Law

Faraday's law

ε = - \frac{d Φ _{B}}{d t}

Statement

The induced emf around a circuit equals minus the rate of change of magnetic flux through it.

Symbols and units

$ε$	induced emf	V
$Φ_{B}$	magnetic flux	Wb

When it applies

Any closed circuit or loop, however the flux through it happens to be changing.

Don't use when

It is the change in flux that matters, not the flux itself — a large but constant field through a stationary loop induces nothing.

Derivation

An empirical law: whenever the magnetic flux linking a circuit changes, by any means, an emf appears around that circuit, proportional to the rate of change.

ε = - \frac{d Φ _{B}}{d t}

Watch out for

It is the CHANGE in flux that matters, not the flux itself — a large but constant field through a stationary loop induces nothing at all.