The **electromotive force (e)** or e.m.f. is the energy provided by a cell or battery per coulomb of charge passing through it, it is **measured in volts (V).** It is equal to the potential difference across the terminals of the cell when no current is flowing.

- e = electromotive force in volts, V
- E = energy in joules, J
- Q = charge in coulombs, C

Batteries and cells have an **internal resistance (r)** which is **measures in ohm’s (W).**When electricity flows round a circuit the internal resistance of the cell itself resists the flow of current and so thermal (heat) energy is wasted in the cell itself.

- e = electromotive force in volts, V
- I = current in amperes, A
- R = resistance of the load in the circuit in ohms, W
- r = internal resistance of the cell in ohms, W

We can rearrange the above equation;

and then to