Electric fields and insulators Will the electric field of an induced dipole in an insulator match the electric field inducing it but in the opposite direction? I have 2 counter theories:
Let's say I place an insulator (and let's make this one a plate as well, just because I want electric fields to be constant to simplify things) above a charged plate, and I want the electric field on the other side of the insulator plate. Would it be 0 because the electric field from the insulator has perfectly matched that of the charged plate?


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*To me, it seems like it would, otherwise I don't understand why if there is a net force on the electrons in the insulator, they wouldn't keep on shifting until the electric fields did indeed match and cancel each other out. 

*On the other hand, I really feel like for most materials it doesn't, since electric fields DO permeate insulators. But if the electric fields aren't canceling out, why don't the electrons inside the insulator keep on shifting until they DO cancel out?

*What about in a salt water solution for example. This is a conductor, so the electric field WILL eventually reach 0. However, what is the big difference in an insulator, if according to everywhere I've read, in an insulator dipoles are still created and the atoms "stretch", it's just the negative charges can't separate? If there is an applied electric field, won't the atoms keep on "stretching" until the field cancels itself out anyways?
Thank you!
 A: In an insulator (dielectric) there are no (very few) mobile charge carriers and so the external electric field distorts the electron shells around the nuclei so that the atom acts like a dipole.
All the electrons are still bound to nuclei but the "centre" of the electron cloud is no longer coincident with the position of the nucleus.  

The induced dipoles are lined up and produce an induced electric field in opposition to the inducing electric field. 
The relative permittivity of the medium is a measure of how well the induced field opposes the external inducing field.
So the higher the permittivity the smaller is the electric field inside the insulator.  
Under the influence of an external electric field the mobile charge carriers in a conductor rearrange themselves (the mobile charge carriers actually move within the lattice) so that the induced electric field that they produce is equal in magnitude and opposite in direction to the external inducing field with the result that there is no electric field inside the conductor - relative permittivity is infinite.
