I'm only midway to finish my introduction to electromagnetism (level around Resnick-Halliday) so I understand the concepts I will talk about, but I never derived them. For example, in the case of a spherical (not empty) conductor or any other solid conductor. If we charge it with extra electrons, they will move until being on the surface of the solid. I understand this result (however not why an electron cannot stay at the center of the sphere while the others go on the surface, but this is not my main question for now at least. I know I'd have to derive that in the formal EM course.). Now if we take the case of a thin and long conductor (like a copper electric cable) and we apply a difference of potential between both extremes of the rod, the free electrons will move through the rod from a surface area to another. I don't understand why the density of electrons is the same for each cross area section. Shouldn't they move only at the surface of the conductor? Why is the case different from the static case I first described? Thanks a lot.