Why cant charge move in an insulator?

[aaaa202]

I am a bit confused about what actually makes something insulating. As far as I know a conductor has free electrons so when you put it in an external E-field these will move to terminate the external field thereby producing a currrent. But then I sat down and thought, why do charges pile up at the surface of a conductor in air?

Why don't they just continue through air. Surely air is almost empty space so why wouldn't they? My book say's it's because air insulates, so can someone tell me what insulative proterty that makes the charges unable to continue through air? or vacuum for that matter

 

[Quinzio]

Well, I'll be not 100% accurate, but I suppose electrons in metals are not really free, but they are "shared" between adjacent atoms which have them in common on their external orbits.
So electrons are free must they must still belong to an atom.
To pull off electrons from metals, you need high voltages. Pieces of metals that emits electrons are called cathodes. You need E-fields of thousands of V/m, and if the cathode is hot is better. You may look how it works in the "old" CRT tubes.

 

[tiny-tim]

hi aaaa202!

how can an electron leave the conductor?

that would make the conductor positively charged, and it would just attract the electron back again, a lot more strongly than the field is trying to pull it out

inside the conductor, the electron is attracted to various positive sites, and doesn't much mind which one it's nearest to … a small field will make it move from one to the next

but an insulator has no such sites for the electron to go to … the electric field would have to be extremely strong (the breakdown voltage for the insulator) to launch the electron through it

 

[azizlwl]

Energy Bands

Recall that the valence shell of an atom represents a band of energy levels and that the valence electrons are confined to that band. When an electron acquires enough additional energy, it can leave the valence shell, become a free electron, and exist in what is known as the conduction band.

The difference in energy between the valence band and the conduction band is called an
energy gap. This is the amount of energy that a valence electron must have in order to jump from the valence band to the conduction band. Once in the conduction band, the electron is free to move throughout the material and is not tied to any given atom.
http://img38.imageshack.us/img38/6591/bandsn.jpg