**Ohm's law** states that resistance and current are proportional to voltage by the equation

- $ V = IR $

by dividing by area, this becomes

- $ \vec{\mathbf{J}} = \sigma \vec{\mathbf{E}} $

where **J** is current density, *σ* is conductivity, and **E** is the electric field strength in the wire.
For AC circuits, this becomes

- $ V_{rms} = I_{rms} \cdot Z $

with *Z* representing impedance. This is a more general form of Ohm's law.

Ohm's law is not always followed. All materials will experience an electrical breakdown (a spark) given a high enough voltage, and some materials, such as diodes, do not follow Ohm's law even at low voltages.