3.1 Resistance

Resistance

1. The resistance R of a material is defined as the ratio V : I, where V is the potential difference across the material and I is the current flowing in it. 
   
2. The SI unit of resistance is the ohm (W). One ohm is the resistance of a material through which a current of one ampere flows when a potential difference of one volt is maintained.

Finding Resistance from the Potential Difference – Current Graph

In the graph of potential difference against the current, the gradient of the graph is equal to the resistance of the resistor.

Example:

The figure above shows the graph of potential difference across a wire against its current. Find the resistance of the wire.

Answer:
Resistance

Ohmic Conductor

(Examples of non-Ohmic conductor)

1. Conductors that obey Ohm’s law are said to be the Ohmic conductor.
2. Examples of Ohmic conductor: Metal, Copper sulphate solution with copper electrodes

Non-Ohmic Conductor

1. Conductors which do not obey Ohm’s law are called non-ohmic conductor.
2. Example: Semiconductor Diode, Vacuum tube diode

Factors Affecting the Resistance 1

The resistance R of a given conductor depends on:

1. its length l,
2. its cross-sectional area A
3. its temperature and
4. the type of material.

Length

Resistance is directly proportional to the length of the conductor.

Cross-Sectional Area

Resistance is inversely proportional to the cross-sectional area of the conductor.

Temperature

A conductor with a higher temperature has higher resistance.

Material

Difference materials have difference resistivity. The resistance of copper wire is lower than iron wire.

Since resistance is directly proportional to the length and inversely proportional to the cross-sectional area of the conductor. If two resistors of the same material have the same temperature, we can relate the resistance of the two resistors by the following equation.