A steel paper clip is brought close to one end of a bar magnet and becomes attracted to it. When the paper clip is removed from near the magnet, it is found to weakly attract small iron filings for a short time, but this effect soon disappears. Explain these observations.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
When the steel clip is brought near the bar magnet, the magnetic field of the magnet aligns the magnetic domains in the clip in the same direction – the clip becomes magnetised by induction.
Because the domains are aligned, the clip itself now has a magnetic field and therefore behaves as a magnet. It is attracted to the bar magnet and can also attract small iron filings.
After the magnet is removed the domains in the clip are still partially aligned for a short time, so the clip can weakly attract iron filings. However, steel is a hard magnetic material and the domains eventually relax back to a random arrangement, so the induced magnetism disappears.
Because the domains are aligned, the clip itself now has a magnetic field and therefore behaves as a magnet. It is attracted to the bar magnet and can also attract small iron filings.
After the magnet is removed the domains in the clip are still partially aligned for a short time, so the clip can weakly attract iron filings. However, steel is a hard magnetic material and the domains eventually relax back to a random arrangement, so the induced magnetism disappears.
Examiner tips
- Use the phrase ‘magnetised by induction’ to show understanding of the process. Mention that the clip now behaves as a magnet and can attract filings. Explain the short‑lived retention of magnetism and the eventual loss due to domain relaxation.
- common_mistakes
- :
- Saying the clip becomes a permanent magnet – it is only temporarily magnetised. Forgetting to mention the alignment of domains or the short‑lived retention. Using vague terms like ‘magnetic field’ without linking to domain alignment.
Mark scheme (4 marks)
- The bar magnet causes the magnetic domains in the steel paper clip to align (in the same direction) / the paper clip is magnetised by induction
- This alignment of domains means the paper clip itself acts as a magnet / has a magnetic field, so it attracts the bar magnet (and later the iron filings)
- When the bar magnet is removed, the paper clip retains some domain alignment for a short time, so it can still weakly attract iron filings
- Steel is a hard magnetic material so it retains magnetism longer than soft iron, but the domains eventually return to a random arrangement so the magnetism disappears
Related
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