Question

This seems like it would be a simple question, but I was using the right hand rule and can't figure out how to apply it to a compass magnet. I know that a compass magnet aligns itself to have it's north pole attract the the earths magnetic south pole (which is geographically north), but how can this be proven using the right hand rule?

Answer 1

can be done with some kind of a "trick". the right hand rule is about moving charges of currents, in a magnetic field.
It is not supposed to be applied on a magnet in a magnetic field.
for this purpuse u can use parts from this http://en.wikipedia.org/wiki/Magnetic_moment

BUT - the fact is that every magnetic field is caused by a current, also the magnetic field of a ar magnet like in the compass.
So u can discover the current in the magnet by it's field, and use the right hand with this information.
Or just use the magnetic moment concept which is a development of the rugular right hand rule.

Answer 2

Torque exerted by an external magnetic field B on a magnetic moment (you compass needle) is :
\[\vec{\tau}=\vec{M}\times \vec{B}_{ext}\]
You can use the right-hand rule to check which way your magnet will be rotating in the magnetic field.