Wikipedia

Ampere

(Redirected fromMilliamp)
For other uses, seeAmpere (disambiguation).

Theampere(/ˈæmpɛər/AM-pair,US:/ˈæmpɪər/AM-peer;[1][2][3]symbol:A),[4]oftenshortenedtoamp,[5]is the unit ofelectric currentin theInternational System of Units(SI). One ampere is equal to 1coulomb(C) moving past a point per second.[6][7][8]It is named afterFrenchmathematician and physicistAndré-Marie Ampère(1775–1836), considered the father ofelectromagnetismalong withDanishphysicistHans Christian Ørsted.

ampere
Demonstration model of a moving ironammeter.As the current through the coil increases, the plunger is drawn further into the coil and the pointer deflects to the right.
General information
Unit systemSI
Unit ofelectric current
SymbolA
Named afterAndré-Marie Ampère

As of the2019 revision of the SI,the ampere is defined by fi xing theelementary chargeeto be exactly1.602176634×10−19C,[6][9]which means an ampere is an electric current equivalent to1019elementary charges moving every1.602176634seconds or6.241509074×1018elementary charges moving in a second. Prior to the redefinition the ampere was defined as the current passing through two parallel wires 1metreapart that produces a magnetic force of2×10−7newtonsper metre.

The earlierCGS systemhas two units of current, one structured similarly to the SI's and the other usingCoulomb's lawas a fundamental relationship, with the CGS unit of charge defined by measuring the force between two charged metal plates. The CGS unit of current is then defined as one unit of charge per second.[10]

History

edit
Main article:International System of Electrical and Magnetic Units

The ampere is named for French physicist and mathematicianAndré-Marie Ampère(1775–1836), who studiedelectromagnetismand laid the foundation ofelectrodynamics.In recognition of Ampère's contributions to the creation of modern electrical science, an international convention, signed at the 1881International Exposition of Electricity,established the ampere as a standard unit of electrical measurement for electric current.

The ampere was originally defined as one tenth of the unit ofelectric currentin thecentimetre–gram–second system of units.That unit, now known as theabampere,was defined as the amount of current that generates a force of twodynesper centimetre of length between two wires one centimetre apart.[11]The size of the unit was chosen so that the units derived from it in theMKSAsystem would be conveniently sized.

The "international ampere" was an early realization of the ampere, defined as the current that would deposit0.001118gramsof silver per second from asilver nitratesolution. Later, more accurate measurements revealed that this current is0.99985A.[12]

Sincepoweris defined as the product of current and voltage, the ampere can alternatively be expressed in terms of the other units using the relationshipI=P/V,and thus 1 A = 1 W/V. Current can be measured by amultimeter,a device that can measure electrical voltage, current, and resistance.

Former definition in the SI

edit

Until 2019, the SI defined the ampere as follows:

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed onemetreapart in vacuum, would produce between these conductors a force equal to2×10−7newtonsper metre of length.[13]: 113 [14]

Ampère's force law[15][16]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.

The SI unit of charge, thecoulomb,was then defined as "the quantity of electricity carried in 1 second by a current of 1 ampere".[13]: 144 Conversely, a current of one ampere is one coulomb of charge going past a given point per second:

In general, chargeQwas determined by steady currentIflowing for a timetasQ=It.

This definition of the ampere was most accurately realised using aKibble balance,but in practice the unit was maintained viaOhm's lawfrom the units ofelectromotive forceandresistance,thevoltand theohm,since the latter two could be tied to physical phenomena that are relatively easy to reproduce, theJosephson effectand thequantum Hall effect,respectively.[17]

Techniques to establish the realisation of an ampere had arelative uncertaintyof approximately a few parts in 107,and involved realisations of the watt, the ohm and the volt.[17]

Present definition

edit

The2019 revision of the SIdefined the ampere by taking the fixed numerical value of theelementary chargeeto be1.602176634×10−19when expressed in the unit C, which is equal to A⋅s, where the second is defined in terms of∆νCs,the unperturbed ground state hyperfine transition frequency of thecaesium-133 atom.[18]

The SI unit of charge, thecoulomb,"is the quantity of electricity carried in 1 second by a current of 1 ampere".[19]Conversely, a current of one ampere is one coulomb of charge going past a given point per second:

In general, chargeQis determined by steady currentIflowing for a timetasQ=It.

Constant, instantaneous and average current are expressed in amperes (as in "the charging current is 1.2 A" ) and the charge accumulated (or passed through a circuit) over a period of time is expressed in coulombs (as in "thebatterycharge is30000C"). The relation of the ampere (C/s) to the coulomb is the same as that of thewatt(J/s) to thejoule.

