Explain why an ohmic conductor has a constant resistance, and describe how the current through it changes when the potential difference across it is doubled.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student investigates a length of resistance wire kept at a constant temperature. She measures the current through the wire for several different values of potential difference.
Model answer (4 marks)
An ohmic conductor has a constant resistance because its temperature is kept constant, so the number of charge carriers and their scattering rate do not change.
For an ohmic conductor at constant temperature, the current is directly proportional to the potential difference.
If the potential difference across the wire is doubled, the current also doubles.
This follows from V = I R; with R constant, V ∝ I, so doubling V doubles I.
For an ohmic conductor at constant temperature, the current is directly proportional to the potential difference.
If the potential difference across the wire is doubled, the current also doubles.
This follows from V = I R; with R constant, V ∝ I, so doubling V doubles I.
Examiner tips
- Use the word ‘directly proportional’ to show understanding of Ohm’s law.
- Mention that the temperature is constant to justify constant R.
- Show the simple algebra V = IR to link V and I.
- Keep the answer concise – 4 marks only.
Common mistakes
- Failing to state that the temperature is constant, so R is constant.
- Confusing the relationship and saying current is inversely proportional to potential difference.
- Not showing the V = IR step or the algebraic reasoning.
Mark scheme (4 marks)
- An ohmic conductor has a constant resistance because its temperature does not change (is kept constant)
- Current is directly proportional to potential difference (for an ohmic conductor at constant temperature)
- When potential difference is doubled, the current also doubles
- This follows from V = IR; since R is constant, doubling V must double I
Key terms in this question
ohmic conductor · potential difference · resistance · current
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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