The ampere (/ˈæmpɛər/, US: /ˈæmpɪər/;[1][2][3] symbol: A[4][5][6][7]) is the unit of electric current in the International System of Units (SI) equivalent to a flow of 1 coulomb of electric charge per second,[4][5] or of 5×1018 elementary charges, e, every 0.801088317 seconds (about 6.241509×1018 e per second).[4][5][6][7][8] It is often shortened to amp,[9] though the SI supports only the use of symbols and deprecates the use of abbreviations for units.
History edit
The ampere is named for French physicist and mathematician André-Marie Ampère (1775–1836),[7][9] who studied electromagnetism and laid the foundation of electrodynamics. In recognition of Ampère's contributions to the creation of modern electrical science, an international convention, signed at the 1881 International Exposition of Electricity, established the ampere as a standard unit of measurement for electric current.
The ampere was originally defined as one tenth of the abampere, a unit of electric current in the centimetre–gram–second system of units defined as the amount of current that generates a force of two dynes per centimetre of length between two wires one centimetre apart.[10] The size of the unit was chosen so that the units derived from it in the MKSA system would be conveniently sized.
The "international ampere" was an early realization of the ampere, defined as the current that would deposit 0.001118 grams of silver per second from a silver nitrate solution.[7][11] Later, more accurate measurements revealed that this current is 0.99985 A.
Former definition in the SI edit
Resolution 2 of the 41st CIPM (1946), which came into effect on the 1st of January 1948,[12] introduced definitions for the SI electrical units, including the ampere, defined 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 one metre apart in vacuum, would produce between these conductors a force equal to 2×10−7 MKS unit of force [i.e. 2×10−7 newtons, the term used in later literature[7][13][14][15]] per metre of length.[16]
Note that while the ampere is here defined with reference to a constant current, the ampere can also be used to measure an alternating current (AC) or other non-constant waveform. AC is typically described using the root mean square current; alternatively one may either measure the instantaneous current at a given point in time or the average current across a period of time.
This definition implicitly fixed the SI value of the vacuum permeability, μ0[12]:
- μ0 = 4π×10−7 N/A2
After the metre was redefined in terms of the speed of light, c, in 1983, the SI value of the vacuum permittivity, ε0, was also fixed exactly by definition of the metre and ampere.[12]
- ε0 = 1/μ0c2 = 10−10/3.59502071494727056π F/m
These figures remain very close, but not exact, approximations of μ0 and ε0 per the post 2019 definition of the ampere. Because the newton is defined as the force needed to induce a 1 metre per second squared acceleration in a 1 kilogram mass,[5][16] and the kilogram was at the time defined as the mass of the international Prototype of the Kilogram (IPK),[7][17][18] the magnitude of the ampere was tied to the square root of the mass of the IPK. Any change in the mass of the IPK was reflected in change to the legal definition of the kilogram and by extension the newton and the ampere.
This definition of the ampere was most accurately realised using a Kibble balance, but is in practice the unit was maintained via Ohm's law from the units of electromotive force and resistance, the volt and the ohm, since the latter two could be tied to physical phenomena that are relatively easy to reproduce, the Josephson effect and the quantum Hall effect, respectively.[19]
Techniques to establish the realisation of an ampere had a relative uncertainty of approximately a few parts in 107, and involved realisations of the watt, the ohm and the volt.[19]
Present definition edit
Resolution 1 of the 26th CGPM (2018), which came into force on the 20th of May 2019,[8] abrogated the explicit definitions of all 7 SI base units and introduced explicitly assigned SI values for 7 physical constants, from which the magnitudes of all SI units could be calculated.[8][5] The definition of the ampere is now implicit in the fixed SI value of the elementary charge:
- e = 1.602176634×10−19 C
With the unit C equal to A⋅s.[8] From which it follows that:
- 1 A = C/s = 1019/1.602176634 e per second ≈ 6241509074460762607.776 e per second
The unit second, s, used above is defined by the assigned value of the unperturbed ground state hyperfine transition frequency of the caesium-133 atom:
- ∆νCs = 9192631770 Hz
With the unit Hz being the inverse second, hence:
- 1 A = 1019/1.602176634 × 9192631770 e ∆νCs = 5×108/0.736410991343003109 e ∆νCs ≈ 678968681.725 e ∆νCs[6]
Because the charge on an electron is exactly -e (electrons are negatively charged), in an electrical system where electrons are the only charge carriers present the flow of charge per unit time expressed in multiples of e is equal to the number of electrons passsing in the direction opposite that of the conventional current per unit time. For instance an electric circuit with 1019 electrons passing a given point in the clockwise direction every 1.602176634 seconds would have a conventional current of 1 ampere in the anticlockwise direction.
