Explain why a compass needle points towards geographical north and describe what happens to the compass needle when it is brought close to a strong bar magnet.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A compass is a small permanent magnet that is free to rotate. It is used for navigation because it aligns with the Earth's magnetic field.
Model answer (4 marks)
A compass needle is a small permanent magnet. The Earth behaves like a giant magnet, producing a magnetic field that runs from the magnetic south pole (geographical north) to the magnetic north pole (geographical south). The north pole of the needle is attracted to the Earth's magnetic south pole, so the needle aligns with the Earth's field and points towards geographical north.
When the needle is brought close to a strong bar magnet, the bar magnet’s field is locally much stronger than the Earth's field. The needle is deflected and aligns with the bar magnet’s field instead of the Earth's, so it no longer points north but points towards or away from the bar magnet’s poles depending on the relative orientation.
When the needle is brought close to a strong bar magnet, the bar magnet’s field is locally much stronger than the Earth's field. The needle is deflected and aligns with the bar magnet’s field instead of the Earth's, so it no longer points north but points towards or away from the bar magnet’s poles depending on the relative orientation.
Examiner tips
- Use the term "giant magnet" to describe the Earth; mention the field direction. State that the needle’s north pole is attracted to geographical north. Explain that the bar magnet’s field dominates locally and causes deflection. Keep the answer concise and use correct terminology.
Common mistakes
- Confusing geographical north with magnetic north. Failing to mention the Earth's magnetic field. Not explaining that the bar magnet’s field is stronger locally.
Mark scheme (4 marks)
- The Earth behaves as / acts like a giant magnet (with a magnetic field surrounding it)
- The north pole of the compass needle is attracted towards geographical north / the compass aligns with the Earth's magnetic field
- When close to a bar magnet, the compass needle is deflected / no longer points north / points towards or away from the bar magnet's poles
- The bar magnet's field is much stronger (locally) than the Earth's field, so the needle aligns with the bar magnet's field rather than the Earth's
Key terms in this question
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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