# 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.

> IB DP Physics Higher Level (2023 syllabus) — D.2 Electric and magnetic fields · Explain · 4 marks

## Mark scheme (4 marks)

1. A current-carrying wire produces a magnetic field that circles around it (using the right-hand rule / Biot–Savart principle).
2. 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).
3. 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).
4. 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

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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