Describe how iron filings and a plotting compass can each be used to show the direction of a magnetic field around a bar magnet, and state one advantage of using a plotting compass rather than iron filings for this purpose.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Iron filings: place a sheet of paper over the bar magnet, sprinkle iron filings on the paper and tap the paper. The filings align along the magnetic field lines, showing the field’s shape.
Plotting compass: place a small compass next to the magnet, mark the direction of the needle, then move the compass along the surface and mark successive directions to trace a field line.
Both methods show that field lines run from the north pole to the south pole.
Advantage of the plotting compass: it indicates the direction of the field at each point, whereas iron filings only reveal the pattern of the lines.
Plotting compass: place a small compass next to the magnet, mark the direction of the needle, then move the compass along the surface and mark successive directions to trace a field line.
Both methods show that field lines run from the north pole to the south pole.
Advantage of the plotting compass: it indicates the direction of the field at each point, whereas iron filings only reveal the pattern of the lines.
Examiner tips
- Use the word ‘describe’ to explain each step clearly; include the key actions (sprinkling filings, tapping paper, marking compass direction).
- Mention the direction of the field lines (north to south) to gain the point on field direction.
- State the specific advantage of the compass (shows direction) to earn the extra mark.
Common mistakes
- Failing to mention that the compass needle points along the field direction.
- Confusing the direction of field lines with the direction of the magnetic force on a north pole.
Mark scheme (4 marks)
- Iron filings method: paper is placed on top of (or over) the bar magnet and iron filings are sprinkled onto the paper; tapping the paper causes the filings to align along the magnetic field lines
- Plotting compass method: the compass is placed next to the magnet and the needle points in the direction of the magnetic field at that point; dots are marked and the compass is moved progressively to trace a field line
- Field lines point / arrows go from north pole to south pole (direction is shown)
- Advantage of plotting compass: it shows the direction of the field (whereas iron filings only show the shape/pattern of the field lines but not the direction)
Key terms in this question
magnetic field · plotting compass · bar magnet
Related
- All Cambridge International IGCSE Physics (0625) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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