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

P=IV = \frac{V^2}{R}

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

V = -\frac{q}{4 \pi \epsilon_0 r}

The electric field is the gradient of the voltage function.