Resistance and Ohm's Law — AQA GCSE Physics
Resistance opposes the flow of current. For some components, current and potential difference are directly proportional.
Ohm's law
$$V = I \times R$$
- V = potential difference (V), I = current (A), R = resistance (Ω).
Rearrange to find resistance (R = V/I) or current (I = V/R).
Ohmic conductors
For an ohmic conductor (such as a fixed resistor or a wire) at a constant temperature, the current is directly proportional to the potential difference. This gives a straight-line graph through the origin on an I–V graph, and the resistance stays constant.
Required practical
You can investigate how the resistance of a wire depends on its length: as the length increases, the resistance increases (resistance is proportional to length). You set up a circuit with an ammeter and voltmeter and calculate R = V/I for different lengths.
Factors affecting resistance
- Length — longer wire, more resistance.
- Temperature — for many components, resistance increases as temperature rises.
Exam tips
- Learn V = IR and practise rearranging.
- For an ohmic conductor at constant temperature, I ∝ V (straight line through origin).
- Resistance of a wire increases with length.
- In the required practical, calculate R = V ÷ I.