Electric potential (Voltage)

Voltage is electric potential energy per unit charge, measured in joules per coulomb ( = volts). It is often referred to as "electric potential", which then must be distinguished from electric potential energy by noting that the "potential" is a "per-unit-charge" quantity. Like mechanical potential energ, the zero of potential can be chosen at any point, so the difference in voltage is the quantity which is physically meaningful. The difference in voltage measured when moving from point A to point B is equal to the work which would have to be done, per unit charge, against the electric field to move the charge from A to B.

Electric potential may be conceived of as "electric pressure". Where this "pressure" is uniform, no current flows and nothing happens. This is similar to why people do not feel normal atmospheric air pressure: there is no difference between the pressure inside the body and outside, so nothing is felt. However, where this electrical pressure varies, it produces an electric field, which will create a force on charged particles.

Mathematically, it is the potential ? (a scalar field) associated with the conservative electric field () that occurs when the magnetic field is time invariant (so that from Faraday's law of induction).

Like any potential function, only the potential difference (voltage) between two points is physically meaningful, since any constant can be added to ? without affecting .

The electric potential is therefore measured in units of energy per unit of electric charge. In SI units, this is:

The electric potential can also be generalized to handle situations with time-varying potential fields, in which case the electric field is not conservative and a potential function cannot be defined everywhere in space. There, an effective potential drop is included, associated with the inductance of the circuit. This generalized potential difference is also called the electromotive force (emf).