What causes the force of attraction between two permanent magnets?

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Discussion Overview

The discussion centers on the underlying mechanisms that cause the force of attraction between two permanent magnets. Participants explore various aspects of magnetism, including atomic behavior, electron spins, and the implications of force laws in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the nature of moving charges in the context of magnetism, referencing the equation F = qv x B.
  • Another participant suggests that atomic currents, arising from spin and angular momentum, can lead to a net magnetic force when organized in certain materials.
  • There is a query regarding the role of valence electrons versus atomic nuclei in generating magnetic forces, with a focus on why the entire piece of metal experiences a force rather than just a current flow.
  • One participant asserts that magnetism in permanent magnets is primarily due to unpaired electron spins, which create a net magnetic field, and notes that this is distinct from the Lorentz force equation.
  • Another participant elaborates on the force law between electron spins, stating it is proportional to the magnetic moment and inversely related to the cube of the separation distance, while also discussing the potential energy of dipoles in a magnetic field.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms of magnetism, particularly regarding the roles of electron spins, atomic currents, and the applicability of various force laws. The discussion remains unresolved with multiple competing perspectives presented.

Contextual Notes

Some participants reference specific force laws and equations, but there is no consensus on the definitions or implications of these concepts, leaving certain assumptions and dependencies unaddressed.

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

The electrons are bound to the atoms.
 
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.
 
krab said:
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.

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 [itex]U = -\vec \mu \cdot \vec B[/itex] 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.
 

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