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| {{Other uses}}
| | == シャオヤンは追いつくために急いだ == |
| {{Infobox Unit
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| | name = Volt
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| | image = [[File:NISTvoltChip.jpg|240px]]
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| | caption = [[Josephson junction]] array chip developed by the [[NIST|National Bureau of Standards]] as a standard volt
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| | standard = [[SI derived unit]]
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| | quantity = [[Electric potential]], [[electromotive force]]
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| | symbol = V
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| | dimension = M·L<sup>2</sup>·T<sup>-3</sup>·I<sup>-1</sup>
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| | namedafter = [[Alessandro Volta]]
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| | extralabel = In [[SI base unit]]s:
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| | extradata = 1 V = 1 [[kilogram|kg]]·[[metre|m]]<sup>2</sup>·[[second|s]]<sup>-3</sup>·[[ampere|A]]<sup>-1</sup>
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| }}
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| The '''volt''' (symbol: {{math|'''V'''}}) is the [[SI]] [[SI derived unit|derived unit]] for [[electric potential]] ([[voltage]]), [[electric potential difference]], 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}}</ref> The volt is named in honour of the Italian physicist [[Alessandro Volta]] (1745–1827), who invented the [[voltaic pile]], possibly the first chemical [[battery (electricity)|battery]].
| | シャオヤンは追いつくために急いだ [http://www.ispsc.edu.ph/nav/japandi/casio-rakuten-6.html casio 腕時計 メンズ]。<br>戦いの技術コートを停止する前に、3段階で<br>は、すぐにパビリオンの上から3の図を席巻しており、その後、3赤いローブ老人に、そしてドンの前のバウショックを曲げる [http://www.ispsc.edu.ph/nav/japandi/casio-rakuten-13.html カシオ腕時計 メンズ]。<br><br>'三長老たちは、戦ってスキルパビリオンは、それを開いてください。「唐ジェンはかすかな手を振った [http://www.ispsc.edu.ph/nav/japandi/casio-rakuten-7.html casio 腕時計 説明書]。<br><br>は手が同時にデビュー道元異なる印刷された文の、3から道路赤色光を結び目、その後、すべての3つのスイープのシャオヤンスイープを見て3長老の雄大な雰囲気で満たされている覆われたこと、聞いたそううなずい手の放牧は、ドアを閉じたパビリオンの「ショット」 [http://www.ispsc.edu.ph/nav/japandi/casio-rakuten-6.html casio 腕時計 メンズ]。<br>表面波紋変動をガンガンこれらHongmangとして<br>「ショット」、とドアが突然投げていることは、すぐにゆっくりと開いて、ちょうどクランチの音でドアをバースト。<br><br>追いつくために迅速シャオヤン·デュオ、に開いたドア、唐ジェンリードミドンのペースラインを参照してください [http://www.ispsc.edu.ph/nav/japandi/casio-rakuten-15.html カシオ ソーラー 腕時計]。 |
| | | 相关的主题文章： |
| == Definition ==
| | <ul> |
| A single 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:
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| | <li>[http://www.fenchurch.org/priv_cgi-bin/ids/postcomment/commentviewer.cgi http://www.fenchurch.org/priv_cgi-bin/ids/postcomment/commentviewer.cgi]</li> |
| :<math>\mbox{V} = \dfrac{\mbox{kg} \cdot \mbox{m}^2}{\mbox{A} \cdot \mbox{s}^{3}}. </math>
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| | | <li>[http://www.lkwew.com/plus/view.php?aid=142137 http://www.lkwew.com/plus/view.php?aid=142137]</li> |
| It can also be expressed as amps×ohms ([[Ohm's law]]), power per unit current ([[Joule heating|Joule's law]]), or energy per unit charge:
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| | <li>[http://www.3716999.com/plus/feedback.php?aid=88 http://www.3716999.com/plus/feedback.php?aid=88]</li> |
| :<math>\text{V} = \text{A} \cdot \Omega= \dfrac{\text{W}}{\text{A}} = \dfrac{\text{J}}{\text{C}}.</math>
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| | | </ul> |
| === Josephson junction definition ===
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| Between 1990 and 1997, the volt was calibrated using the [[Josephson effect]] for exact voltage-to-frequency conversion, combined with cesium-133 time reference, as decided by the 18th [[General Conference on Weights and Measures]]. The following value for the [[Josephson constant]] is used:
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| :''K''<sub>{J-90}</sub> = 2''e''/''h'' = 0.4835979 GHz/µV,
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| where ''e'' is the [[elementary charge]] and ''h'' is the [[Planck constant]].
