[gumby]

> Many feed wagons have strong magnets under the discharge chute..

Even though I understand it's 1/r^2 this summoned a vision of a cow magnetically stuck to the discharge tube at her neck, with the head pushed up and mooing unhappily.

[marcosdumay]

I think the force is actually 1/r^4.

The magnetic field is proportional to 1/r^3. But the linear force is not proportional to it. (Rotations are.)

[planede]

It's 1/r^4 for magnetic dipoles, where both have fixed a fixed dipole moment. So it's 1/r^4 between two permanent magnets.

Where one object is a permanent magnet and the other is some unmagnetized ferromagnetic/paramagnetic piece metal, then the dipole moment of that piece of metal also depends on the distance. Assuming it's proportional to the magnetic field of the other dipole (~1/r^3) then the force is going to be ~1/r^7 for this pair of objects.

[marcosdumay]

Oh, that's right.

But magnetization is famously path-dependent, so now I'm tending to believe the force will be closer to 1/r^7 when moving on the direction of the magnet, but 1/r^4 when moving away. And not exactly any of those functions.

[planede]

Do you mean that the ferromagnetic material gets magnetized and retains some magnetization while moving away?

[marcosdumay]

Yes.