Potential Difference (p.d) across a component is a measure of the electrical energy transferred in the component.
Potential Difference is measured in volts (V).
It is the amount of energy (in joules) given to each coulomb of charge as it passes through the component.
P.d is measured by a voltmeter, which must be connected between the two points, in parallel with any circuit elements between the points. The figure below shows a voltmeter in parallel with an element.
The electrons moving round a circuit have some potential energy. As electrons move around a circuit, they transfer energy to the various components in the circuit. For example, when the electrons move through a lamp they transfer some of their energy to the lamp.
The amount of energy that a unit of charge (a coulomb) transfers between one point and another (the number of joules per coulomb) is called the potential difference (p.d.). Potential difference is measured in volts and so it is often referred to as voltage.
If the potential difference across a lamp, say, is 1 volt, then each coulomb of charge that passes through the lamp will transfer 1 joule of energy to the lamp.