Current, resistance and potential difference

Resistance is how much a component opposes the flow of current. The current through a component depends on both the potential difference across it and its resistance. A bigger pd pushes more current; a bigger resistance lets less through.

You must recall this equation:

potential difference = current × resistance, or V = IR

Example: a 6 Ω resistor has a pd of 12 V across it. Find the current.

1. Rearrange: I = V ÷ R
2. I = 12 ÷ 6
3. I = 2 A

This practical shows how the length of a wire affects its resistance.

1. Tape a length of thin constantan wire along a metre ruler.
2. Connect it in series with a cell and an ammeter, using crocodile clips. Connect a voltmeter across the wire between the clips.
3. Set the clips 10 cm apart. Switch on, record the current and pd, then switch off.
4. Calculate the resistance with R = V ÷ I.
5. Repeat every 10 cm up to 100 cm.

The circuit, with the test wire drawn as a resistor:

Plot resistance against length. You get a straight line: resistance is directly proportional to length, so doubling the length doubles the resistance.

The same circuit can test resistors joined in series and in parallel.

• Independent variable: the length of the wire.
• Dependent variable: the resistance of the wire (worked out from V and I).
• Control variables: the material and thickness of the wire, and its temperature.

Hazard: the wire can get hot enough to burn you, so use a low pd and don't touch it while the current is on.

Improving accuracy: switch off between readings so the wire doesn't heat up, because a hotter wire has a higher resistance. Measure the length from the inside edges of the clips, and repeat each reading to find a mean.