E.m.f. of a Source

E.m.f. of a Source

A battery gives energy to charge.

E.m.f. tells us how much energy each coulomb receives from the source.

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.

Source Role

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. vs P.d.

E.m.f. describes energy supplied per coulomb by a source.

Potential difference across a component describes energy transferred per coulomb by that component.

Terminal Voltage

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.

E.m.f. of a Source — Advanced

E.m.f. represents work done per unit charge by non-electrostatic processes inside the source.

For a real source delivering current, internal resistance causes a difference between e.m.f. and terminal voltage.