### Resistance

- 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.
- 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.

### Ohmic Conductor

(Examples of non-Ohmic conductor) |

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

### Non-Ohmic Conductor

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

### Factors Affecting the Resistance 1

The resistance R of a given conductor depends on:

- its length l,
- its cross-sectional area A
- its temperature and
- 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.

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Regards

Ganpati-Wires