When a current-carrying wire is placed in a magnetic field, it can experience a force. Explain why reversing the direction of the current in the wire causes the direction of this force to reverse.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An engineer is testing a simple electric motor. She notices that when she reverses the current supplied to a wire held between the poles of a magnet, the wire moves in the opposite direction to before.
Model answer (4 marks)
The force on a current‑carrying wire in a magnetic field is produced by the interaction between the magnetic field of the magnet and the magnetic field created by the current in the wire.
Reversing the current reverses the direction of the magnetic field produced by the wire.
The interaction between the reversed wire field and the magnet's field now acts in the opposite sense.
This reversed interaction produces a force on the wire in the opposite direction.
Reversing the current reverses the direction of the magnetic field produced by the wire.
The interaction between the reversed wire field and the magnet's field now acts in the opposite sense.
This reversed interaction produces a force on the wire in the opposite direction.
Examiner tips
- Use the term "magnetic field" and "interaction" to show understanding of the Lorentz force.
- Show the chain of cause and effect: current → wire field → interaction → force direction.
- Mention that the force is given by λI×B×L and that reversing I changes the sign of the force.
Common mistakes
- Confusing the direction of the current with the direction of the force without explaining the magnetic field reversal.
- Using vague language such as "the wire moves" instead of describing the magnetic field interaction.
- Omitting the role of the magnet’s magnetic field in the interaction.
Mark scheme (4 marks)
- The force on the wire is produced by the interaction between the magnetic field of the magnet and the magnetic field created by the current in the wire.
- Reversing the current reverses the direction of the magnetic field produced by the wire.
- The interaction between the reversed wire field and the magnet's field now acts in the opposite sense.
- This reversed interaction produces a force on the wire in the opposite direction.
Key terms in this question
current · magnetic field · force
Related
- All WJEC GCSE Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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