Potential difference

In the physics of electrical circuits, the term potential difference or p.d. is sometimes used as an old-fashioned synonym for the modern quantity known as "the voltage (difference) between two positions in an electrical circuit". Following the discovery of the electron by J.J. Thomson in 1897, and later discoveries about electron behaviour and the role of electrons in the conduction of electricity in metals, it is now known that a "voltage difference" (as measured with a voltmeter) is not the same scientific quantity as the pre-atomic-era physical quantity "electric potential difference" (discussed, for example, by Maxwell in the 1891 edition of textbook. A treatise on electricity and magnetism (Vol. 1). Oxford: Clarendon. first printed 1891, reprinted 1998. ISBN 0-19-850373-3. {{cite book}}: Check date values in: |date= (help) In the context of electrical circuits, use of the term "potential difference" as a synonym for voltage (difference) is dropping out of use. This may be partly because science has no name (other than voltage) for the potential concerned, partly because of the possibility of confusion between the terms "potential difference" and "electric potential difference", which nowadays refer to different physical things. Use of the term "potential difference" as a synonym for voltage (difference) should be regarded as obsolescent/obsolete, and it is recommended that it should not be employed.

For further information on the science involved, see the articles on Voltage and Electric potential.

It is quite common for physics undergraduates either to be taught (incorrectly) that "electric potential difference" and "voltage difference" mean the same thing, or for the issue of "what voltage really is" to be avoided (presumably, on the grounds that this is too complicated for them to understand). Some older textbooks are also incorrect or ambiguous, or do not discuss the issue. However, some advanced solid-state textbooks (e.g., Ashcroft and Merrin Solid State Physics. New York: Holt, Reinhart and Winston. 1976. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help) - see section on Thermoelectric Power) do clearly acknowledge that in principle voltmeters do not measure electric potential difference.

Part of the problem with electricity is that real electric currents involve the flow of electrons in the opposite direction to conventional current; another part of the problem is that electrons are subject to "chemical" effects as well as "electrostatic" effects. This kind of difficulty does not arise in many other areas of physics (e.g., the theory of gravitation): in these areas, there is an unique definition of the related potential, and "potential difference" can be defined without ambiguity as the difference in the related potential.