Location of free charge in insulators I'm going through the introductory section to Electrostatics in Materials in Griffiths, and I have a question that I can't seem to find a satisfactory answer to.
If I have an insulator with free charge, is it necessarily confined to the surface?
In the case of a conductor, Gauss' law immediately gives a "yes", because no electric field can exist in a conductor, leading us to conclude that there is no free charge inside the surface. But insulators can have electric fields inside them. Does this mean that free charge can exist inside the volume? Or does the free charge still move to the surface of the insultator?
 A: Dielectrics can be both conductors and insulators. For a capacitor plate, an insulator is the best thing to use. It is due to this insulation property that means no free charges, that creates an electric field to counter the one between the plates due to some surface charge density. 
If you are talking about conductors used as dielectrics, then surely, the internal field is zero (from Gauss' law as you said). But then, this is the reason why conductors are not used as dielectrics, because of the charge leakage as current in capacitors. 
A: The question is not entirely clear; I wonder if you are really asking about bound charge (polarization charge). 
Free charge is charge that is free to move macroscopically; bound charge is charge that can only move on a microscopic or submicroscopic scale.  Free charge ends up on the surface of a dielectric medium under static conditions. 
In a uniform dielectric slab subjected to a uniform electric field, bound charge is displaced in proportion to the strength of the electric field, but there is no net bound charge density except at the surfaces of the slab.
However net density of bound charge is confined to the surface of a dielectric only if the dielectric and the electric field is uniform. See this Feynman lecture. If, for example, there are two slabs of different dielectric media laminated together and an electric field is imposed across the combined slab, free charge will exist at the interface between the two.  Or, if a slab of material were prepared containing a gradient of ratio of two substances with different dielectric constant - so that the net dielectric constant in the material has a gradient - and an electric field is applied across the slab, bound charge will be distributed throughout the volume of the slab.