As a base unit edit
The ampere is one of 7 SI base units alongside the second, metre, kilogram, kelvin, mole, and candela that, until 2019, had explicit definitions in terms of their physical realisation and where used to derive values for all other SI units.[5][7][20] These SI derived units can either be given special names and symbols e.g. watt (W), joule (J), newton (N), etc. or simply be named after their derivation, e.g. square metre, metre per second, etc. 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 | watt per ampere | 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 capacitance | farad | F | coulomb per volt | s4⋅A2⋅kg−1⋅m−2 |
| Electrical inductance | henry | H | ohm second | kg⋅m2⋅s−2⋅A−2 |
| Magnetic flux | weber | Wb | volt second | kg⋅m2⋅s−2⋅A−1 |
| Magnetic flux density | tesla | T | weber per square metre | kg⋅s−2⋅A−1 |
As of the 2019 redefinition of the SI base units all explicit base unit definitions have been abrogated and the magnitudes of all SI units are implicit in the fixed SI values of 7 physical constants (none of them dimentionally equivalent to any SI base unit),[8] however the base unit/derived unit distinction remains in use with both terms still appearing frequently in official BIPM publications.[5][20] The ampere can be expressed in terms of the derived units in multiple ways, for example:
There are also some derived units frequently used in the context of electrical engineering and electrical appliances, that can be defined independently of the ampere, notably the hertz, watt, lumen, and lux.
SI prefixes edit
Like other SI units, the ampere can be modified by adding a prefix that multiplies it by a power of 10.
| Submultiples | Multiples | ||||
|---|---|---|---|---|---|
| Value | SI symbol | Name | Value | SI symbol | Name |
| 10−1 A | dA | deciampere | 101 A | daA | decaampere |
| 10−2 A | cA | centiampere | 102 A | hA | hectoampere |
| 10−3 A | mA | milliampere | 103 A | kA | kiloampere |
| 10−6 A | μA | microampere | 106 A | MA | megaampere |
| 10−9 A | nA | nanoampere | 109 A | GA | gigaampere |
| 10−12 A | pA | picoampere | 1012 A | TA | teraampere |
| 10−15 A | fA | femtoampere | 1015 A | PA | petaampere |
| 10−18 A | aA | attoampere | 1018 A | EA | exaampere |
| 10−21 A | zA | zeptoampere | 1021 A | ZA | zettaampere |
| 10−24 A | yA | yoctoampere | 1024 A | YA | yottaampere |
| 10−27 A | rA | rontoampere | 1027 A | RA | ronnaampere |
| 10−30 A | qA | quectoampere | 1030 A | QA | quettaampere |
See also edit
- Ammeter
- Ampacity (current-carrying capacity)
- Electric current
- Electric shock
- Hydraulic analogy
- Magnetic constant
- Orders of magnitude (current)
References edit
- ^ Jones, Daniel (2011). Roach, Peter; Setter, Jane; Esling, John (eds.). Cambridge English Pronouncing Dictionary (18th ed.). Cambridge University Press. ISBN 978-0-521-15255-6.
- ^ Wells, John C. (2008). Longman Pronunciation Dictionary (3rd ed.). Longman. ISBN 978-1-4058-8118-0.
- ^ "ampere". Merriam-Webster.com Dictionary. Retrieved 29 September 2020.
- ^ a b c BIPM (20 May 2019). "Mise en pratique for the definition of the ampere in the SI". BIPM. Retrieved 18 February 2022.
- ^ a b c d e f g "9th edition of the SI Brochure". BIPM.org. BIPM. Retrieved 24 August 2022.
- ^ a b c "BIPM web page for ampere". BIPM.org. BIPM. Retrieved 10 August 2022.
- ^ a b c d e f g "A Turning Point for Humanity: Redefining the World's Measurement System". Nist. 12 May 2018. Retrieved 27 August 2022.
- ^ a b c d e "Resolution 1 of the 26th CGPM (2018)".
- ^ a b "ampere (A)". www.npl.co.uk. Retrieved 21 May 2019.
- ^ 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 from the original on 14 February 2002
- ^ History of the ampere, Sizes, 1 April 2014, archived from the original on 20 October 2016, retrieved 29 January 2017
- ^ a b c "Historical perspective: Unit of electric current, ampere". BIPM.org. BIPM. Retrieved 23 August 2022.
- ^ "8th edition of the SI Brochure (text in English)" (PDF). 2006. p. 130. Archived from the original (PDF) on 14 August 2017. Retrieved 21 November 2011.
- ^ Monk, Paul MS (2004), Physical Chemistry: Understanding our Chemical World, John Wiley & Sons, ISBN 0-471-49180-2, archived from the original on 2 January 2014
- ^ Base unit definitions: Ampere Archived 25 April 2017 at the Wayback Machine Physics.nist.gov. Retrieved on 28 September 2010.
- ^ a b "Resolution 2 of the 41st CIPM (1946)". BIPM.org. BIPM. Retrieved 23 August 2022.
- ^ "Historical perspective: Unit of mass, kilogram". BIPM.org. BIPM. Retrieved 23 August 2022.
- ^ Beyond the Kilogram: Redefining the International System of Units, US: National Institute of Standards and Technology, 2006, archived from the original on 21 March 2008, retrieved 3 December 2008.
- ^ a b "Appendix 2: Practical realisation of unit definitions: Electrical quantities", SI brochure, BIPM, archived from the original on 14 April 2013.
- ^ a b "Web page for SI base units". BIPM.org. BIPM. Retrieved 22 August 2022.