Science required practicals
How do you do the resistance required practical?
Tape a wire to a metre ruler and pass a small current through it. Use crocodile clips to connect different lengths, measure the current with an ammeter and the potential difference (p.d.) with a voltmeter, then work out R = V ÷ I. Resistance against length gives a straight line: a longer wire has more resistance. The second part shows that resistors in series add up, while two in parallel have less resistance than either one alone.
Last updated 23 September 2026 · Written and fact-checked by the GCSEwiz team
What does the resistance practical investigate?
It looks at two things that change the resistance in a circuit. The first is the length of a wire: you measure the resistance of longer and longer pieces of the same wire. The second is how resistors are joined: you compare two resistors in series with the same two in parallel.
The wire has to stay at a steady temperature, because a metal wire's resistance goes up as it warms. It is a required practical on AQA GCSE Physics and on AQA GCSE Combined Science: Trilogy.
What equipment do you need?
You need a simple circuit with an ammeter and a voltmeter in it, plus a length of resistance wire fixed to a ruler.
- A low-voltage power supply or battery pack, and a switch
- An ammeter and a voltmeter (or two multimeters)
- Connecting leads and two crocodile clips
- About a metre of thin resistance wire (constantan is common), taped along a metre ruler
- Two resistors for the series and parallel part
What is the method?
Part one measures the resistance of different lengths of the same wire. Part two measures two resistors on their own, then in series, then in parallel.
- Tape the wire along the metre ruler, keeping it straight and tight.
- Connect the power supply, switch, ammeter and wire in series, with a crocodile clip on the wire at the 0 cm mark and a second clip you can slide along it.
- Connect the voltmeter across the wire, to the two crocodile clips.
- Put the sliding clip at 10 cm. Close the switch, record the current and the p.d., then open the switch straight away.
- Move the clip to 20 cm, 30 cm and so on up to 100 cm, taking readings at each length.
- Repeat the whole set and work out a mean resistance for each length.
- For part two, take the wire out. Record V and I for each resistor on its own, then for the two in series, then for the two side by side in parallel.
What are the variables?
In part one you change the length of the wire and work out its resistance from the current and the p.d. Anything else that affects a wire's resistance stays fixed. In part two the thing you change is how the resistors are joined, and you measure the total resistance.
| Variable | In this practical |
|---|---|
| Independent variable | The length of wire between the crocodile clips |
| Dependent variable | The resistance of that length, worked out from V and I |
| Control variable | The material of the wire (use the same piece throughout) |
| Control variable | The thickness of the wire |
| Control variable | The temperature of the wire, kept steady with a small current and the switch opened between readings |
How do you work out and present the results?
Use R = V ÷ I, with the p.d. in volts, the current in amps and the resistance in ohms (Ω).
Worked example: with the clips 50 cm apart, the voltmeter reads 1.2 V and the ammeter reads 0.40 A, so R = 1.2 ÷ 0.40 = 3.0 Ω. Doubling the length to 100 cm should roughly double it, to about 6.0 Ω.
Plot resistance (y-axis) against length (x-axis) and draw a line of best fit. It should be a straight line through the origin, which shows resistance is directly proportional to length. The gradient is the resistance per centimetre (or per metre) of that wire. If the line crosses the resistance axis a little above zero, the extra usually comes from the connections at the crocodile clips: a systematic error that adds the same amount to every reading.
For two 10 Ω resistors in series, the readings might be 3.0 V and 0.15 A, so R = 3.0 ÷ 0.15 = 20 Ω, the two added together. In parallel the readings might be 3.0 V and 0.60 A, so R = 3.0 ÷ 0.60 = 5.0 Ω, less than either resistor on its own. In parallel the charge has more than one path, so more current flows for the same p.d.
How do you make it accurate and safe?
The thing to guard against is heating. A current warms the wire, and a warmer wire has more resistance, so keep the current small and open the switch as soon as you have each pair of readings. Measure each length from the point where the clip actually touches the wire, and keep the wire straight along the ruler.
On safety: a thin wire can get hot enough to burn if the current is too big, so use a low voltage and don't touch the wire while the switch is closed. Keep the two crocodile clips from touching each other, which would short out the wire and let a large current flow.
What do exam questions ask about it?
You might be asked to draw the circuit with the right symbols, or to plot the graph from a results table and say what it shows. Series and parallel questions usually ask you to compare the total resistances and explain the difference.
Here is one we wrote in the exam style: "Explain why the student opened the switch between readings. (2 marks)" A good answer links the current to the wire heating up, and the heating to a rise in resistance that would spoil the results.
Common mistakes
- Putting the voltmeter in series. It goes across the length of wire being tested.
- Working out I ÷ V. Resistance is the p.d. divided by the current.
- Leaving the switch closed between readings. The wire warms up, and later readings come out too high.
- Measuring the length from the end of the ruler. Measure between the two clips.
- Adding resistances in parallel. Only resistances in series add up; two in parallel have less resistance than either one.