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I've been reading A Student's Guide to Maxwell's Equations by Daniel Fleisch, and he states:

in nonconducting materials (called "insulators" or "dielectrics"), charge does not move freely, but may be slightly displaced from their equilibrium positions.

However, he then describes the effect of an electric field on an insulator, and states:

dielectrics become "polarized" when placed in an electric field, which means that positive and negative charges are displaced from their original positions.

How is this different from the behavior of conductors? Is the only difference the extent to which charges are displaced?

Additionally, if the ability to reduce the amplitude of an electric field by creating an opposing electric field increases capacitance, then why aren't conductors used in capacitors? Wouldn't conductors have the same effect only stronger, since the polarization of charges would occur to a much greater extent and create a stronger electric field opposing the external electric field?

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In conductor material, there are free electrons, they can move to one side of conductor. The whole conductor body is polarized. In insulator, there is no free elections, under electric field, the electrons can only move little distance in their equilibrium positions. Molecules or atoms are polarized.

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