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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.

References

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+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.
-{{For|electric potential difference|Voltage}}
+{{cite book
+  | authorlink = J.C. Maxwell
+  | title = A treatise on electricity and magnetism (Vol. 1)
+  | publisher = Clarendon
+  | date = first printed 1891, reprinted 1998
+  | location = Oxford
+  | isbn = 0-19-850373-3
+}}
+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]].
-In [[physics]], the '''potential difference''' or '''p.d.''' between two points is the difference of the points' [[scalar potential]],  equivalent to the  line integral of the [[field strength]] between the two points.
+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
-There are various types of potential difference  related to the amount of [[energy]] required to move an object from one place to another against various types of [[force]]s.
+{{cite book
+  | authorlink = N.W. Ashcroft
+  | coauthors = Mermin, N.D.
+  | title = Solid State Physics
+  | publisher = Holt, Reinhart and Winston
+  | date = 1976
+  | location = New York
+}}
+- 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.
-* in [[electrodynamics]], the potential difference corresponds to the  amount of [[Work (thermodynamics)|work]] that would need to be done on a unit [[electric charge]] to move it from one point to the other against an [[electric field]].<ref>[http://www.sasked.gov.sk.ca/docs/physics/u3b23phy.html Physics 30 - Electricity - Electric Potential Difference<!-- Bot generated title -->]</ref>  P.d. is synonymous with [[voltage]] <ref>[http://www.powerstream.com/1922/battery_1922_WITTE/batteryfiles/definitions.htm Battery - Definitions<!-- Bot generated title -->]</ref> and is measured in [[volt]]s. It is the line integral of the [[electric field strength]] between two points.<ref>Pitt, V.H. (ed.), ''The Penguin Dictionary of Physics'', Penguin Books Ltd., 1977, ISBN 0140510710</ref> Also, if ''v'' is the p.d. in volts, ''w'' is the work in [[joule]]s (J), and ''q'' is the charge in [[coulomb]]s (C), then:
-*:<math> v = { w \over q } {\mathrm{J} \over \mathrm{C}} \ \mathrm{or} \ {w \over q} \mathrm{V}  </math>
-*:{{main|Voltage}}
-* In [[mechanics]], the ''[[gravitational potential]] difference'' between two points on Earth is related to the energy that would be required to move a unit [[mass]] from one point to the other against the Earth's [[gravity|gravitational field]]. Units: joules per kilogram.
-* In fluid systems the ''potential difference'' is the difference in [[pressure]]. Units: [[pascal (unit)|pascal]]s.
-* In thermal systems the ''potential difference'' is the difference in [[temperature]]. Units: [[kelvin]]s.
-In some engineering fields, "potential" is sometimes described as the '[[across variable]]', whereas [[flux]] is the '[[through variable]]'.  The product of the flux and the potential difference is the [[Power (physics)|power]], which is the time rate of change of [[energy]].
 ==References==