# Force between magnetic poles

Jack123
Hi, I was wondering if anyone could tell me how to calculate the attractive force between a pair of magnets. I orignally thought that this would involve a really simple formula (something of the 1 over r squared variety) but have struggled to find any equations dealling with the force between poles; they all seem to associate magnetic forces to charged particles.

The only formula I have found is located at this address:

http://geophysics.ou.edu/solid_earth/notes/mag_basic/mag_basic.html [Broken]

In my experiment I was examining how the force of attraction between a solenoid and bar magnet of known strength (0.01 T) depended on current and number of turns of the solenoid as well distance between the two. I reasoned that the field of a solenoid is in effect the same as a bar magnet so I should be able to use the above formula.

However, the force I calculated was tiny, despite the fact that I could physically feel the attraction when I suspended the magnet over the solenoid. When I measured the force I found it to be on the order of around a tenth of a Newton, hundreds of times greater than the number I had obtained from the above equation.

So what am I doing wrong?

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MagnetDave
The result is heavily dependent on the geometry and material of the situation. A "bar" magnet encompasses a wide variety of things that all perform very differently. If you envision your bar as a thin sheet, with the direction of magnetization in the thin axis, it will be close to useless. Conversely, if you make a baton, with the DOM in the long direction, it's quite powerful. Similarly, a Neo magnet will be different than a Samarium magnet, will be different than an Alnico...

In short, the situation is not very amenable to a quick-and-dirty formula. You should look into FEMM, which is a simple, free, 2D finite element code so that you can at least get some order-of-magnitude level calculation done.