Lecture 8: Polarization, Dielectrics, The Van de Graaff, More on Capacitors
recorded by: Massachusetts Institute of Technology, MIT
published: Oct. 10, 2008, recorded: February 2002, views: 22407
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"Electric fields can induce dipoles in insulators.
Electrons and insulators are bound to the atoms and to the molecules, unlike conductors, where they can freely move, and when I apply an external field -- for instance, a field in this direction, then even though the molecules or the atoms may be completely spherical, they will become a little bit elongated in the sense that the electrons will spend a little bit more time there than they used to, and so this part become negatively charged and this part becomes positively charged, and that creates a dipole.
I discussed that with you, already, during the first lecture, because there's something quite remarkable about this, that if you have an insulator -- notice the pluses and the minuses indicate neutral atoms -- and if now, I apply an electric field, which comes down from the top, then, you see a slight shift of the electrons, they spend a little bit more time up than down, and what you see now is, you see a layer of negative charge being created at the top, and a layer of positive charge being created at the bottom.
That's the result of induction, we call that also, sometimes, polarization. You are polarizing, in a way, the electric charge..."
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