Required practical 15 · Physics · Physics Paper 1, both tiers
Resistance of a wire and of resistor combinations
Investigate how the length of a wire affects its resistance, and the resistance of resistors in series and in parallel.
The exam trap. Keep the current small so the wire stays cool. Convert mA to A before R = V ÷ I.
Variables
- Independent (you change)
- Length of wire (or the arrangement of resistors)
- Dependent (you measure)
- Resistance, from R = V ÷ I
- Control (you keep the same)
- Material and diameter of the wire
- Temperature of the wire (small current, switch off between readings)
- The same meters and supply
Apparatus
- Constantan wire on a metre rule
- Crocodile clips
- Ammeter and voltmeter
- Low-voltage d.c. supply and switch
- Resistors for series and parallel tests
Method, and why each step matters
Set up the ammeter in series and the voltmeter in parallel with the length of wire being tested.
Why An ammeter measures the current through; a voltmeter the potential difference across.
Attach one clip at 0 cm and the other at 10 cm. Close the switch, read V and I, then open it.
Why Switching off between readings stops the wire heating up.
Repeat for lengths up to 100 cm.
Why A range of lengths shows the pattern.
Calculate R = V ÷ I for each length and plot R against length.
Why A straight line through the origin shows direct proportion.
Measure the total resistance of two resistors in series, then in parallel.
Why Series resistance adds; parallel is less than the smallest.
Convert milliamps to amps before calculating.
Why The equation needs SI units.
Risks
| Hazard | How to reduce the risk |
|---|---|
| The wire can get hot. | Use a small current and switch off between readings. |
A worked set of results
| Length in cm | V in V | I in A | R in Ω |
|---|---|---|---|
| 20 | 0.24 | 0.40 | 0.6 |
| 40 | 0.48 | 0.40 | 1.2 |
| 60 | 0.72 | 0.40 | 1.8 |
| 80 | 0.96 | 0.40 | 2.4 |
| 100 | 1.20 | 0.40 | 3.0 |
- Resistance is directly proportional to length.
- A line that misses the origin shows a zero error, for example from the contact at the clip.
Mistakes that cost marks
Connecting the voltmeter in series.
Instead The voltmeter goes in parallel with the component.
Using milliamps directly in R = V ÷ I.
Instead Convert: 400 mA = 0.40 A.
Drawing correct symbols in a circuit that is not a complete loop.
Instead Check the circuit is a closed path from one terminal to the other.
Practice questions on this practical
- A student measures the resistance of a length of wire.In Circuit symbols, charge, current and resistance
- A student measures the resistance of different lengths of the same wire at the same temperature.In Circuit symbols, charge, current and resistance
- A student is given a battery, an ammeter, a voltmeter, a metre rule, crocodile clips and a long piece of constantan wire taped to the metre rule.In Circuit symbols, charge, current and resistance