ampere | |
---|---|

General information | |

Unit system | SI |

Unit of | electric current |

Symbol | A |

Named after | André-Marie Ampère |

The **ampere** ( /ˈæmpɛər/ *AM-pair*, US: /ˈæmpɪər/ *AM-peer*;^{ [1] }^{ [2] }^{ [3] } symbol: **A**),^{ [4] } often shortened to **amp**,^{ [5] } is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 coulomb (C) moving past a point in 1 second.^{ [6] }^{ [7] }^{ [8] } It is named after French mathematician and physicist André-Marie Ampère (1775–1836), considered the father of electromagnetism along with Danish physicist Hans Christian Ørsted.

- History
- Former definition in the SI
- Present definition
- Units derived from the ampere
- SI prefixes
- See also
- References
- External links

As of the 2019 redefinition of the SI base units, the ampere is defined by fixing the elementary charge `e` to be exactly 1.602176634×10^{−19} C,^{ [6] }^{ [9] } which means an ampere is an electric current equivalent to 10^{19} elementary charges moving every 1.602176634 seconds or 6.241509074×10^{18} elementary charges moving in a second. Prior to the redefinition the ampere was defined as the current passing through two parallel wires 1 metre apart that produces a magnetic force of 2×10^{−7} newtons per metre.

The earlier CGS system has two units of current, one structured similarly to the SI's and the other using Coulomb's law as 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] }

The ampere is named for French physicist and mathematician André-Marie Ampère (1775–1836), 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 electrical measurement for electric current.

The ampere was originally defined as one tenth of the unit of electric current in the centimetre–gram–second system of units. That unit, now known as the abampere, was defined as the amount of current that generates a force of two dynes per 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 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. Later, more accurate measurements revealed that this current is 0.99985 A.^{ [12] }

Since power is defined as the product of current and voltage, the ampere can alternatively be expressed in terms of the other units using the relationship *I* = *P*/*V*, and thus 1 A = 1 W/V. Current can be measured by a multimeter, a device that can measure electrical voltage, current, and resistance.

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 one metre apart in vacuum, would produce between these conductors a force equal to 2×10

^{−7}newtons per 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, the coulomb, 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, charge Q was determined by steady current I flowing for a time t as *Q* = *It*.

This definition of the ampere was most accurately realised using a Kibble balance, but 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.^{ [17] }

Techniques to establish the realisation of an ampere had a relative uncertainty of approximately a few parts in 10^{7}, and involved realisations of the watt, the ohm and the volt.^{ [17] }

The 2019 redefinition of the SI base units defined the ampere by taking the fixed numerical value of the elementary charge e to be 1.602176634×10^{−19} when expressed in the unit C, which is equal to A⋅s, where the second is defined in terms of ∆*ν*_{Cs}, the unperturbed ground state hyperfine transition frequency of the caesium-133 atom.^{ [18] }

The SI unit of charge, the coulomb, "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, charge Q is determined by steady current I flowing for a time t as *Q* = *I**t*.

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 "the battery charge is 30000 C"). The relation of the ampere (C/s) to the coulomb is the same as that of the watt (J/s) to the joule.

The international system of units (SI) is based on seven SI base units the second, metre, kilogram, kelvin, ampere, mole, and candela representing seven fundamental types of physical quantity, or "dimensions", (time, length, mass, temperature, electric current, amount of substance, and luminous intensity respectively) with all other SI units being defined using these. These SI derived units can 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 | joule per coulomb | kg⋅m^{2}⋅s^{−3}⋅A^{−1} |

Electrical resistance | ohm | Ω | volt per ampere | kg⋅m^{2}⋅s^{−3}⋅A^{−2} |

Electrical conductance | siemens | S | ampere per volt or inverse ohm | s^{3}⋅A^{2}⋅kg^{−1}⋅m^{−2} |

Electrical inductance | henry | H | ohm second | kg⋅m^{2}⋅s^{−2}⋅A^{−2} |

Electrical capacitance | farad | F | coulomb per volt | s^{4}⋅A^{2}⋅kg^{−1}⋅m^{−2} |

