When you add charge to an insulator the electrons stay in the same place whereas in a conductor they spread apart. Why is this? What force is making the excess charge stay in one place in an insulator?
It is also the electric force, but caused by atoms/molecules of the insulating solid.
Such an electric force is also found in conductors; the difference between conductors and insulators is in its implications on the mobility of charge carriers.
And this is fully governed by quantum theory of solids: in short, in insulators an electron can not get into motion without either:
- another electron moving in the exactly opposite direction, or
- a big amount of energy, which is very unlikely to be gained through atomic-scale movement in a "normal" electric field, nor by thermal fluctuations.
So the charges usually stay where you put them in an insulator. If you heat it up, if it absorbs UV radiation, or if the charge accumulated is strong enough to form a discharge (https://en.wikipedia.org/wiki/Lichtenberg_figure ), the repulsing charges will try to behave like in a conductor and get towards the surface.