# 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?

1. 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.

2. 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?

3. 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!

• The applied electric field is just not strong enough to separate electrons from its nucleus to the point where the now induced field would cancel out the applied field completely. Under normal conditions insulators cancel some of the applied field inside. Apply a strong enough field you will get a zero field inside. Aug 14, 2018 at 11:34
• But I don't understand. If it's not canceled out inside, then isn't is still strong enough to keep on creating the dipoles until the field DOES become 0? It's like (for an oversimplified example) I hang a block from a rope (the electrons being the block) and suspend it. If the rope (attraction to the atomic nucleus) is holding the block up against gravity, I would say the net force on that block is 0. If I have a bunch of blocks, the entire force on all of the blocks would also be 0. @Global Aug 14, 2018 at 11:54
• Actually, the applied E field keeps on stretching atoms till the applied force and the now induced force cancel. It just happens instantaneously, maybe of the order of $10^{-8} secs$. When you apply a very strong field to an insulator where the field is zero inside, the insulator becomes a conductor. Aug 14, 2018 at 12:04
• The conductors and the insulators, BOTH, experience the same type of force due to an applied external field i.e. like an insulator, a conductor do face a force on it. The expressions might be different but they are the same type of force. Does that make any sense or not? Aug 14, 2018 at 13:47
• Yea, I know its the same type of force, what I don't understand its why the insulator doesn't balance out the electric field inside of it once the molecules polarize Aug 14, 2018 at 15:14