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'{{other uses}} {{Infobox Unit | name = Volt | image = [[File:NISTvoltChip.jpg|240px]] | caption = [[Josephson voltage standard]] chip developed by the [[NIST|National Bureau of Standards]] as a standard volt | standard = [[SI derived unit]] | quantity = [[Electric potential]], [[electromotive force]] | symbol = V | dimension = M·L²·T⁻³·I | namedafter = [[Alessandro Volta]] | extralabel = In [[SI base unit]]s: | extradata = [[kilogram|kg]]·[[metre|m]]²·[[second|s]]⁻³·[[ampere|A]]⁻¹ }} The '''volt''' (symbol: V) is the [[SI derived unit|derived unit]] for [[electric potential]], [[electric potential difference]] ([[voltage]]), and [[electromotive force]].<ref>{{cite web| url = http://www.bipm.org/en/si/si_brochure/chapter2/2-2/table3.html| title = SI Brochure, Table 3 (Section 2.2.2)| accessdate = 2007-07-29| year = 2006| publisher = BIPM| deadurl = yes| archiveurl = https://web.archive.org/web/20070618123613/http://www.bipm.org/en/si/si_brochure/chapter2/2-2/table3.html| archivedate = 2007-06-18| df = }}</ref> It is named after the Italian physicist [[Alessandro Volta]] (1745–1827). == Definition == One volt is defined as the difference in [[electric potential]] between two points of a [[electrical conductor|conducting wire]] when an [[electric current]] of one [[ampere]] dissipates one [[watt]] of [[power (physics)|power]] between those points.<ref>[http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf BIPM SI Brochure: Appendix 1, p. 144]</ref> It is also equal to the potential difference between two parallel, infinite planes spaced 1 [[meter]] apart that create an [[electric field]] of 1 [[Newton (unit)|newton]] per [[coulomb]]. Additionally, it is the potential difference between two points that will impart one [[joule]] of [[energy]] per [[coulomb]] of charge that passes through it. It can be expressed in terms of SI base units ([[metre|m]], [[kilogram|kg]], [[second|s]], and [[ampere|A]]) as :<math alt="volt equals kilogram times meter squared per ampere per second cubed"> \text{V} = \frac{\text{potential energy}}{\text{charge}} = \frac{\text{J}}{\text{C}} = \frac{\text{kg} {\cdot} \text{m}^2}{\text{A} {\cdot} \text{s}^3}.</math> It can also be expressed as amperes times [[ohm]]s (current times resistance, [[Ohm's law]]), watts per ampere (power per unit current, definition of electric power), or joules per coulomb (energy per unit charge), which is also equivalent to [[electronvolt]]s per [[elementary charge]]: :<math alt="volt equals ampere times ohm, watt per ampere, and joules per coulomb"> \text{V} = \text{A} {\cdot} \Omega = \frac{\text{W}}{\text{A}} = \frac{\text{J}}{\text{C}} = \frac{\text{eV}}{e}.</math> === Josephson junction definition === {{main|Josephson voltage standard}} The "[[Conventional electrical unit|conventional]]" volt, V₉₀, defined in 1988 by the 18th [[General Conference on Weights and Measures]] and in use from 1990, is implemented using the [[Josephson effect]] for exact frequency-to-voltage conversion, combined with the [[Caesium standard|caesium frequency standard]]. For the [[Josephson constant]], ''K''_J = 2''e''/''h'' (where ''e'' is the [[elementary charge]] and ''h'' is the [[Planck constant]]), the "conventional" value ''K''_J-90 is used: :<math alt="K J-90 equals 0.4835979 gigahertz per microvolt"> K_\text{J-90} = 0.4835979\,\frac{\text{GHz}}{\mu\text{V}}.</math> This standard is typically realized using a series-connected array of several thousand or tens of thousands of [[Junction (semiconductor)|junctions]], excited by microwave signals between 10 and 80&nbsp;GHz (depending on the array design).