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| This is typically used with an array of several thousand or tens of thousands of [[Junction (semiconductor)|junctions]], excited by microwave signals between 10 and 80 GHz (depending on the array design).<ref>{{Citation |url=http://www.nist.gov/cgi-bin/view_pub.cgi?pub_id=15238 |title=1 Volt DC Programmable Josephson Voltage Standard |first1=Charles J. |last1=Burroughs |first2=Samuel P. |last2=Benz |first3=Todd E. |last4=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}}</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 |month=February |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''<sub>J</sub> and ''R''<sub>K</sub> 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 }}</ref>
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| == Water flow analogy ==
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| In the ''[[hydraulic analogy|water flow analogy]]'' sometimes used to explain electric circuits by comparing them to water-filled pipes, [[voltage]] (difference in electric potential) is likened to difference in water [[pressure]].
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| The relationship between voltage and current is defined (in ohmic devices) by [[Ohm's Law]].
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| == Common voltages ==
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| [[File:Electronic multi meter.jpg|thumb|250px| A [[multimeter]] can be used to measure the voltage between two positions.]]
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| [[File:BateriaR14.jpg|150px|thumb|1.5 V C-cell batteries]]
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| Nominal voltages of familiar sources:
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| * [[Nerve cell]] [[resting potential]]: around −75 mV<ref>Bullock, Orkand, and Grinnell, pp. 150–151; Junge, pp. 89–90; Schmidt-Nielsen, p. 484</ref>
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| * Single-cell, rechargeable [[Nickel metal hydride battery|NiMH]] or [[Nickel-cadmium battery|NiCd]] battery: 1.2 V
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| * [[Mercury battery]]: 1.355 V
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| * Single-cell, non-rechargeable [[alkaline battery]] (e.g., [[Battery (electricity)#Common battery sizes|AAA, AA, C and D cells]]): 1.5 V
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| * [[Lithium iron phosphate battery|LiFePO<sub>4</sub>]] rechargeable battery: 3.3 V
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| * [[Lithium polymer]] rechargeable battery: 3.75 V (see [[Rechargeable battery#Table of rechargeable battery technologies]])
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| * [[Transistor-transistor logic]]/[[CMOS]] (TTL) power supply: 5 V
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| * USB: 5 V DC
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| * [[PP3 battery]]: 9 V
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| * [[Automobile]] electrical system: nominal 12 V, about 11.8 V discharged, 12.8 V charged, and 13.8–14.4 V while charging (vehicle running).
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| * Household [[mains electricity]]: 230 V [[Root mean square|RMS]] in Europe, Asia and Africa, 120 V RMS in North America, 100 V RMS in Japan (see [[List of countries with mains power plugs, voltages and frequencies]])
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| * [[Truck]]s/[[lorries]]: 24 V DC
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| * [[Rapid transit]] [[third rail]]: 600–750 V (see [[List of current systems for electric rail traction]])
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| * High-speed train overhead power lines: [[25 kV AC|25 kV RMS at 50 Hz]], but see the [[list of current systems for electric rail traction]] and [[25 kV AC#60 Hz|25 kV at 60 Hz]] for exceptions.
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| * High-voltage [[electric power transmission]] lines: 110 kV RMS and up (1.15 MV RMS was the record as of 2005{{Citation needed|date=November 2009}})
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| * [[Lightning]]: Varies greatly, often around 100 MV.
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| Note: Where ''RMS'' ([[root mean square]]) is stated above, the peak voltage is <math>\sqrt{2}</math> times greater than the RMS voltage for a [[sinusoidal]] signal centered around zero voltage.
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| == History ==
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| [[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 is [[zinc]] and [[silver]]. In the 1880s, the International Electrical Congress, now the [[International Electrotechnical Commission]] (IEC), approved the volt as the unit for electromotive force. They made the volt equal to 10<sup>8</sup> [[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=http://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.
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| 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.
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| 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.
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| {{SI unit lowercase|Alessandro Volta|volt|V}}
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| == See also ==
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| {{multicol}}
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| * [[Ampere]]
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| * [[Electric potential difference]]
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| * [[List of current systems for electric rail traction|Rail traction voltage]]
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| {{multicol-break}}
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| * [[Ohm]]
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| * [[Orders of magnitude (voltage)]]
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| * [[SI electromagnetism units]]
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| * [[SI prefix]] for unit prefixes
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| {{multicol-break}}
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| * [[Voltage]]
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| * [[Voltmeter]]
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| * [[Watt]]
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| {{multicol-end}}
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| == Notes and references ==
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| {{Reflist|2}}
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| == External links ==
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| {{Wiktionary}}
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| * [http://seaus.free.fr/spip.php?article964 History of the electrical units.]
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| {{SI units}}
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| [[Category:SI derived units]]
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| [[Category:Units of electrical potential]]
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