electromotive force

In physics, the energy supplied by a source of electric power in driving a unit charge around the circuit. The unit is the volt. The term is therefore used to describe the voltage produced by an electric battery or generator in an electrical circuit.

A difference in charge between two points in a material can be created when an external energy source such as a battery causes electrons to move so that there is an excess of electrons at one point and a deficiency at a second point. This difference in charge gives rise to electrical potential energy known as emf. It is the emf that causes a current to flow through a circuit.

When the source is connected in a circuit some of the energy it supplies will be lost in driving current across its own internal resistance, and so its terminal voltage (the potential difference across its terminals) will be less than its emf. If a source's terminal voltage is V volts, the current it supplies to a circuit I amperes, and its internal resistance r ohms, then its emf E can be expressed as: E = V + Ir or, where R is the total circuit resistance, as: E = I(R + r)