Explain how a plotting compass can be used to map the magnetic field around a bar magnet, and describe the pattern of field lines that would be produced.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student places a bar magnet flat on a piece of paper and uses a plotting compass to investigate the magnetic field around it.
Model answer (5 marks)
Place the plotting compass close to one pole of the bar magnet and note the direction the needle points. Move the compass to a new position, record the needle direction again and join the successive points with a line – this line is a magnetic field line. Repeat this process around the magnet. The field lines run from the north pole to the south pole of the bar magnet. They are most closely spaced near the poles, indicating the field is strongest there. The lines never cross each other.
Examiner tips
- Use the compass to mark successive points and join them to show a continuous line.
- Show that lines start at the north pole and end at the south pole.
- Explain the density of lines near the poles and that lines never cross.
Common mistakes
- Failing to describe that lines start at N and end at S.
- Not mentioning that lines are densest near the poles.
- Assuming lines can cross or ignoring the need to join points to form a line.
Mark scheme (5 marks)
- Place the plotting compass near one pole of the bar magnet and mark the direction the needle points
- Move the compass to the new marked position and repeat, joining the dots to form a field line
- The field lines go from the north pole to the south pole of the bar magnet
- Field lines are closest together (most dense) near the poles, showing the field is strongest there
- Field lines never cross each other
Key terms in this question
plotting compass · magnetic field · field lines
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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