- Electromotive force (e.m.f.) is the force that causes a current of electricity to flow.

- The potential difference (p.d.) V, between two points in a circuit is the electrical pressure r voltage requited to drive the current between them.

- The volt is a unit of potential difference and electromotive force. It is defined as the difference of potential across a resistance of 1 ohm crying a current of 1 ampere.

**Electron volt :**

Electron volt is a unit in terms of which the energies of atomic particles are expressed. It is the work done when an electron, whose charge is e coulombs, is moved in an electric field through a potential difference of 1 volt against the force (newtons) acting on the charge.

Thus 1 electron volt = 1.6021 x 10^{-19} J.