A current produces magnetic flux, and changing that current can therefore induce an electromotive force. Inductance measures this coupling between current and magnetic flux.

For a linear inductor,

$$\lambda=LI,$$

where $\lambda$ is the flux linkage and $L$ is the inductance.

The SI unit is the henry:

$$1,\mathrm H=1,\mathrm{V,s/A}.$$

Self-induced voltage

Faraday's law gives the voltage associated with a changing current:

$$v_L=L\frac{dI}{dt}$$

for the passive sign convention.

The induced effect opposes changes in current, consistent with Lenz's law. An inductor therefore allows steady current ideally with zero voltage drop while resisting rapid changes in current.

Stored magnetic energy

An ideal linear inductor stores energy in its magnetic field:

$$U=rac12LI^2.$$

Geometry and materials

Inductance depends on conductor geometry and on the magnetic properties of the surrounding material. Coils and magnetic cores are used to increase flux linkage for a given current.

Inductance is not electrical resistance: an ideal inductor stores and returns energy rather than dissipating it.