A straight wire carrying an electric current is placed at right angles to a uniform magnetic field. Explain how the direction of the force on the wire can be reversed, and describe what happens to the size of the force if the current in the wire is doubled.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The force on a current‑carrying wire in a magnetic field is given by
F = I L × B.
• If the direction of the current I is reversed, the vector I changes sign, so the cross‑product changes sign and the force reverses direction.
• If the magnetic field B is reversed, the cross‑product also changes sign, giving the same reversal of force.
• Doubling the current (I → 2I) doubles the magnitude of the force (F → 2F) because the force is directly proportional to I.
The force is produced by the interaction between the magnetic field created by the current in the wire and the external magnetic field – the motor effect.
F = I L × B.
• If the direction of the current I is reversed, the vector I changes sign, so the cross‑product changes sign and the force reverses direction.
• If the magnetic field B is reversed, the cross‑product also changes sign, giving the same reversal of force.
• Doubling the current (I → 2I) doubles the magnitude of the force (F → 2F) because the force is directly proportional to I.
The force is produced by the interaction between the magnetic field created by the current in the wire and the external magnetic field – the motor effect.
Examiner tips
- Use the formula F = ILB and the right‑hand rule to show the direction change clearly.
- State that the force is proportional to I to justify the doubling effect.
- Mention the motor effect to show understanding of the underlying interaction.
Common mistakes
- Confusing reversal of current with reversal of magnetic field – both give the same effect but students may not state both.
- Failing to explain that the force magnitude is proportional to the current, so doubling I doubles F.
- Using vague language like "the force changes" without specifying direction or proportionality.
Mark scheme (4 marks)
- Reversing the direction of the current reverses the direction of the force on the wire
- Reversing the direction of the magnetic field also reverses the direction of the force on the wire
- Doubling the current doubles the size of the force
- The force on the wire is caused by the interaction between the magnetic field of the current and the external magnetic field (the motor effect)
Key terms in this question
magnetic field · current · force
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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