# Explain why two long, parallel wires carrying currents in the same direction attract each other.

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

> Two long, straight parallel conducting wires are separated by a small distance. Each wire carries a steady direct current flowing in the same direction.

## Mark scheme (4 marks)

1. Each current-carrying wire produces a magnetic field in the space around it.
2. The magnetic field lines produced by one wire are circular and directed (using the right-hand rule / corkscrew rule) such that the field at the location of the second wire is directed perpendicularly between the two wires.
3. The second wire, carrying a current in that magnetic field, experiences a force given by F = BIL; the direction of the force (by Fleming's left-hand rule or F = IL × B) is towards the first wire.
4. By Newton's third law (or by symmetry), each wire exerts an equal and opposite attractive force on the other, so both wires are pulled towards each other.

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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