross-section and heat it. A transformer core is instead a stack of thin sheets of silicon steel, typically 0.23 to 0.35 mm, insulated from each other: each sheet cuts the loops short, the eddy loss falls roughly with the square of the sheet thickness, and the silicon raises the steel's resistivity. Motor and generator cores are built the same way, and at high frequency cores are made of ferrite, a ceramic that barely conducts.
The field they make can be read. A metal detector's coil sends a changing field into the ground; a buried coin carries eddy currents whose own field reaches a receiving coil (or, in a pulse-induction detector, makes the coil's field die away more slowly after each pulse). Eddy-current testing uses the same effect to find cracks in aircraft parts and pipes, because a crack cuts the paths the currents would follow.