Explain why two long, parallel wires carrying currents in the same direction attract each other, whilst two long, parallel wires carrying currents in opposite directions repel each other.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A current‑carrying wire produces a magnetic field that circles around it (right‑hand rule). The magnetic field of wire A at the position of wire B exerts a force on the current in wire B (F = BIL). If the currents are in the same direction, the field lines between the wires are opposite, so the net field between them is weaker and the force on each wire is directed toward the other – the wires attract. If the currents are opposite, the field lines between the wires are in the same direction, the net field is stronger and the force on each wire is directed away – the wires repel.
Examiner tips
- Use the right‑hand rule to describe the field direction; mention F= BIL; explain field direction between wires for each case; link field direction to force direction.
- Show the reasoning for attraction/repulsion in one sentence each to use all marks.
Common mistakes
- Confusing the direction of the magnetic field with the direction of the force; not specifying that the force is on the current in the other wire.
- Using the wrong sign for the magnetic field direction between the wires; not mentioning the right‑hand rule or F= BIL.
Mark scheme (4 marks)
- A current-carrying wire produces a magnetic field that circles around it (using the right-hand rule / Biot–Savart principle).
- The second wire, carrying a current, experiences a force due to the magnetic field of the first wire (F = BIL or force on a current-carrying conductor in a magnetic field).
- For currents in the same direction, the magnetic fields between the wires point in opposite directions, so the net field between them is weaker, and the force on each wire is directed towards the other (attraction).
- For currents in opposite directions, the magnetic fields between the wires point in the same direction, reinforcing each other, and the force on each wire is directed away from the other (repulsion).
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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