When revisiting Ohm's Law, I wondered why (I know it's experimentally derived) the voltage change between two points would depend on current. I could understand this dependence with total energy dissipated in moving a current across a resistor but I can not understand why the P.D. (i.e. the work done per unit charge) depends on the current being passed. Essentially, I'm asking why does the amount of energy needed to move a single unit charge depend on how many charge carriers there are with it?