Eddy Current
Definition
Swirling loops of current induced inside chunks of metal by changing magnetic fields — useful in brakes, wasteful in transformers.
Technical definition
Circulating currents induced in bulk conductors by time-varying magnetic flux (Faraday's law). They dissipate energy as heat (I²R losses) — minimized in transformers via laminated cores — but are harnessed in induction heating and eddy-current brakes.
Example
Dropping a magnet through a copper pipe: eddy currents in the pipe oppose the fall, and the magnet drifts down slowly as if through syrup.
Common confusion
Eddy currents are not a flaw in the physics — they are Faraday's law working as designed. Engineers fight them in transformers (laminated cores) but exploit them in induction cooktops and magnetic brakes.