Put a charge displaced from origin by unit distance on z axis. Now, if you try to find out about potential in space, you can usualy read, "the potential inside a sphere is proportional to r^l, and outside a sphere is proportional to (1/r)^(l+1)". I can't understand what sphere? Is it a sphere around the origin where charge is on its surface? Is it a sphere around the charge, of radius which in magnitude equals the distance between charge and origin? Is it a sphere between origin and charge? I can't combine the position of sphere and r^l inside, (1/r)^(l+1) proportionality outside sphere. If someone could draw it and explain why does the potential depends on r^l inside, and (1/r)^(l+1) outside sphere, and also, what about the surface of sphere, and first of all, what sphere are we talking about?