e.m.f.

e.m.f. stands for electromotive force, given the symbol E and has the SI units of Volts, V.

e.m.f. however, is not a force but an increase in potential (energy). Cells, batteries, dynamos, solar pannels etc produce a potential difference that can induce current. This potential differnce requires a different name from the voltage across a resistor. A resistor causes a decrease in potential as energy leaves the circuit normally in the form of heat.

Definition

The e.m.f. of a source (a device which supplies electrical energy) is defined as the energy converted to electrical energy when unit of charge (i.e. 1 coulomb) passes through it.

Terminal potential difference

Definition

The terminal p.d. of a source is the potential difference across its terminals, it is often given the symbol V.

Internal resistance

While a cell gives electrons energy to flow and form a current, some of that energy is used up by movement in the cell itself. This is why batterries become warm after extented use. The cell itself has a resistance which the electrons have to overcome, this is called the internal resistance and is given the symbol r or R_internal, and has the SI units of ohms.

The result of this is that as more current is drawn the terminal p.d. will drop. We model this by thinking of a cell as a source of e.m.f. in series with a resistor whose resistance is the internal resistance.

When there is no current flowing then the terminal p.d. is the same as the e.m.f. as there is no potential drop over the internal resistance (using V_internal=IR_internal when I=0).

When current flows however energy (potential) is lost overcoming internal resistance and the terminal voltage decreases. The e.m.f., internal resistance and current drawn can be linked by the following equation:

V = E - IR_internal

The electric potential difference (V) is the work done per unit charge as a charge is moved be- tween two points in an electric field.