Even when a voltage source is not connected to a circuit, it still maintains an electrical potential between its two terminals. This illustration shows two wires attached to a 12V battery, and depicts the field lines (gray pattern) and the lines of equal potential (yellow). Charge does not flow on the surface of the wires because the field lines are perpendicular to the wires, nor inside the wires the field because the field is zero there. Select any two points in the space around or between the wires, and the potential difference between those two points is shown by the number of equipotential lines between them. The potential difference between the wires is 12V, and the potential difference between two points on the same wire is zero.