Topic:Current, Voltage and Resistance
The ability of an energy source (e.g a battery) to produce a current within a conductor is a measure of its electromotive force (e.m.f). Whenever an e.m.f is applied to a circuit a potential difference (p.d) exists. Both e.m.f and p.d are measured in volts (V). In many practical circuits there is only one e.m.f present (the battery or supply) however, a p.d. will be developed across each component in the circuit.
For any conductor, the current flowing is directly proportional to the e.m.f applied. The current flowing will also be dependent on the physical diamensions (length and cross-sectional area) and material of which the conductor is composed. The amount of current that will flow in a conductor when a given e.m.f is applied is inversely proportional to its resistance. Resistance, therefore, may be thought of a an opposition to current flow; the higher the resistance the lower the current that will flow (assuming that the applied e.m.f remains constant)