Induktionslagen

Induktionslagen, or induction laws, are a set of physical principles describing how changing magnetic fields induce electric phenomena. The best known are Faraday's law of electromagnetic induction and Lenz's law, which are unified in the Maxwell–Faraday equation ∇×E = −∂B/∂t.

Faraday's law states that the electromotive force (emf) around a closed circuit equals the negative rate of change of the magnetic flux through the circuit: ε = −dΦB/dt. The magnetic flux is ΦB = ∫ B · dA over the surface spanned by the circuit. A time-varying magnetic field or a moving loop changes ΦB and induces ε.

Lenz's law specifies the direction of the induced current such that the resulting magnetic field opposes the change in flux. This opposition is what enforces energy conservation and is encapsulated by the negative sign in Faraday's law.

In circuits and devices, inductance quantifies this effect. Self-inductance L relates ΦB to the current in the same circuit, ΦB = L I, and the induced emf is ε = −L dI/dt. Mutual inductance M describes coupling between circuits, with ε2 = −M dI1/dt. The time variation of current in one loop thus induces a voltage in another.

Applications include transformers, inductors, electric generators, and motors. Motional emf, ε = Bℓv, arises when a conductor moves in a magnetic field. The induction laws are often treated in the quasi-static regime, where displacement currents are neglected.