I don't quite understand how to use rho charge per volume. I understand the applications of lamda and sigma as sigma is useful Gauss's Law and lambda is easy when dealing with wires and such. However, how the hell do you use charge per volume? I mean charges only occupy the surfaces of conductors... What law or physical concept can I use to determine how a sphere with volume charge density rho would effect well anything. Basically, what laws or tricks do I use when working with rho? How could a sphere have a volume charge density when all the charges are on the surface to begin with?