# Magnetic dipole interaction

by tris_d
Tags: dipole, interaction, magnetic
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 P: 162 Two permanent magnets are on the table some distance apart and having some arbitrary orientation relative to each other. When we let go of them they will rotate and translate until they stick together. I am looking for suitable equations to model this interaction on a computer. Also, if anyone knows some software that can already do this please let me know.
 P: 665 I would suggest to treat each magnet as a dumbbell of fixed length with all the mass concentrated in the poles. It's then possible to model all of the forces on each pole and iterate the motion. Some possible motions could be chaotic though.
P: 162
 Quote by AJ Bentley I would suggest to treat each magnet as a dumbbell of fixed length with all the mass concentrated in the poles. It's then possible to model all of the forces on each pole and iterate the motion. Some possible motions could be chaotic though.
You mean to treat each pole separately? I don't think there is such equation since magnetic monopoles are not supposed to exist, but that does seem to be the right direction to approach this problem.

So far I only found this equation:
http://en.wikipedia.org/wiki/Magnetic_dipole_moment

The problem is that only tells me how much will they attract, but not how much will they rotate and how much will they translate. You suggestion might solve this problem as instead of one force I would have two, and that would hopefully model rotation and thus possibly solve the whole problem. All I need now is some equation for it. Do we have equation for magnetic monopoles, or can it be derived from the equation above?

 P: 665 Magnetic dipole interaction The basic equation of magnetic force (the inverse square law) is based on the concept of a monopole.
P: 162
 Quote by AJ Bentley The basic equation of magnetic force (the inverse square law) is based on the concept of a monopole.

You mean Biot-Savart law, magnetic field due to moving charge? That's cylindrical magnetic field rather than "spherical", and is defined by the velocity vector. Right in front and behind magnetic field goes to zero as it gets aligned with the velocity vector (doughnut), so I don't think that would work as I don't think that's how individual poles of a magnetic dipole look like.
 P: 665 No - back before the dawn of time, when the unit of magnetisation was the Oersted. The world was a simpler place.
P: 162
 Quote by AJ Bentley No - back before the dawn of time, when the unit of magnetisation was the Oersted. The world was a simpler place.
What do you mean "no"? What equation are you talking about?
 P: 665 Seems like the knowledge has been lost. lol! The magnetic field from a monopole follows the coulomb law just like the electric field. Originally these two subjects were treated identically. You can substitute a pair of electric charges for your magnetic dipole and except for a few constants the mathematics are identical.
P: 162
 Quote by AJ Bentley Seems like the knowledge has been lost. lol! The magnetic field from a monopole follows the coulomb law just like the electric field. Originally these two subjects were treated identically. You can substitute a pair of electric charges for your magnetic dipole and except for a few constants the mathematics are identical.
I see. But the geometry should be different, field lines of an electric field are straight and radial, but magnetic field lines are circular. Still, the force lines might come up to be about the same, so it actually might work. It certainly is the best solution I have so far. Cheers!
P: 665
 Quote by tris_d I see. But the geometry should be different, field lines of an electric field are straight and radial, but magnetic field lines are circular. Still, the force lines might come up to be about the same, so it actually might work. It certainly is the best solution I have so far. Cheers!
The geometry is identical. The fields are identical. Maxwell's equations are symmetric (when monopoles are allowed) . Electric filed lines are not necessarily straight (Curl E ≠ 0). Everything is exactly the same except for the values of a few constants.
P: 162
 Quote by AJ Bentley The geometry is identical. The fields are identical. Maxwell's equations are symmetric (when monopoles are allowed) . Electric filed lines are not necessarily straight (Curl E ≠ 0). Everything is exactly the same except for the values of a few constants.
That's wonderful.

Ok, so if I wanted to replace the two permanent magnets with two electrons and their intrinsic dipole magnetic moment, how would I get the value I need to use for my monopoles?
 P: 665 Well, the only change is that you use μ (permeability) in place of ε (permittivity) You can use ε.μ = 1/c2 so it's just a case of sticking in a factor of c2
P: 162
 Quote by AJ Bentley Well, the only change is that you use μ (permeability) in place of ε (permittivity) You can use ε.μ = 1/c2 so it's just a case of sticking in a factor of c2
That's a bit too much for me. I expected some actual number as electron's dipole moment is constant value. I think magnetic dipole moment drops with inverse cube not inverse square law, and both poles are taken into account, so I expected it would be more complex to obtain the value for each pole separately.

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