Electric potential or electrical potential is a measure of the amount of electrical energy available at a given point in space or point in an electrical circuit. It is measured in volts. A volt is defined as the amount of energy (in joules) that will be gained by an electric charge of one coulomb as it moves from one point in space to another. It is also the measure of the continuous electric power (in watts) that is generated as a current on one ampere flows from one point to another.
People often use the word "voltage" to refer to this electric potential difference, since it is a much simpler term. An old-fashioned term for the same phenomenon was "electromotive force". One still sees references to "E.M.F.".
Voltage appears in many extremely common formulas relating to such things as resistance, current, and power. For example, one can calculate voltage as power divided by current, or work (or energy) divided by electric charge.
Electric potential is, for all practical purposes, measured as a difference between one point in space and another. If a coulomb of charge gains 3 joules moving from point X to point Y, then we say that point X is 3 volts higher in potential than Y. If it gains another 2 joules moving from Y to Z, Y is 2 volts higher than Z. The principle of conservation of energy means that X is 5 volts higher than Z. We can thereby assign voltages to any point in space, or in a circuit, with the understanding that these voltages are all relative to each other, and there is no "absolute" measure of voltage. This is the essence of Kirchoff's voltage law.
Usually a convention is made that some point, called "ground", is assigned an electric potential of zero, so that all other points in a circuit can have voltages assigned to them.
- Schwarz, Stephen E. and William G. Oldham. Electrical Engineering: An Introduction, 2e. Oxford University Press: 1993.
- The International System of Units, 8th edition. Organisation Intergouvernementale de la Convention du Mètre, 2006.