E.m.f. of A Source

The electromotive force (e.m.f.) of a source is the energy supplied by the source per unit charge passing through it. ε = WQ Unit: volt (V) Despite its name, e.m.f. is not a force.

A cell or battery converts another form of energy into electrical energy. For example, a chemical cell converts chemical energy into electrical energy. An e.m.f. of 1.5 V means the source supplies approximately 1.5 J of energy per coulomb.

E.m.f. describes energy supplied per coulomb by a source. Potential difference across a component describes energy transferred per coulomb by that component.

A real source may have internal resistance. When current flows, some energy is transferred inside the source, so terminal voltage can be less than its e.m.f. A simple model is: **V = ε − Ir** where `r` is internal resistance.