Faraday’s law states that the induced voltage around a closed circuit equals the negative rate of change of magnetic flux through the circuit.
In equation form, the law is commonly written as ε = −dΦB/dt for a single loop, where ε is induced electromotive force and ΦB is magnetic flux. For a coil with many turns, the induced voltage is proportional to the number of turns. The minus sign expresses Lenz’s law: the induced effect opposes the change that produces it.
Michael Faraday established the experimental basis of the law in 1831. He showed that a changing current in one coil could induce a transient current in a nearby coil, and that moving a magnet relative to a coil could also produce an induced current.
Faraday’s law explains transformers, generators, induction cooktops, microphones, guitar pickups, and many wireless charging systems. A steady magnetic field through a stationary loop does not by itself induce voltage; the magnetic flux must change.