An electric current is a flow of free electrons through a conductor. The size of current flowing through a conductor for a given applied voltage depends on:
• The number of free electrons.
• The opposition to free movement of the electrons caused by the structure of the material.
These two factors taken together give an effective opposition to current flow which is called resistance. To simplify matters it is usual to ignore the second factor and equate good conductors to a large number of free electrons and poor conductors to fewer free electrons. Hence, a good conductor is a material which has low resistance, i.e. a large number of free electrons, and allows a large current to flow. Conversely a poor conductor has a high resistance, i.e. few free electrons and allows only a small current to flow for the same applied voltage. Because the value of the current flowing is determined by the resistance in the circuit, current flow can be controlled by varying the resistance.
Even the best conductors have resistance.
FACTORS AFFECTING RESISTANCE
The four factors that affect the resistance of a wire conductor are:
• Material. Some materials conduct better than others.
• Length ( ). Resistance is directly proportional to length, thus if the length is doubled (other factors remaining constant), resistance is doubled.
• Cross Sectional Area (A). Resistance is inversely proportional to A. Thus if the cross sectional area is doubled, resistance is halved.
• Temperature. Temperature affects the number of free electrons and hence resistance.
UNITS OF RESISTANCE
Resistance is measured in ohms, symbol ω(omega). The resistance of a piece of material is one ohm if a potential difference of one volt applied across it causes a current of one ampere to flow.