Electrical resistance is a measure of the opposition to the flow of electric current in a conductor. It is defined by the formula:

The ohm is the electrical resistance to a current of 1 A flowing through a conductor when a voltage of 1 V is applied across its ends.

If two wires, A and B have electrical resistances of 6 Ω and 12 Ω respectively, then

• A voltage of 6 V (each coulomb of charge must be given 61 of energy) is required to cause a current of 1 A to flow through A.
• A voltage of 12 V (each coulomb must be given 12 J) is required to cause a current of 1 A to flow through B.

RESISTORS

These are devices specially made to provide resistance. Placed in a simple circuit, they each reduce the current flowing. Meanwhile in more complex circuits found in radios and TVs, they are used to keep currents and p. d. s at levels needed for the proper functioning of the circuit components.

The rheostat or variable resistor provides variable resistance and so can be used to vary the current flowing in a circuit.

Resistances measure up to several million ohms and so higher units are commonly used.

Some units commonly used include the kilohm (k Ω) and the megohm (M Ω).

1 k Ω = 1000 Ω             and                     1M Ω = 1000 000 Ω

FACTORS AFFECTING THE RESISTANCE OF A CONDUCTOR

1. The length of the conductor: The longer the conductor, the greater the resistance it offers to current holding other factors constant. That is, resistance is directly proportional to the length of the conductor.

OHM'S LAW

In 1826, Georg Ohm carried out experiments with different metal wires and came out with an important relationship which is now known as Ohm's Law. The law states:

The current flowing across a metallic conductor is directly proportional to the p.d. across its ends, provided temperatures and other physical conditions remain constant.

• For materials like copper and other metallic conductors, the plot of I against V is a straight line. So they obey Ohm's Law and are called Ohmic conductors.
• For the filament lamp and semi-conductor devices, I against V do not give straight lines. So they do not obey Ohm's Law and are called non-ohmic conductors.

If a conductor obeys ohm's law, the resistance can be determined experimentally. A plot of voltage (y-axis) against current (x-axis) will give a straight line whose gradient is the resistance of the conductor.