Lenz’s law states that an induced current opposes the change in magnetic flux that caused it.
The law gives the direction of the induced current in electromagnetic induction. If a magnet’s north pole approaches a conducting loop, the induced current creates a magnetic field that repels the approaching pole. If the magnet moves away, the induced field reverses to oppose that decrease in flux.
Lenz’s law is represented by the minus sign in Faraday’s law of induction. That sign is essential: without opposition, induction would produce energy without an input, violating conservation of energy. The mechanical work needed to move a magnet through a coil becomes electrical energy and often heat.
The law is named after German physicist Heinrich Lenz, who formulated it in 1834. It applies broadly to conductive loops, generators, transformers, eddy currents, and magnetic braking systems.