Charge moving in an electric circuit gives rise to a current, Naturally, it must take some work, or energy, for the charge to move between two points in a circuit, say, from point a to point b. The total work per unit charge associated with the motion of charge between two points is called voltage. Thus, the units of voltage are those of energy per unit charge.

The voltage, or potential difference, between two points in a circuit indicates the energy required to move charge from one point to the other.

The role played by a voltage source in an electric circuit is equivalent to that played by the force of gravity.

Raising a mass with respect to a reference surface increases its potential energy. This potential energy can be converted to kinetic energy when the object moves to a lower position relative to the reference surface.

The voltage, or potential difference, across a voltage source plays an analogous role, raising the electrical potential of the circuit, so that charge can move in the circuit, converting the potential energy within the voltage source to electric power.