Units derived from the ampere

edit

Theinternational system of units(SI) is based on sevenSI base unitsthesecond,metre,kilogram,kelvin,ampere,mole,andcandelarepresenting seven fundamental types of physical quantity, or"dimensions",(time,length,mass,temperature,electric current,amount of substance,andluminous intensityrespectively) with all other SI units being defined using these. TheseSI derived unitscan either be given special names e.g. watt, volt,lux,etc. or defined in terms of others, e.g.metre per second.The units with special names derived from the ampere are:

Quantity Unit Symbol Meaning In SI base units
Electric charge coulomb C ampere second A⋅s
Electric potential difference volt V jouleper coulomb kg⋅m2⋅s−3⋅A−1
Electrical resistance ohm Ω volt per ampere kg⋅m2⋅s−3⋅A−2
Electrical conductance siemens S ampere per volt or inverse ohm s3⋅A2⋅kg−1⋅m−2
Electrical inductance henry H ohm second kg⋅m2⋅s−2⋅A−2
Electrical capacitance farad F coulomb per volt s4⋅A2⋅kg−1⋅m−2
Magnetic flux weber Wb volt second kg⋅m2⋅s−2⋅A−1
Magnetic flux density tesla T weber persquare metre kg⋅s−2⋅A−1

There are also some SI units that are frequently used in the context ofelectrical engineeringand electrical appliances, but are defined independently of the ampere, notably thehertz,joule, watt, candela,lumen,and lux.

SI prefixes

edit
Main article:Orders of magnitude (current)

Like other SI units, the ampere can be modified by adding aprefixthat multiplies it by apower of 10.

SI multiples of ampere (A)
Submultiples Multiples
Value SI symbol Name Value SI symbol Name
10−1A dA deciampere 101A daA decaampere
10−2A cA centiampere 102A hA hectoampere
10−3A mA milliampere 103A kA kiloampere
10−6A μA microampere 106A MA megaampere
10−9A nA nanoampere 109A GA gigaampere
10−12A pA picoampere 1012A TA teraampere
10−15A fA femtoampere 1015A PA petaampere
10−18A aA attoampere 1018A EA exaampere
10−21A zA zeptoampere 1021A ZA zettaampere
10−24A yA yoctoampere 1024A YA yottaampere
10−27A rA rontoampere 1027A RA ronnaampere
10−30A qA quectoampere 1030A QA quettaampere

See also

edit

References

edit
  1. ^Jones, Daniel(2011),Roach, Peter;Setter, Jane;Esling, John(eds.),Cambridge English Pronouncing Dictionary(18th ed.), Cambridge University Press,ISBN978-0-521-15255-6{{citation}}:CS1 maint: overridden setting (link)
  2. ^Wells, John C.(2008),Longman Pronunciation Dictionary(3rd ed.), Longman,ISBN978-1-4058-8118-0
  3. ^"ampere",Merriam-Webster Dictionary,Merriam-Webster,retrieved29 September2020
  4. ^"2. SI base units",SI brochure(8th ed.), BIPM,archivedfrom the original on 7 October 2014,retrieved19 November2011
  5. ^SI supports only the use of symbols and deprecates the use of abbreviations for units."Bureau International des Poids et Mesures"(PDF),2006, p. 130, archived fromthe original(PDF)on 14 August 2017,retrieved21 November2011
  6. ^abBIPM (20 May 2019),"Mise en pratique for the definition of the ampere in the SI",BIPM,retrieved18 February2022
  7. ^"2.1. Unit of electric current (ampere)",SI brochure(8th ed.), BIPM,archivedfrom the original on 3 February 2012,retrieved19 November2011
  8. ^"Base unit definitions: Ampere",Physics.nist.gov,archived fromthe originalon 25 April 2017,retrieved28 September2010
  9. ^Draft Resolution A "On the revision of the International System of units (SI)" to be submitted to the CGPM at its 26th meeting (2018)(PDF),archived fromthe original(PDF)on 29 April 2018,retrieved28 October2018
  10. ^Bodanis, David(2005),Electric Universe,New York: Three Rivers Press,ISBN978-0-307-33598-2
  11. ^Kowalski, L (1986),"A short history of the SI units in electricity",The Physics Teacher,24(2), Montclair: 97–99,Bibcode:1986PhTea..24...97K,doi:10.1119/1.2341955,archived fromthe originalon 14 February 2002
  12. ^History of the ampere,Sizes, 1 April 2014,archivedfrom the original on 20 October 2016,retrieved20 September2023
  13. ^abInternational Bureau of Weights and Measures(2006),The International System of Units (SI)(PDF)(8th ed.),ISBN92-822-2213-6,archived(PDF)from the original on 4 June 2021,retrieved16 December2021
  14. ^Monk, Paul MS (2004),Physical Chemistry: Understanding our Chemical World,John Wiley & Sons,ISBN0-471-49180-2,archivedfrom the original on 2 January 2014
  15. ^Serway, Raymond A; Jewett, JW (2006),Serway's principles of physics: a calculus based text(Fourth ed.), Belmont, CA: Thompson Brooks/Cole, p. 746,ISBN0-53449143-X,archivedfrom the original on 21 June 2013
  16. ^Beyond the Kilogram: Redefining the International System of Units,US:National Institute of Standards and Technology,2006,archivedfrom the original on 21 March 2008,retrieved3 December2008
  17. ^ab"Appendix 2: Practical realisation of unit definitions: Electrical quantities",SI brochure,BIPM, archived fromthe originalon 14 April 2013
  18. ^"ampere (A)",npl.co.uk,retrieved21 May2019
  19. ^The International System of Units (SI)(PDF)(8th ed.),Bureau International des Poids et Mesures,2006, p. 144,archived(PDF)from the original on 5 November 2013.
edit
Retrieved from "https://en.wikipedia.org/w/index.php?title=Ampere&oldid=1255109911"