The law stating that an induced electromotive force opposes the change that produced it is Lenz’s law.
Lenz’s law determines the direction of an induced current or voltage. If magnetic flux through a circuit increases, the induced current creates a magnetic field that counters that increase; if the flux decreases, the induced field acts to oppose the decrease. This is the physical meaning of the minus sign in Faraday’s law, ε = −dΦB/dt.
The law was formulated by the Baltic German physicist Heinrich Lenz in 1834. It is a consequence of conservation of energy: if an induced current reinforced the original change, energy could appear without an input. Instead, work must be done to move a magnet toward a conducting loop or to change the current in a nearby coil.
Lenz’s law is often confused with Faraday’s law. Faraday’s law quantifies the magnitude of the induced electromotive force, while Lenz’s law specifies its opposing direction. It also explains magnetic braking and the direction of back electromotive force in inductors.