<ref name=ieee-josephson>{{Citation |url=http://ieeexplore.ieee.org/xpl/articleDetails.jsp?reload=true&arnumber=783938 |title=1 Volt DC Programmable Josephson Voltage Standard |first1=Charles J. |last1=Burroughs |first2=Samuel P. |last2=Bent |first3=Todd E. |last3=Harvey |first4=Clark A. |last4=Hamilton |journal=IEEE Transactions on Applied Superconductivity |date=1999-06-01 |volume=9 |number=3 |pages=4145–4149 |issn=1051-8223 |publisher=[[Institute of Electrical and Electronics Engineers]] (IEEE) |doi=10.1109/77.783938 |accessdate=2014-06-27|bibcode=1999ITAS....9.4145B }}</ref> Empirically, several experiments have shown that the method is independent of device design, material, measurement setup, etc., and no correction terms are required in a practical implementation.<ref>{{Citation |title=Current status of the quantum metrology triangle |first=Mark W |last=Keller |url=http://qdev.boulder.nist.gov/817.03/pubs/downloads/set/Metrologia%2045,%20102.pdf |journal=Metrologia |volume=45 |number=1 |pages=102–109 |date=2008-01-18 |issn=0026-1394 |doi=10.1088/0026-1394/45/1/014 |quote=Theoretically, there are no current predictions for any correction terms. Empirically, several experiments have shown that ''K''_J and ''R''_K are independent of device design, material, measurement setup, etc. This demonstration of universality is consistent with the exactness of the relations, but does not prove it outright. |bibcode=2008Metro..45..102K |access-date=2010-04-11 |archive-url=https://web.archive.org/web/20100527094953/http://qdev.boulder.nist.gov/817.03/pubs/downloads/set/Metrologia%2045,%20102.pdf |archive-date=2010-05-27 |dead-url=yes |df= }}</ref> == Water-flow analogy == In the ''[[hydraulic analogy|water-flow analogy]]'', sometimes used to explain electric circuits by comparing them with water-filled pipes, [[voltage]] (difference in electric potential) is likened to difference in water [[pressure]]. [[electric current|Current]] is proportional to the diameter of the pipe or the amount of water flowing at that pressure. A [[resistor]] would be a reduced diameter somewhere in the piping and a [[capacitor]]/[[inductor]] could be likened to a "U" shaped pipe where a higher water level on one side could store energy temporarily. The relationship between voltage and current is defined (in ohmic devices like [[resistor]]s) by [[Ohm's law]]. Ohm's Law is analogous to the [[Hagen–Poiseuille equation]], as both are linear models relating [[flux]] and [[potential]] in their respective systems. == Common voltages == [[File:Electronic multi meter.jpg|thumb|250px| A [[multimeter]] can be used to measure the voltage between two positions.]] [[File:BateriaR14.jpg|150px|thumb|1.5&nbsp;V C-cell batteries]] The voltage produced by each [[electrochemical cell]] in a [[battery (electricity)|battery]] is determined by the chemistry of that cell. See {{Section link|Galvanic cell|Cell voltage}}. Cells can be combined in series for multiples of that voltage, or additional circuitry added to adjust the voltage to a different level. Mechanical generators can usually be constructed to any voltage in a range of feasibility. Nominal voltages of familiar sources: * [[Nerve cell]] [[resting potential]]: ~75&nbsp;mV<ref>Bullock, Orkand, and Grinnell, pp. 150–151; Junge, pp. 89–90; Schmidt-Nielsen, p. 484</ref> * Single-cell, rechargeable [[Nickel metal hydride battery|NiMH]]<ref>{{cite book|last1=Hill|first1=Paul Horowitz; Winfield|last2=Winfield|first2=Hill|title=The Art of Electronics|date=2015|publisher=Cambridge Univ. Press|location=Cambridge [u.a.]|isbn=978-0-521-809269|page=689|edition=3.