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Which is stronger? eletrical or magnetic fields?

  1. Nov 10, 2006 #1
    A single stationary electron's magnetic flux caused by it's spin gives it a north and south pole right? does that mean it's north pole would be attracted to the south pole of a hypothetical nearby stationary electron?

    how would this attractive force compare to it's electric force of repulsion at equal distances? whats the ratio?
  2. jcsd
  3. Nov 10, 2006 #2


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    It's a strange way of stating it the way you do, but yes, there is a "magnetic dipole interaction" between electrons. It's pretty weak, but spectroscopically visible. However, the main effect doesn't come from the magnetic dipole interaction between different electrons, but rather between the intrinsic magnetic moment of the electron related to its spin (the thing you talk about), and the more conventional magnetic moment of the same electron, because it is in orbit (a circulating charge also generates a magnetic field). This effect is called "spin-orbit coupling" and is part of the fine structure of the spectra of atoms.
  4. Nov 10, 2006 #3
    so precisely how much stronger is an eletrical force to a magnetic force between 2 stationary electrons? 1:100?

    Remember that they are stationar but still have a magnetic field caused by their spin.
    Last edited: Nov 10, 2006
  5. Nov 10, 2006 #4


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    The electron magnetic moment equals - sqrt(1/2(1+1/2)) e hbar / (m c)

    From this you can calculate the magnetic field generated by the magnetic dipole at a certain distance and angle, and the energy another (identical) magnetic dipole has.

    You can find the magnetic dipole field here: http://en.wikipedia.org/wiki/Dipole

    The gradient of this energy wrt the place where the second dipole is, will give you the force experienced due to the dipole-dipole interaction.

    You can compare this to the Coulomb interaction.
  6. Nov 10, 2006 #5
    Actually (in solids at least), the "exchange interaction" that causes spins to line up is a result of the exclusion principle coupled with the Coulomb interaction. It's energetically favorable for the electron-electron two-body state to be anti-symmetric in real space, because it reduces the Coulomb interaction, but this causes the spins to be symmetric, and therefore they have to line up. This is what is perceived as the "dipole-dipole interaction" that leads to magnetism, but the pure dipole-dipole interaction is entirely too weak to account for the temperatures that we see the phase transition at.

    So that should give you a sense of scale of the interaction: at equal distances, some sort of exclusion effect is far more powerful than the actual dipole-dipole interaction.
  7. Nov 10, 2006 #6
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