In my chapter about electric fields in matter my book derives and expression for the potential due to the polarization of a dielectric material. For that you find that the polarization is equal to the potential of a collection of negative charges on the surface and positive charges inside the volume. I want to know: Do these charges represent genuine, physical charges or are the expression merely a mathematical tool? My book (Griffiths) certainly seems to believe that bound charges are indeed something physical. On the other hand I keep seeing pictures like the one attached, which wants to explain why putting a dielectric between two conductors gives a different capacitance for the capacitor they make. AND... Here both plus and minus charges are smeared over the surface of the material.