# How do I relate the direction of a compass needle to the direction of a current?

1. A compass needle is placed above the wire points with its N-pole toward the east. In what direction is the current flowing?
2. If a compass is put underneath the wire, in which direction will the needle point?

I have quite a few problems on this for my Physics homework tonight, and I am utterly confused. The book says nothing about direction besides stating the right hand rule. What confuses me is that the compass needle points with its N-pole towards the east. So would that change the direction of the current? And then also, what is the difference between the magnetic field above and below the wire?

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Potential answers are asked to concentrate on the conceptual issues here in keeping with our homework policy. – dmckee May 8 '12 at 17:17
– tmac May 8 '12 at 17:38

The right hand rule is a trick for remembering the direction of a magnetic field generated by a current in a wire. The compass is showing you what direction the magnetic field is in, so you can use the right hand rule to work out whether the current is flowing north to south or south to north.

There's a good diagram of the magnetic field around a wire here. This should help you work out why the compass might point ina different direction above and below the wire.

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Thank you! That helps a lot. – Cara May 8 '12 at 17:33
If the answer that @JohnRennie posted was helpful (it certainly seems like it was, it's a good answer), you should mark it as accepted and upvote it! This will help keep the site working better for everyone. – tmac May 8 '12 at 17:38
@tmac: of course you're correct, but I should mention it takes 15 rep to upvote, so brand-new users often don't have the ability to cast upvotes. – David Z May 8 '12 at 17:42
@DavidZaslavsky doh! So many little rules! Thanks for the reminder. – tmac May 8 '12 at 17:45
I've casually glossed over the fact I can never remember whether a compass needle points with the direction of flow or against it :-) – John Rennie May 8 '12 at 17:58

If you put a magnetic needle above a current carrying wire as shown in the first diagram, it will experience a torque as shown and will be aligned as shown in the second diagram. This convention can be remembered by remembering the right hand thumb rule. The direction of the curl of the fingers gives the direction towards which the N-pole is repelled or the "direction" of the magnetic field lines. So, a magnetic needle put below the wire will have its N-pole aligned in the opposite direction to that shown in the second diagram.

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