Magnetic flux | weber | Wb | volt second | kg⋅m^{2}⋅s^{−2}⋅A^{−1} |

Magnetic flux density | tesla | T | weber per square metre | kg⋅s^{−2}⋅A^{−1} |

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

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 | 10^{1} A | daA | decaampere |

10^{−2} A | cA | centiampere | 10^{2} A | hA | hectoampere |

10^{−3} A | mA | milliampere | 10^{3} A | kA | kiloampere |

10^{−6} A | μA | microampere | 10^{6} A | MA | megaampere |

10^{−9} A | nA | nanoampere | 10^{9} A | GA | gigaampere |

10^{−12} A | pA | picoampere | 10^{12} A | TA | teraampere |

10^{−15} A | fA | femtoampere | 10^{15} A | PA | petaampere |

10^{−18} A | aA | attoampere | 10^{18} A | EA | exaampere |

10^{−21} A | zA | zeptoampere | 10^{21} A | ZA | zettaampere |

10^{−24} A | yA | yoctoampere | 10^{24} A | YA | yottaampere |

10^{−27} A | rA | rontoampere | 10^{27} A | RA | ronnaampere |

10^{−30} A | qA | quectoampere | 10^{30} A | QA | quettaampere |

- Ammeter
- Ampacity (current-carrying capacity)
- Electric current
- Electric shock
- Hydraulic analogy
- Magnetic constant
- Orders of magnitude (current)

The **centimetre–gram–second system of units** is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways in which the CGS system was extended to cover electromagnetism.

The **joule** is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of one newton displaces a mass through a distance of one metre in the direction of that force. It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule (1818–1889).

The **International System of Units**, internationally known by the abbreviation **SI**, is the modern form of the metric system and the world's most widely used system of measurement. Coordinated by the International Bureau of Weights and Measures it is the only system of measurement with an official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce.

The **SI base units** are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all other SI units can be derived. The units and their physical quantities are the second for time, the metre for length or distance, the kilogram for mass, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity. The SI base units are a fundamental part of modern metrology, and thus part of the foundation of modern science and technology.

The **volt** is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI).

The **caesium standard** is a primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency. The first caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK. and promoted worldwide by Gernot M. R. Winkler of the United States Naval Observatory.

The **metric system** is a system of measurement that is a decimal system. The current international standard for the metric system is the International System of Units, in which all units can be expressed in terms of seven base units. The units that serve as the SI base units are the metre, kilogram, second, ampere, kelvin, mole, and candela.

The **coulomb** (symbol: **C**) is the unit of electric charge in the International System of Units (SI). It is equal to the electric charge delivered by a 1 ampere current in 1 second and is defined in terms of the elementary charge *e*, at about 6.241509×10^{18} *e*.

In physics, the **weber** is the unit of magnetic flux in the International System of Units (SI). The unit is derived from the relationship 1 Wb = 1 V⋅s (volt-second). A magnetic flux density of 1 Wb/m^{2} is one tesla.

**Vacuum permittivity**, commonly denoted ** ε_{0}**, is the value of the absolute dielectric permittivity of classical vacuum. It may also be referred to as the

The **ohm** is the unit of electrical resistance in the International System of Units (SI). It is named after German physicist Georg Ohm. Various empirically derived standard units for electrical resistance were developed in connection with early telegraphy practice, and the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time, and of a convenient scale for practical work as early as 1861.

The **vacuum magnetic permeability**, also known as the **magnetic constant**, is the magnetic permeability in a classical vacuum. It is a physical constant, conventionally written as ** μ_{0}**. Its purpose is to quantify the strength of the magnetic field emitted by an electric current. Expressed in terms of SI base units, it has the unit kg⋅m⋅s

In electromagnetism, the **impedance of free space**, *Z*_{0}, is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is,

A **conventional electrical unit** is a unit of measurement in the field of electricity which is based on the so-called "conventional values" of the Josephson constant, the von Klitzing constant agreed by the International Committee for Weights and Measures (CIPM) in 1988, as well as Δ*ν*_{Cs} used to define the second. These units are very similar in scale to their corresponding SI units, but are not identical because of the different values used for the constants. They are distinguished from the corresponding SI units by setting the symbol in italic typeface and adding a subscript "90" – e.g., the conventional volt has the symbol *V*_{90} – as they came into international use on 1 January 1990.

