View source for Ampere

{{Other uses}}
{{Infobox Unit
| bgcolour = [[#0000FF]]
| name = Ampere
| image = [[File:Amperemeter hg.jpg|200px]] 
| caption = Demonstration model of a moving iron ammeter. As the current through the coil increases, the plunger is drawn further into the coil and the pointer deflects to the right. 
| standard = [[SI base unit]]
| quantity = [[Electric current]]
| symbol = A
| dimension = I
| namedafter = [[André-Marie Ampère]]
| units1 = 
| inunits1 = 
| units2 = 
| inunits2 =
}}

The '''ampere''' (SI [[Units of measurement|unit]] symbol: {{math|A}}; SI [[Dimension of a physical quantity|dimension]] symbol: {{math|I}}), often [[Clipping (morphology)|shortened]] to '''amp''',<ref name=BIPM2006>SI supports only the use of symbols and deprecates the use of abbreviations for units.{{cite web | format = [[PDF]] | url= http://www.bipm.fr/utils/common/pdf/si_brochure_8_en.pdf |title=Bureau International des Poids et Mesures |year=2006  |page=130 | accessdate =2011-11-21}}</ref> is the [[International System of Units|SI]] unit of [[electric current]]<ref name="BIPMdefinition">{{Citation | chapter = 2.1. Unit of electric current (ampere)|url= http://www.bipm.org/en/si/si_brochure/chapter2/2-1/ampere.html | title =SI brochure | edition = 8th |publisher= BIPM|accessdate= 19 November 2011}}</ref><ref>[http://physics.nist.gov/cuu/Units/ampere.html Base unit definitions: Ampere]. Physics.nist.gov. Retrieved on 2010-09-28.</ref>  (quantity symbol: {{mvar|I}}, {{mvar|i}})<ref>{{Citation | chapter =2. SI base units|url=http://www.bipm.org/en/si/si_brochure/chapter2/2-1/ | title = SI brochure | edition = 8th | publisher = BIPM|accessdate= 19 November 2011}}</ref> and is one of the seven<ref>The other six are the [[metre]], [[kelvin]], [[second]], [[Mole (unit)|mole]], [[candela]], and [[kilogram]]</ref> [[SI base unit]]s. It is named after [[André-Marie Ampère]] (1775–1836), French mathematician and physicist, considered the father of [[electrodynamics]].

In practical terms, the ampere is a measure of the amount of [[electric charge]] passing a point in an electric circuit per unit time, with {{val|6.241|e=18}} [[electron]]s (or one [[coulomb]]) per second constituting one ampere.<ref>{{Citation | author-link = David Bodanis| last = Bodanis | first = David | year = 2005 | title = Electric Universe | place = New York | publisher = Three Rivers Press | ISBN = 978-0-307-33598-2}}</ref>

The practical definition may lead to confusion with the definition of the [[coulomb]] (i.e., 1 ampere-second) and the [[ampere-hour]] (A·h), but in practical terms this means that measures of a constant current (e.g., the nominal flow of charge per second through a simple circuit) will be defined in amperes (e.g., "a 20 mA circuit") and the flow of charge through a circuit over a period of time will be defined in coulombs (e.g., "a variable-current circuit that flows a total of 10 coulombs over 5 seconds"). In this way, amperes can be viewed as a flow rate, i.e. number of (charged) particles transiting per unit time, and coulombs simply as the number of particles.

