1 Road Safety And Automobile Association Patrol In Great Britain

RESISTANCE AND OHM'S LAW
The resistance of a conductor or of a circuit is a very important quantity. It indicates how much the conductor or circuit resists a flow of electrons, that is, how much it resists a current.
The potential difference between the ends of a circuit divided by the current through the circuit gives the resistance of the circuit. The definition is:



The unit of resistance is the ohm.
The physicist George Ohm discovered in 1826 that if the temperature of a metal conductor is kept constant, then potential difference/current is a constant. By our definition of resistance, the conductor has a resistance of 18£2. It can be proved by experiment that, if the potential difference is doubled, the current is doubled. If the potential difference increases by ten times, the current increases by ten times. The current is proportional to the potential difference. This is the form in which George Ohm expressed his law: 'Where the temperature of a metallic conductor remains constant, the current through it is proportional to the potential difference between its ends.' The temperature is mentioned because it is important. If the temperature of the conductor changes, then V/^{1 r} is not constant. For most metal conductors, if the temperature rises the resistance rises, and if the temperature falls the resistance falls. This is very important in electrical machinery. Very many electrical machines have coils of copper wire in their components.
A current in the wire causes its temperature to rise, and so the resistance of the wire rises. Designers of electrical machinery have to work out what the change in resistance will be when the machine heats up, and they have to allow for it. Figure 1 shows some electrical conductors, which are each 1 km long. They are of different cross-section areas, and this affects their electrical resistance. A copper conductor 1 km long of cross-section area 1mm² has a resistance of 18£i. A 1 km copper conductor of cross-section 2 mm² has a resistance of 9ii. Doubling the cross-section area halves the resistance. A copper conductor 1 km long of cross-section area four times the first conductor, that is, 4 mm², has a resistance of 4.5S2, that is V* of the resistance of the first conductor. Similarly, the copper conductor with ten times the cross-section area of the first has only 1/10 of the resistance of the first. If the cross-section area of a conductor is increased, the resistance is decreased in proportion. In other words, the resistance of a conductor is inversely proportional to its cross-section area.

Topic # 9