AQA GCSE Physics · Electricity
Resistance required practical
Investigate the factors that affect resistance: the length of a wire, and resistors in series and parallel.
You measure the current through a wire and the potential difference across it at different lengths. Resistance is directly proportional to length, so the graph is a straight line through the origin.
Equipment
- Resistance wire (e.g. constantan) taped to a metre ruler
- Two crocodile clips
- Ammeter and voltmeter
- Low-voltage power supply (or a cell) and switch
- Connecting leads
- Two identical resistors (for series and parallel)
Variables
- Independent you change
- Length of the wire
- Dependent you measure
- Resistance (from potential difference ÷ current)
- Control you keep the same
- Same wire (material and thickness)
- Low current, so the temperature stays constant
- Same power supply setting
Method
- 1
Set up the circuit: the ammeter in series with the wire, and the voltmeter in parallel across the length being tested.
- 2
Put one crocodile clip at 0 cm and the other at 20 cm.
- 3
Switch on, read the ammeter and voltmeter, then switch off straight away so the wire doesn’t heat up.
- 4
Repeat for 40, 60, 80 and 100 cm.
- 5
Calculate resistance R = V ÷ I for each length and plot R against length.
- 6
Then replace the wire with two resistors, first in series and then in parallel, and find the total resistance each time.
Worked example
Constantan wire, measured with the clips moved along a metre ruler.
| Length (cm) | p.d. (V) | Current (A) | Resistance (Ω) |
|---|---|---|---|
| 20 | 0.31 | 0.52 | 0.60 |
| 40 | 0.58 | 0.48 | 1.21 |
| 60 | 0.80 | 0.45 | 1.78 |
| 80 | 0.98 | 0.40 | 2.45 |
| 100 | 1.15 | 0.38 | 3.03 |
Working
R = V ÷ I
60 cm: 0.80 ÷ 0.45 = 1.78 Ω
Gradient ≈ 3.03 ÷ 100 = 0.03 Ω per cm
Conclusion
Resistance is directly proportional to length: the line is straight and passes through the origin, so doubling the length doubles the resistance. Two resistors in series give a larger total resistance; in parallel the total is smaller than either one.
Risks
- Wire can get hot at high currentUse a low voltage and switch off between readings.
- Short circuitCheck the circuit before switching on.
Common mistakes
- Connecting the voltmeter in series. It goes in parallel across the wire; the ammeter goes in series.
- Leaving the current on. The wire heats up, which increases its resistance and spoils a fair test.
- Measuring length from the end of the ruler instead of from the clip, which adds a zero error.
Better results
- Use a lower current or switch off between readings to keep the temperature constant.
- Repeat each length and calculate a mean resistance.
- Make sure the clips touch the wire at the exact marks.
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7 exam questions on this practical.
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