Explain what happens to a plotting compass when it is placed at different positions around a bar magnet, and describe how this can be used to draw the magnetic field pattern around the magnet.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student places a plotting compass at several positions around a bar magnet on a piece of paper and marks the direction the needle points each time.
Model answer (5 marks)
The needle of a plotting compass aligns with the magnetic field at the point where it is placed.
The north pole of the needle points away from the north pole of the bar magnet and toward its south pole – i.e. the field lines run from the north to the south pole outside the magnet.
At each position the student marks the tip of the needle (or the position of the needle) with a dot, then moves the compass to a new point and repeats the process.
When the dots are joined, a line is formed that represents a magnetic field line; arrows are added to indicate the direction of the field.
Because the field is strongest near the poles, the field lines are drawn closest together in those regions, showing the density of the field.
The north pole of the needle points away from the north pole of the bar magnet and toward its south pole – i.e. the field lines run from the north to the south pole outside the magnet.
At each position the student marks the tip of the needle (or the position of the needle) with a dot, then moves the compass to a new point and repeats the process.
When the dots are joined, a line is formed that represents a magnetic field line; arrows are added to indicate the direction of the field.
Because the field is strongest near the poles, the field lines are drawn closest together in those regions, showing the density of the field.
Examiner tips
- Use the phrase ‘the needle aligns with the magnetic field’ to gain the first point. Show the direction of the field by stating the needle’s north pole points from the magnet’s north to its south. Explain the dot‑and‑move method and that joining the dots gives a field line with arrows. Mention that lines are drawn closest together near the poles to indicate field strength.
Common mistakes
- Confusing the direction of the field with the direction of the compass needle’s north pole. Failing to describe the dot‑and‑move procedure or the need for arrows on the field lines. Not noting that field lines are densest near the poles, which shows the field is strongest there.
Mark scheme (5 marks)
- The compass needle points in the direction of the magnetic field at that position (the needle aligns with the field line).
- The north pole of the compass needle points from the north pole of the bar magnet toward the south pole (field lines go from north to south outside the magnet).
- A dot is marked at the tip of the compass needle (or the position is marked), then the compass is moved to that new position and the process is repeated.
- Joining the sequence of dots produces a field line, and arrows must be added to show direction.
- The field lines are closest together (most dense) near the poles, showing the field is strongest there.
Key terms in this question
magnetic field · plotting compass
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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