Volt
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{{Otheruses4|the [[SI derived unit]]|the [[plug-in hybrid]] [[concept car]]|Chevrolet Volt}}
{{Redirect|Volts|the [[AC/DC]] album|Volts (album)}}
[[Image:NISTvoltChip.jpg|thumb|Josephson junction array chip developed by [[NIST]] as a standard volt.]]
The '''volt''' (symbol: '''V''') is the [[SI]] [[SI derived unit|derived unit]] of electric [[potential difference]] or [[electromotive force]].<ref name="BIPM Definition">{{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|date= [[2006]]| publisher = BIPM}}</ref><ref>Rudolf F. Graf, "Volt", Dictionary of Electronics; Radio Shack, 1974-75. Fort Worth, Texas. ISBN B000AMFOZY</ref> It is named in honor of the [[Lombards|Lombard]] [[physicist]] [[Alessandro Volta]] (1745–1827), who invented the [[voltaic pile]], the first chemical [[battery (electricity)|battery]].
== Definition ==
The volt is defined as the [[potential difference]] across a conductor when a [[Electric current|current]] of one [[ampere]] dissipates one [[watt]] of [[power (physics)|power]].<ref>[http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf BIPM SI Brochure: Appendix 1, p. 144]</ref> Hence, it is the base SI representation [[metre|m]]<sup>2</sup> · [[kilogram|kg]] · [[second|s]]<sup>-3</sup> · [[ampere|A]]<sup>-1</sup>, which can be equally represented as one [[joule]] of [[energy]] per [[coulomb]] of charge, J/C.
:<math>\mbox{V} = \dfrac{\mbox{W}}{\mbox{A}} = \dfrac{\mbox{J}}{\mbox{C}} = \dfrac{\mbox{N} \cdot \mbox{m} }{\mbox{A} \cdot \mbox{s}} = \dfrac{\mbox{kg} \cdot \mbox{m}^2}{\mbox{A} \cdot \mbox{s}^{3}}</math>
===Josephson junction definition===
Since 1990 the volt is maintained internationally for practical measurement using the [[Josephson effect]], where a conventional value is used for the [[Josephson constant]], fixed by the 18th [[General Conference on Weights and Measures]] as
:'''K<sub>{J-90}</sub>''' = 0.4835979 GHz/µV.
== Hydraulic analogy ==
In the ''[[hydraulic analogy]]'' sometimes used to explain electric circuits by comparing them to water-filled pipes, [[voltage]] is likened to water [[pressure]] – it determines how fast the electrons will travel through the circuit. Current (in amperes), in the same analogy, is a measure of the volume of water that flows past a given point per unit time ([[volumetric flow rate]]). The flow rate is determined by the width of the pipe (analogous to [[electrical resistance]]) and the pressure difference between the front end of the pipe and the exit ([[potential difference]] or [[voltage]]). The analogy extends to power dissipation: the power given up by the water flow is equal to flow rate times pressure, just as the power dissipated in a resistor is equal to current times the voltage drop across the resistor (amperes x volts = watts).
The relationship between voltage and current (in ohmic devices) is defined by [[Ohm's Law]].
== Common voltages ==
[[Image:Electronic multi meter.jpg|thumb|250px| A [[multimeter]] can be used to measure the voltage between two points]]
[[Image:BateriaR14.jpg|150px|thumb|1.5 V C-cell batteries]]
Nominal voltages of familiar sources:
* [[Nerve cell]] [[action potential]]: around 75 mV<ref>The dual effect of membrane potential on sodium conductance in the giant axon of Loligo. ''J Physiol'' 1952 Apr;116(4):497-506. PMID 14946715</ref>
* Single-cell, rechargeable [[Nickel metal hydride battery|NiMH]] or [[Nickel-cadmium battery|NiCd]] battery: 1.2 V
* [[Mercury battery]]: 1.355 V
* Single-cell, non-rechargeable [[alkaline battery]] (e.g. [[Battery (electricity)#Common battery sizes|AAA, AA, C and D cells]]): 1.5 V
* [[Lithium polymer]] rechargeable battery: 3.75 V
* [[Transistor-transistor logic]]/[[CMOS]] (TTL) power supply: 5 V
* [[PP3 battery]]: 9 V
* [[Automobile]] electrical system: "12 V", about 11.8 V discharged, 12.8 V charged, and 13.8-14.4 V while charging (vehicle running).
* Household [[mains electricity]]: 240 V RMS in Australia, 230 V 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]])
* [[Rapid transit]] [[third rail]]: 600 to 750 V (see [[List of current systems for electric rail traction]])
* High speed train overhead power lines: 25 kV RMS at 50 Hz, but see the [[list of current systems for electric rail traction]] for exceptions.
* High voltage [[electric power transmission]] lines: 110 kV RMS and up (1150 kV RMS was the record as of 2005)
* [[Lightning]]: Varies greatly, often around 100 MV.
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.
== History of the volt ==
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 the 1880s, the International Electrical Congress, now the [[International Electrotechnical Commission]] (IEC), approved the volt for electromotive force. At that time, the volt was defined as the potential difference across a conductor when a current of one [[ampere]] dissipates one [[watt]] of power.
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 cell]]s'''. The United States used a design called the [[Weston cell]] from 1905 to 1972.
{{SI unit lowercase|Alessandro Volta|volt|V}}
== See also ==
{{Wiktionary}}
* [[SI]] for unit prefixes
* [[SI electromagnetism units]]
* [[Voltage]]
* [[Voltmeter]]
* [[Watt]]
* [[Ampere]]
==References==
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==External links==
* [http://video.google.com/videoplay?docid=5645396659673218353&q=Physics+for+Future+Presidents+Electricity&total=5&start=0&num=10&so=0&type=search&plindex=0#0h20m30s Google Video Lecture about Electricity and Volt by Richard A. Muller]
* [http://www.aqdi.com/rms.htm What is RMS?]
[[Category:SI derived units]]
[[Category:Units of electrical potential]]
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