A current-carrying wire is held at right angles to a uniform magnetic field between two permanent magnets. Explain why the wire experiences a force, and describe two ways in which the magnitude of this force could be increased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A straight conducting wire carries a current of 2 A and is placed perpendicular to the magnetic field between the poles of two permanent magnets. The wire is observed to move.
Model answer (5 marks)
A current of 2 A in the wire creates a magnetic field around it (right‑hand rule). This field interacts with the magnetic field of the permanent magnets, producing a Lorentz force on the wire.
The force is perpendicular to both the direction of the current and the magnetic field (right‑hand rule).
The magnitude of the force can be increased:
1. by increasing the current through the wire (F∝I).
2. by increasing the magnetic flux density of the field, e.g. using stronger magnets (F∝B).
The force is perpendicular to both the direction of the current and the magnetic field (right‑hand rule).
The magnitude of the force can be increased:
1. by increasing the current through the wire (F∝I).
2. by increasing the magnetic flux density of the field, e.g. using stronger magnets (F∝B).
Examiner tips
- Use the right‑hand rule to show the force direction. Mention the Lorentz force equation F=IL×B. Show the two methods to increase force clearly.
- common_mistakes
- :
- Confusing the direction of the force with the direction of the magnetic field. Forgetting that the force is proportional to both I and B. Not specifying that the field must be uniform or that the wire is perpendicular.
Mark scheme (5 marks)
- The current in the wire produces a magnetic field around the wire
- This magnetic field interacts with the magnetic field of the permanent magnets
- The resultant force on the wire is perpendicular to both the current direction and the magnetic field direction
- The force can be increased by increasing the current through the wire
- The force can be increased by increasing the magnetic flux density of the field (e.g. using stronger magnets)
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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