A student has two identical-looking steel rods. One rod is a permanent magnet and the other is unmagnetised. The student is only allowed to use the two rods themselves to find out which one is the permanent magnet. Explain how the student can identify the permanent magnet.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. Hold one rod still and bring an end of the other rod close to each end of the first rod in turn.
2. If the two rods attract at both ends, at least one is a magnet.
3. To find which one is the permanent magnet, test the middle of one rod against the end of the other.
4. The rod whose end repels the end of the other rod (or whose end attracts the middle of the other rod) is the permanent magnet.
2. If the two rods attract at both ends, at least one is a magnet.
3. To find which one is the permanent magnet, test the middle of one rod against the end of the other.
4. The rod whose end repels the end of the other rod (or whose end attracts the middle of the other rod) is the permanent magnet.
Examiner tips
- Use the command word ‘Explain’ to describe the step‑by‑step procedure, not just state the result.
- Show the reasoning that attraction at the middle indicates a permanent magnet.
- Mention that the unmagnetised rod is attracted to the magnet but never repels.
- Keep the answer concise – 4 points only.
Mark scheme (4 marks)
- Hold one rod still and bring an end of the second rod close to each end of the first rod in turn (or a described systematic testing method)
- If the rods attract at both ends (or repel at one end), at least one is a magnet; but to distinguish them, test the middle of one rod against the end of the other
- The unmagnetised rod will always attract the permanent magnet (because it becomes induced/temporarily magnetised), so attraction alone does not identify the permanent magnet
- The permanent magnet is the rod whose end repels the end of the other rod, OR the permanent magnet is the one that attracts the middle of the other rod (whereas a non-magnet's middle shows no attraction to either rod)
Key terms in this question
permanent magnet · unmagnetised
Related
- All WJEC GCSE Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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