# Force between two magnets

1. Nov 20, 2004

### so-crates

What causes the force of attraction between two permanent magnets? I'm well aware that F = qv x B, but what charges are moving?

2. Nov 20, 2004

### Tide

Charges are moving at the atomic level. In most materials the atomic currents (i.e. from spin & angular momentum) are randomly oriented so the net force is zero. However, in some materials, the atomic currents can become organized (correlated) on a macroscopic level and allow for a net magnetic force on the material.

3. Nov 20, 2004

### so-crates

So it is the valence electrons in the metal, and not the nuclei? If it just the electrons, why does this cause a force on the entire piece of metal and not just cause a current to flow ?

4. Nov 20, 2004

### Tide

The electrons are bound to the atoms.

5. Nov 20, 2004

### krab

Magnetism in permanent magnets results from the net magnetic field created by unpaired electron spins. These are not necessarily moving; some of the magnetism can come from orbital motion, but this is not the main effect. Electron spins are dipoles and there is a force law between these spins that is proportional to the magnetic moment of each and to the reciprocal of the cube of the separation. This force law is not F=qvxB.

6. Nov 21, 2004

### Tide

Actually, the strength of the magnetic field produced by the dipole varies inversely with the cube of distance and the potential energy of a dipole in a magnetic field is $U = -\vec \mu \cdot \vec B$ so the force exerted by one dipole on the other will vary as the gradient of U.

BTW - if we think about the magnetic moment of an electron classically then it amounts to a rotating charge, i.e. it constitutes an electrical current. Ultimately, these notions and forces between dipoles derive from the basic Lorentz force.