A student has two identical-looking metal rods. One rod is a permanent bar magnet and the other rod is made of unmagnetised iron. Without using any other equipment, the student can work out which rod is the magnet by using only the two rods. Explain how the student could do this, and explain the result they would observe to identify which rod is the magnet.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. Bring one rod’s end close to the middle of the other rod.
2. If the end is from the permanent magnet, the middle of the iron rod is attracted.
3. If the end is from the iron rod, the middle of the magnet shows little or no attraction.
4. Thus the rod whose centre does not attract the other rod’s end is the permanent magnet.
5. The result is that the magnet’s centre is non‑magnetic, whereas the iron rod’s centre is attracted to the magnet’s pole.
2. If the end is from the permanent magnet, the middle of the iron rod is attracted.
3. If the end is from the iron rod, the middle of the magnet shows little or no attraction.
4. Thus the rod whose centre does not attract the other rod’s end is the permanent magnet.
5. The result is that the magnet’s centre is non‑magnetic, whereas the iron rod’s centre is attracted to the magnet’s pole.
Examiner tips
- Use the command word ‘explain’ to describe the procedure and the observed result. Include the key point that the magnet’s centre is non‑magnetic. Use the phrase ‘little or no attraction’ to match the mark scheme. Keep the answer concise and to the point.
Common mistakes
- Confusing the roles of the rods and describing attraction at both ends.\nAssuming the iron rod can repel the magnet.\nForgetting to mention that the magnet’s centre shows no pull.
Mark scheme (5 marks)
- The student should bring the end of one rod close to the middle (or along the length) of the other rod
- A magnet attracts at its poles but has little or no magnetic force at its centre/middle
- If both rods are brought end-to-end they will always attract (because iron is always attracted to a magnet, and two magnets can attract or repel)
- If the magnet rod's end is placed against the centre of the unmagnetised iron rod, there will be attraction
- If the unmagnetised iron rod's end is placed against the centre of the magnet rod, there will be little or no attraction, identifying the magnet rod as the one whose centre shows no pull
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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