Explain why a current-carrying wire placed between the poles of a permanent magnet experiences a force, and describe what happens to the size of this force when the current in the wire is increased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A straight wire is connected to a battery and placed horizontally between the north and south poles of a U-shaped permanent magnet so that the wire is at right angles to the magnetic field.
Model answer (5 marks)
A current in the wire creates a magnetic field that circles the wire (right‑hand rule). This field meets the uniform field of the magnet. Where the two fields combine, the resultant field is not uniform, so a magnetic force acts on the current‑carrying wire. The force is perpendicular to both the current direction and the magnetic field (motor effect). If the current is increased, the magnetic field of the wire becomes stronger, the interaction with the magnet’s field becomes stronger, and therefore the magnitude of the force on the wire increases.
Examiner tips
- Use the right‑hand rule to describe the wire’s field direction.
- Explain that the non‑uniform resultant field gives a net force.
- State that the force is perpendicular to both I and B.
- Show that F∝I by mentioning the increase in the wire’s field.
Mark scheme (5 marks)
- The current in the wire produces its own magnetic field around the wire
- The magnetic field of the wire interacts with the magnetic field of the permanent magnet
- This interaction produces a resultant/combined magnetic field that is not uniform
- The force on the wire acts at right angles to both the current and the magnetic field (motor effect)
- Increasing the current increases the size of the force on the wire
Key terms in this question
current · permanent magnet · force
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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