**Voltage** (denoted as *V*) is a measure of the potential energy difference per unit of charge. The SI unit for voltage is the volt, equal to one joule per coulomb. Voltage is related to resistance (*R*) and current (*I*) by the formula

and to power (*P*) by

For a point charge *q*, the voltage at a distance *r* is equal to

The electric field is the gradient of the voltage function.