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Faraday's law of induction describes a basic law of electromagnetism, which is involved in the working of transformers, inductors, and many forms of electrical generators. The law states:

The induced electromotive force or EMF in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.

Electromagnetic induction was discovered by Michael Faraday in 1831, and, independently and at the same time, by Joseph Henry. The above law is sometimes also stated as - The EMF generated is proportional to the rate at which flux is linked.

Faraday formulated the law quantitatively in the form:

$\mathcal{E} = - {{d\Phi_B} \over dt}$.

where

$\mathcal{E}$ is the electromotive force (EMF) in volts ΦB is the magnetic flux through the circuit (in webers).

The direction of the electromotive force (the negative sign in the above formula) is given by Lenz's law. The meaning of "flux through the circuit" is elaborated upon in the examples below.

Traditionally, two different ways of changing the flux through a circuit are recognized. In the case of transformer EMF the idea is to alter the field itself, for example by changing the current originating the field (as in a transformer). In the case of motional EMF, the idea is to move all or part of the circuit through the magnetic field, for example, as in a homopolar generator.