Resistance of a Wire
Get to grips with Resistance of a Wire for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Investigate how the resistance of a wire depends on its length, keeping the current low to avoid heating effects.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
This required practical investigates how the length of a wire affects its resistance.
A length of resistance wire is connected in a circuit with an ammeter in series and a voltmeter in parallel across it. The current and potential difference are measured for different lengths, and the resistance is calculated using R = V ÷ I at each length.
The resistance is directly proportional to the length, giving a straight line through the origin. The circuit should be disconnected between readings, because current heats the wire and a hotter wire has a higher resistance, which would make the results invalid.
Revision notes
The method
Connect a length of resistance wire in a circuit with an ammeter in series and a voltmeter in parallel across the wire.
Use crocodile clips to vary the length. Record the current and potential difference at each length, and calculate resistance using R = V ÷ I.
The expected result
Resistance is directly proportional to length.
A graph of resistance against length is a straight line through the origin. Doubling the length doubles the resistance.
Avoiding heating
Disconnect the circuit between readings, or use a switch, so current flows only while measuring.
Current heats the wire, and a hotter wire has a higher resistance. If the wire is allowed to heat up, later readings would be too high and the results would be invalid.
Key points
- Resistance is directly proportional to length.
- The graph is a straight line through the origin.
- Calculate resistance using R = V ÷ I.
- The ammeter goes in series with the wire.
- The voltmeter goes in parallel across the wire.
- Disconnect between readings to avoid heating.
Worked examples
Example 1
A wire has a potential difference of 4.5 V across it and a current of 0.3 A through it. Calculate its resistance. [2 marks]
Model answer
Resistance = potential difference ÷ current = 4.5 ÷ 0.3 (1 mark) = 15 Ω (1 mark).
Example 2
Explain why the circuit should be disconnected between readings. [2 marks]
Model answer
Current flowing through the wire heats it up (1 mark), and a hotter wire has a higher resistance, so later readings would be too high and the results invalid (1 mark).
Example 3
Describe the relationship between the length of a wire and its resistance. [2 marks]
Model answer
Resistance is directly proportional to length (1 mark), so a graph of resistance against length is a straight line passing through the origin (1 mark).
Common mistakes
Leaving the circuit connected throughout.
The wire heats up, raising its resistance and invalidating later readings.
Connecting the voltmeter in series.
It must be in parallel across the wire.
Saying resistance is inversely proportional to length.
It is directly proportional — longer wire means more resistance.
Not calculating resistance from V and I.
The resistance must be calculated at each length using R = V ÷ I.
Exam tips
- Say directly proportional and describe the graph.
- Explain the heating problem when asked about accuracy.
- Check ammeter in series and voltmeter in parallel.
- Calculate resistance separately at each length.
Key terms
- Resistance wire
- Wire used to investigate resistance, often nichrome.
- Directly proportional
- Increasing at a constant ratio, giving a line through the origin.
- Crocodile clip
- A connector used to vary the length of wire in the circuit.
- Invalid
- Not measuring what was intended, due to an uncontrolled variable.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.