The **International System of Electrical and Magnetic Units** is an obsolete system of units used for measuring electrical and magnetic quantities. It was proposed as a system of practical **international units** by unanimous recommendation at the International Electrical Congress, discussed at other Congresses, and finally adopted at the International Conference on Electric Units and Standards in London in 1908. It was rendered obsolete by the inclusion of electromagnetic units in the International System of Units (SI) at the 9th General Conference on Weights and Measures in 1948.

In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artifacts such as the standard kilogram. Effective 20 May 2019, the 144th anniversary of the Metre Convention, the kilogram, ampere, kelvin, and mole are now defined by setting exact numerical values, when expressed in SI units, for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The second, metre, and candela had previously been redefined using physical constants. The four new definitions aimed to improve the SI without changing the value of any units, ensuring continuity with existing measurements. In November 2018, the 26th General Conference on Weights and Measures (CGPM) unanimously approved these changes, which the International Committee for Weights and Measures (CIPM) had proposed earlier that year after determining that previously agreed conditions for the change had been met. These conditions were satisfied by a series of experiments that measured the constants to high accuracy relative to the old SI definitions, and were the culmination of decades of research.

The **siemens** is the unit of electric conductance, electric susceptance, and electric admittance in the International System of Units (SI). Conductance, susceptance, and admittance are the reciprocals of resistance, reactance, and impedance respectively; hence one siemens is equal to the reciprocal of one ohm and is also referred to as the *mho*. The siemens was adopted by the IEC in 1935, and the 14th General Conference on Weights and Measures approved the addition of the siemens as a derived unit in 1971.

The history of the metric system began during the Age of Enlightenment with measures of length and weight derived from nature, along with their decimal multiples and fractions. The system became the standard of France and Europe within half a century. Other measures with unity ratios were added, and the system went on to be adopted across the world.

A **coherent system of units** is a system of units of measurement used to express physical quantities that are defined in such a way that the equations relating the numerical values expressed in the units of the system have exactly the same form, including numerical factors, as the corresponding equations directly relating the quantities. It is a system in which every quantity has a unique unit, or one that does not use conversion factors.

- ↑ 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. - ↑ "2. SI base units",
*SI brochure*(8th ed.), BIPM, archived from the original on 7 October 2014, retrieved 19 November 2011 - ↑ 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 from the original (PDF) on 14 August 2017. Retrieved 21 November 2011.
- 1 2 BIPM (20 May 2019). "Mise en pratique for the definition of the ampere in the SI".
*BIPM*. Retrieved 18 February 2022. - ↑ "2.1. Unit of electric current (ampere)",
*SI brochure*(8th ed.), BIPM, archived from the original on 3 February 2012, retrieved 19 November 2011 - ↑ Base unit definitions: Ampere Archived 25 April 2017 at the Wayback Machine Physics.nist.gov. Retrieved on 28 September 2010.
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*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 from the original (PDF) on 29 April 2018, retrieved 28 October 2018 - ↑ Bodanis, David (2005),
*Electric Universe*, New York: Three Rivers Press, ISBN 978-0-307-33598-2 - ↑ Kowalski, L (1986), "A short history of the SI units in electricity",
*The Physics Teacher*, Montclair,**24**(2): 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 20 September 2023 - 1 2 International Bureau of Weights and Measures (2006),
*The International System of Units (SI)*(PDF) (8th ed.), ISBN 92-822-2213-6, archived (PDF) from the original on 4 June 2021, retrieved 16 December 2021 - ↑ 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 - ↑ Serway, Raymond A; Jewett, JW (2006).
*Serway's principles of physics: a calculus based text*(Fourth ed.). Belmont, CA: Thompson Brooks/Cole. p. 746. ISBN 0-53449143-X. Archived from the original on 21 June 2013. - ↑
*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. - 1 2 "Appendix 2: Practical realisation of unit definitions: Electrical quantities",
*SI brochure*, BIPM, archived from the original on 14 April 2013. - ↑ "ampere (A)".
*www.npl.co.uk*. Retrieved 21 May 2019. - ↑
*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.

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