== Definition ==
[[File:Ampere-def-en.svg|thumb|Illustration of the definition of the ampere unit]]
[[Ampère's force law]]<ref name=Serway>{{cite book
| first1 =Raymond A | last1 = Serway | last2 = Jewett | first2 = JW
|title=Serway's principles of physics: a calculus based text
|url=http://books.google.com/books?id=1DZz341Pp50C&pg=RA1-PA746&dq=wire+%22magnetic+force%22&lr=&as_brr=0&sig=4vMV_CH6Nm8ZkgjtDJFlupekYoA#PRA1-PA746,M1 |publisher=Thompson Brooks/Cole
|edition=Fourth
|location=Belmont, CA
|year=2006
|page=746
|isbn = 0-53449143-X}}</ref><ref name = "Beyond">{{Citation | url = http://www.nist.gov/public_affairs/newsfromnist_beyond_the_kilogram.htm | title = Beyond the Kilogram: Redefining the International System of Units | year = 2006 | publisher = [[National Institute of Standards and Technology]] | place = USA | accessdate = March 2008| archiveurl= http://web.archive.org/web/20080321221139/http://www.nist.gov/public_affairs/newsfromnist_beyond_the_kilogram.htm| archivedate= 21 March 2008 <!--DASHBot-->| deadurl= no}}.</ref> states that there is an attractive or repulsive force between two parallel wires carrying an electric current. This force is used in the formal definition of the ampere, which states that it is "the constant current that will produce an attractive force of 2&nbsp;&times;&nbsp;10<sup>–7</sup> [[newton (unit)|newton]] per metre of length between two straight, parallel conductors of infinite length and negligible circular [[cross section (geometry)|cross section]] placed one [[metre]] apart in a [[vacuum]]".<ref name= "BIPMdefinition" /><ref>{{Citation | first = Paul MS | last = Monk | title = Physical Chemistry: Understanding our Chemical World | publisher = John Wiley & Sons | year = 2004 | ISBN = 0-471-49180-2 | url = http://books.google.com/books?vid=ISBN0471491802&id=LupAi35QjhoC&pg=PA16&lpg=PA16&ots=IMiGyIL-67&dq=ampere+definition+si&sig=9Y0k0wgvymmLNYFMcXodwJZwvAM}}.</ref>

The [[SI]] unit of charge, the [[coulomb]], "is the quantity of electricity carried in 1 second by a current of 1 ampere".<ref>{{Citation | publisher = [[Bureau International des Poids et Mesures]] | year = 2006 | url = http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf | title = The International System of Units (SI) | edition = 8th | page = 144 | format = PDF}}.</ref> Conversely, a current of one ampere is one coulomb of charge going past a given point per second:
:<math>\rm 1\ A=1\tfrac C s.</math>
In general, charge ''Q'' is determined by steady current ''I'' flowing for a time ''t'' as ''Q'' = ''It''.

== History ==
The ampere was originally defined as one tenth of the [[Centimetre gram second system of units|CGS system]] electromagnetic unit of current (now known as the [[abampere]]), the amount of current that generates a force of two [[dyne]]s per centimetre of length between two wires one centimetre apart.<ref>{{Citation | url = http://alpha.montclair.edu/~kowalskiL/SI/SI_PAGE.HTML | title = A short history of the SI units in electricity | publisher = Montclair | last = Kowalski | first = L}}.</ref> The size of the unit was chosen so that the units derived from it in the [[International System of Units|MKSA system]] would be conveniently sized.

The "international ampere" was an early realization of the ampere, defined as the current that would deposit {{val|fmt=commas|0.001118000}} grams of silver per second from a [[silver nitrate]] solution.<ref>{{Citation | url = http://www.sizes.com/units/ampHist.htm | title = History of the ampere | publisher = Sizes}}</ref> Later, more accurate measurements revealed that this current is 0.99985&nbsp;A.

== Realization ==
The standard ampere is most accurately realized using a [[watt balance]], but is in practice maintained via [[Ohm's law]] from the units of [[electromotive force]] and [[electrical resistance|resistance]], the [[volt]] and the [[ohm]], since the latter two can be tied to physical phenomena that are relatively easy to reproduce, the [[Josephson junction]] and the [[quantum Hall effect]], respectively.<ref name = "Electrical quantities">{{Citation | url = http://www.bipm.org/en/si/si_brochure/appendix2/electrical.html | title = SI brochure | chapter = Appendix 2: Practical realization of unit definitions: Electrical quantities | publisher = BIPM}}.</ref>

At present, techniques to establish the realization of an ampere have a [[relative uncertainty]] of approximately a few parts in 10<sup>7</sup>, and involve realizations of the watt, the ohm and the volt.<ref name= "Electrical quantities" />

