A student places a plotting compass at several positions around a bar magnet. Describe what the student would observe about the direction and spacing of the compass needle at different positions, and explain what these observations reveal about the magnetic field surrounding the magnet.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student is investigating the magnetic field around a bar magnet using small plotting compasses placed at various distances and positions around the magnet.
Model answer (5 marks)
1. The needle points away from the north pole and towards the south pole, i.e. along the magnetic field lines.
2. At different positions the needle’s direction changes, showing that the field lines curve around the magnet.
3. Near the poles the needles are close together, indicating a high density of field lines.
4. This close spacing reveals a stronger magnetic field close to the poles.
5. Far from the magnet the needles are spaced further apart, showing the field lines spread out and the field strength decreases with distance.
2. At different positions the needle’s direction changes, showing that the field lines curve around the magnet.
3. Near the poles the needles are close together, indicating a high density of field lines.
4. This close spacing reveals a stronger magnetic field close to the poles.
5. Far from the magnet the needles are spaced further apart, showing the field lines spread out and the field strength decreases with distance.
Examiner tips
- Use the exact phrase "points from north to south" to gain full credit for point 1. Show the change in direction explicitly for point 2. Mention the relationship between spacing and field strength for points 3‑5. Keep each point brief and to the point.
Common mistakes
- Writing that the needle points towards the north pole (reverse direction). Failing to mention that the field lines curve. Confusing spacing of needles with spacing of field lines. Omitting the link between close spacing and stronger field.
Mark scheme (5 marks)
- The compass needle points from north to south (along the field lines) / the needle points away from the north pole and towards the south pole of the magnet.
- The direction of the compass needle changes at different positions around the magnet, showing the field lines curve around the magnet.
- The compass needles are closer together (more concentrated) near the poles / nearer the magnet.
- Greater concentration of field lines (closer compass needle positions) indicates a stronger magnetic field in that region.
- As distance from the magnet increases, the spacing between compass needle directions increases / field lines become more spread out, showing the field strength decreases with distance.
Key terms in this question
magnetic field · plotting compass
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
More Waves questions
- A rescue worker uses an electromagnet attached to a crane to separate iron and s…
- A technician wants to build the strongest possible solenoid for use in a medical…
- An engineer is designing a solenoid for use in an electronic locking system. Exp…
- A student is building an electromagnet for a school project. They wind a coil of…