}}</ref> or [[Nickel-cadmium battery|NiCd]] battery: 1.2&nbsp;V * Single-cell, non-rechargeable (e.g., [[Battery (electricity)#Common battery sizes|AAA, AA, C and D cells]]): [[alkaline battery]]: 1.5&nbsp;V;<ref>{{cite web |url= http://www.ti.com/lit/an/slva194/slva194.pdf |title= Single-cell Battery Discharge Characteristics Using the TPS61070 Boost Converter |author= SK Loo and Keith Keller |publisher= Texas Instruments |date= Aug 2004}}</ref> [[zinc-carbon battery]]: 1.56&nbsp;V if fresh and unused * [[Lithium iron phosphate battery|LiFePO₄]] rechargeable battery: 3.3&nbsp;V * [[Cobalt]]-based [[Lithium polymer]] rechargeable battery: 3.75&nbsp;V (see [[Comparison of commercial battery types]]) * [[Transistor-transistor logic]]/[[CMOS]] (TTL) power supply: 5&nbsp;V * [[USB]]: 5&nbsp;V DC * [[PP3 battery]]: 9&nbsp;V * [[Automotive battery|Automobile battery]] systems are 2.1 volts per cell; a "12V" battery is 6 cells or 12.6V; a "24V" battery is 12 cells or 25.2V. Some antique vehicles use "6V" 3-cell batteries or 6.3 volts. * [[Electric vehicle]] battery: 400&nbsp;V when fully charged<ref>{{citation |url=https://www.tesla.com/sites/default/files/downloads/20130214_ModelS_Emergency_Response_Guide.pdf |title=2012-2013 Model S Emergency Response Guide |access-date=2017-08-06}}</ref> * Household [[mains electricity]] AC: (see [[List of countries with mains power plugs, voltages and frequencies]]) ** 100&nbsp;V in Japan ** 120&nbsp;V in North America, ** 230&nbsp;V in Europe, Asia, Africa and Australia * [[Rapid transit]] [[third rail]]: 600–750&nbsp;V (see [[List of railway electrification systems]]) * High-speed train overhead power lines: [[25 kV AC|25&nbsp;kV at 50&nbsp;Hz]], but see the [[List of railway electrification systems]] and [[25 kV AC#60 Hz|25&nbsp;kV at 60&nbsp;Hz]] for exceptions. * High-voltage [[electric power transmission]] lines: 110&nbsp;kV and up (1.15&nbsp;MV was the record as of 2005{{Citation needed|date=November 2009}}) * [[Lightning]]: Varies greatly, often around 100&nbsp;MV. == History == {{refimprove section|date=June 2014}} [[Image:Alessandro Volta.jpeg|140px|thumb|Alessandro Volta]] In 1800, as the result of a professional disagreement over the galvanic response advocated by [[Luigi Galvani]], [[Alessandro Volta]] developed the so-called [[voltaic pile]], a forerunner of the [[Battery (electricity)|battery]], which produced a steady electric [[current (electricity)|current]]. Volta had determined that the most effective pair of dissimilar metals to produce electricity was [[zinc]] and [[silver]]. In 1861, [[Latimer Clark]] and Sir [[Charles Tilston Bright|Charles Bright]] coined the name "volt" for the unit of resistance.<ref>As names for units of various electrical quantities, Bright and Clark suggested "ohma" for voltage, "farad" for charge, "galvat" for current, and "volt" for resistance. See: * Latimer Clark and Sir Charles Bright (1861) [https://www.biodiversitylibrary.org/item/93052#page/483/mode/1up "On the formation of standards of electrical quantity and resistance,"] ''Report of the Thirty-first Meeting of the British Association for the Advancement of Science'' (Manchester, England: September 1861), section: Mathematics and Physics, pp. 37-38. * Latimer Clark and Sir Charles Bright (November 9, 1861) [https://babel.hathitrust.org/cgi/pt?id=nyp.33433090837166;view=1up;seq=15 "Measurement of electrical quantities and resistance,"] ''The Electrician'', '''1''' (1) : 3–4.</ref> By 1873, the British Association for the Advancement of Science had defined the volt, ohm, and farad.