== Proposed future definition ==
{{Main|New SI definitions}}
Rather than a definition in terms of the force between two current-carrying wires, it has been proposed to define the ampere in terms of the rate of flow of elementary charges.<ref name = "Beyond" />  Since a [[coulomb]] is approximately equal to {{val |6.2415093|e=18}} [[elementary charge]]s (such as [[electron]]s), one ampere is approximately equivalent to {{val|fmt=commas|6.2415093|e=18}} elementary charges moving past a boundary in one second, or the reciprocal of the value of the elementary charges in coulombs.<ref>{{Citation | url = http://physics.nist.gov/cgi-bin/cuu/Value?e | publisher = NIST | place = USA | title = Physics | contribution = Value}}.</ref> The proposed change would define 1 A as being the current in the direction of flow of a particular number of elementary charges per second. In 2005, the [[International Committee for Weights and Measures]] (CIPM) agreed to study the proposed change. The [[New SI definitions|new definition]] is expected to be formally proposed at the 25th [[General Conference on Weights and Measures]] (CGPM) in 2014.<ref>{{cite press release
| url = http://www.bipm.org/utils/en/pdf/Press_release_resolution_1_CGPM.pdf
| title = General Conference on Weights and Measures approves possible changes to the International System of Units, including redefinition of the kilogram. 
| publisher = [[General Conference on Weights and Measures]]
| location = Sèvres, France
| date = 23 October 2011
| accessdate = 25 October 2011}}</ref>

== Everyday examples ==
{{main|Orders of magnitude (current)}}
The current drawn by typical constant-voltage energy distribution systems is usually dictated by the power ([[watt]]s) consumed by the system and the operating voltage. For this reason the examples given below are grouped by voltage level.

=== Portable gadgets ===
*Hearing aid (typically 1&nbsp;mW at 1.4&nbsp;V): 0.7&nbsp;mA

=== Motor vehicles – 12 V DC ===
A typical motor vehicle has a 12&nbsp;V battery. The various accessories that are powered by the battery might include:
*Instrument panel light (typically 2&nbsp;W): 166&nbsp;mA.
*Headlights (typically 60&nbsp;W): 5&nbsp;A each.
*Starter Motor (typically 1–2&nbsp;kW): 80-160&nbsp;A

=== North American domestic supply – 120 V AC ===
Most United States, Canada and Mexico domestic power suppliers run at 120&nbsp;V.

Household [[circuit breaker]]s typically provide a maximum of 15&nbsp;A or 20&nbsp;A of current to a given set of outlets.
*22-inch/56-centimeter portable television (35&nbsp;W): 290&nbsp;mA
*Tungsten light bulb (60–100&nbsp;W): 500–830&nbsp;mA
*Toaster, kettle (2&nbsp;kW): 16.6&nbsp;A
*Immersion heater (4.6&nbsp;kW): 38.3&nbsp;A

=== European & Commonwealth domestic supply – 230-240 V AC ===
Most European domestic power supplies run at 230&nbsp;V, and most Commonwealth domestic power supplies run at 240&nbsp;V. The current drawn by a particular European or Commonwealth appliance (in Europe or a Commonwealth country) will be less than for an equivalent North American appliance.<ref group="Note">The formula for power is given by
:<math>
P(t) = I(t) \cdot V(t) \,
</math>
so it follows that if the voltage is doubled and the power remains the same, the current will be halved.</ref> Typical circuit breakers will provide 16&nbsp;A.

The current drawn by a number of typical appliances are:
*22-inch/56-centimeter Portable Television (35&nbsp;W): 145–150&nbsp;mA
*Tungsten light bulb (60–100&nbsp;W): 240–450&nbsp;mA
*Compact Fluorescent Lamp (11–30&nbsp;W): 56–112&nbsp;mA
*Toaster, kettle (2&nbsp;kW): 9&nbsp;A
*Immersion heater (4.6&nbsp;kW): 19-20&nbsp;A

== See also ==
* [[Ammeter]]
* [[Ampacity]] (current-carrying capacity)
* [[Electric current]]
* [[Electric shock]]
* [[Hydraulic analogy]]
* [[Magnetic constant]]

== Notes ==
{{Reflist | group = "Note"}}

== References ==
{{Reflist|colwidth=30em}}

== External links ==
* [http://physics.nist.gov/cuu/ The NIST Reference on Constants, Units, and Uncertainty]
* [http://alpha.montclair.edu/~kowalskiL/SI/SI_PAGE.HTML  A short history of the SI units in electricity]{{Dead link|date=April 2012}}
* [http://physics.nist.gov/cuu/Units/ampere.html NIST ''Definition of ampere and μ<sub>0</sub>'']

{{SI units}}

{{Use dmy dates|date=June 2013}}

[[Category:SI base units]]
[[Category:Units of electric current]]