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Homework Help: Polarizing an insulator/dieelectric

  1. May 18, 2012 #1
    As far as I can understand polarizing an object refers to putting an insulator in an electric field, such that the nucleus and electrons are pulled away from each other until they establish a field strong enough to prevent them from being pulled further apart.
    If this is correct, I would like to ask a few things about polarization that I don't quite understand:

    1) When you pull an electron away from a nucleus doesn't the electrostatic interaction weaken between them? As such how can they establish this "equilibrium" (as my book calls it) where their attraction cancels the external which tries to rip the apart?

    2) It is clear that a polarized object establishes a dipole, i.e. a field pointing in the same direction as the external field. But, why is this such an important thing to stipulate? After all every atom with a positive nucleus and negative electrons must at some point be a dipole, since it is not like the nucleus and electron have annihilated each other? What is so special about the proces where the material gets polarized (further) by an external field?
  2. jcsd
  3. May 21, 2012 #2

    rude man

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    An electron spinning around a nucleus (if you accept this picture, which BTW quantum physics has refuted), ON THE AVERAGE there is no dipole moment without the application of an external field since the orientation of the electron wrt to the nucleus is randomly time-varying.

    Speaking of quantum physics, that is what you need to understand your first question. There are other forces involved besides the electrostatic one, and besides, as I said the model of nucleus with electrons spinning around it is really not rigorous.
  4. May 22, 2012 #3


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    This is essentially correct, but (classically) the electrons move in orbits around the nuclei, and those orbits get elongated along the axis of the applied field. There are also many atoms bound together into molecules and many molecules bound together inhto any macroscopic material. The exact electric fields and motiions of charges inside a macroscopic material are immensely complicated and impossible (at least for my puny human brain) to visualize.

    Also, as rudeman points out, this is only the classical picture and QM (and indeed experiment!) tells us it is incorrect. However, it works very well for calculating fields and polarizations of most macroscopic materials, and that is all you would use classical Electrodynamics for anyway.

    When the electron is on one side of the nucleus, the applied field will tend to push it closer to its partner, while on the other side, it will tend to push it further apart. The field between the electron and nucleus will always tend to draw the two together. The average effect is to squeeze the orbit into an ellipse with the nuclei an one focus.

    The average dipole moment (averaged over any number of orbital periods) of an point charge in uniform circular motion about another point charge is zero, since it constantly changes direction. When you talk about classical polarization, you are really talking about average dipoles moments (averaged over time and space).
    Last edited: May 22, 2012
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