<ref>Sir W. Thomson, et al. (1873) [https://www.biodiversitylibrary.org/page/29853513#page/324/mode/1up "First report of the Committee for the Selection and Nomenclature of Dynamical and Electrical Units,"] ''Report of the 43rd Meeting of the British Association for the Advancement of Science'' (Bradford, September 1873), pp. 222-225. From p. 223: "The "ohm," as represented by the original standard coil, is approximately 10⁹ C.G.S. units of resistance ; the "volt" is approximately 10⁸ C.G.S. units of electromotive force ; and the "farad" is approximately 1/10⁹ of the C.G.S. unit of capacity."</ref> In 1881, the International Electrical Congress, now the [[International Electrotechnical Commission]] (IEC), approved the volt as the unit for electromotive force.<ref>(Anon.) (September 24, 1881) [https://babel.hathitrust.org/cgi/pt?id=nyp.33433090837489;view=1up;seq=309 "The Electrical Congress,"] ''The Electrician'', '''7''' : 297.</ref> They made the volt equal to 10⁸ [[cgs units]] of voltage, the cgs system at the time being the customary system of units in science. They chose such a ratio because the cgs unit of voltage is inconveniently small and one volt in this definition is approximately the emf of a [[Daniell cell]], the standard source of voltage in the telegraph systems of the day.<ref name=Hamer>{{cite book |title=Standard Cells: Their Construction, Maintenance, and Characteristics |publisher=US National Bureau of Standards |last=Hamer |first=Walter J. |date=January 15, 1965 |series=National Bureau of Standards Monograph #84 |url=https://www.nist.gov/calibrations/upload/mn84.pdf}}</ref> At that time, the volt was defined as the potential difference [i.e., what is nowadays called the "voltage (difference)"] across a conductor when a current of one [[ampere]] dissipates one [[watt]] of power. [[File:PSM V85 D521 Group photograph of herman helmholtz and academic friends.png|left|thumb|Group photograph of [[Hermann von Helmholtz|Hermann Helmholtz]], his wife (seated) and academic friends [[Hugo Kronecker]] (left), [[Thomas Corwin Mendenhall]] (right), [[Henry Villard]] (center) during the International Electrical Congress]] The "international volt" was defined in 1893 as 1/1.434 of the [[Electromotive force|emf]] of a [[Clark cell]]. This definition was abandoned in 1908 in favor of a definition based on the international [[ohm]] and international ampere until the entire set of "reproducible units" was abandoned in 1948. Prior to the development of the Josephson junction voltage standard, the volt was maintained in national laboratories using specially constructed batteries called "[[Weston cell|standard cells]]". The United States used a design called the [[Weston cell]] from 1905 to 1972. A [[proposed redefinition of SI base units]], including defining the value of the [[elementary charge]], is expected to take effect in 2019.<ref name=draft-resolution-A> {{citation |title=Draft Resolution A "On the revision of the International System of units (SI)" to be submitted to the CGPM at its 26th meeting (2018) |url=https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN.pdf }}</ref> == See also == {{Portal|Energy}} {{div col|colwidth=24em}} * [[Orders of magnitude (voltage)]] * [[List of railway electrification systems|Rail traction voltage]] * [[SI electromagnetism units]] * [[SI prefix]] for unit prefixes * [[Railway electrification system#Standardised voltages|Standardised railway voltages]] * [[Voltmeter]] {{div col end}} == References == {{Reflist|40em}} == External links == {{Wiktionary}} * [http://histoires-de-sciences.over-blog.fr/2013/11/electrical-units-history.html History of the electrical units.] {{SI units}} [[Category:SI derived units]] [[Category:Units of electrical potential]] [[Category:Alessandro Volta]]'