TABLE OF CONTENTS Conductors have free electrons or other mobile charges in their interior. When a conductor is placed in an electric field, the free electrons rearrange themselves, settling into a new equilibrium configuration such that the electric field is totally canceled everywhere within the conductor. Insulators have no free electrons or other free charges. When an insulator is placed in an electric field, the field will not be canceled, at least not totally. Insulators have bound charges. In response to the electric field, some limited motion of these bound charges occurs, and this leads to a partial cancellation of the electric field. The bound charges rearrange themselves in such a way that there is a slight separation between the average positions of the positive and the negative charges; such a separation is called a polarization of the charge distribution. Materials that acquire a polarization when placed in an electric field are called dielectrics.
We can summarize the effect of the electric field on a dielectric material by saying that dipoles are produced in the material. Of course, since what really is involved here is a charge separation, not only dipoles but also quadrupoles and higher multipoles will be produced; but it usually is a good approximation to ignore everything except dipoles. One mechanism for the production of dipoles in a dielectric material is the distortion of the charge distribution within each atom. If we place an atom in an electric field, the forces exerted by this electric...
