# How to calculate the magnetic dipole moment of a bar magnet?

I want to calculate the force between two magnets. In a previous Phys.SE question, I was told that I needed to use the dipole-dipole interaction, but that equation depends on $$m$$, the magnetic dipole moment of the magnet.

How do I calculate $$m$$? I found a source that said $$m = p \ell$$ where $$p$$ is the "magnetic pole strength" and $$\ell$$ is the length of the magnet, but I haven't found anything that says how to compute $$p$$.

• Given the amount of emphasis we put on the electrostatic force in introductory courses I am always a bit amused by how studiously we avoid this problem. I don't think I've even seen it treated. That said, I am pretty sure that one would have to measure rather than calculate this value for any given physical example magnet. Once could then compute forces involving two of them to his or her hearts content. – dmckee --- ex-moderator kitten Oct 25 '13 at 1:14
• Hint; a tightly wound solenoid with a metered current flowing through it should provide a pole of known strength. – dmckee --- ex-moderator kitten Oct 25 '13 at 1:51
• @dmckee I have an idea , I already know the B of the magnet , if we have a solenoid with given turns and radius , we can easily calculate the current needed to give us B , then if there's a formula relating current to pole strength we can use and find out about. So , how to know the pole strength when current flow in a solenoid? – user28324 Oct 25 '13 at 7:04
• This question was driving me nuts for a while until I found this article. Basically they show a way that you can measure it. Check it out: academia.edu/2560530/… Cheers – user84928 Jul 2 '15 at 6:51