A voltage source, V, drives an electric current, I , through resistor, R, the three quantities obeying Ohm's law: V = IR.

The voltage of a battery or power cell denotes the increase in electrical potential energy per unit charge moved from the lower (negative) to the higher (positive) pole. The situation of charges at the positive pole is analogous to that of water at the top of a water wheel (at an old saw-mill, for example). In the former case, the charges have been elevated by the chemical reactions in the battery to a state of higher electrical potential energy. In the latter case, water has been elevated by the weather to a state of higher gravitational potential energy. The work performed by the water as it passes through the water wheel from a higher to lower potential energy state is analogous the work performed by the charges in the resistor of the circuit.