The ampere (//, US: //; symbol: A), often shortened to amp, is the SI base unit of electric current, named after French mathematician and physicist André-Marie Ampère (1775–1836), considered the father of electromagnetism along with Danish physicist Hans Christian Ørsted.
As of the 2019 redefinition of the SI base units, the ampere is defined by setting the magnitude of the elementary charge to 1.602176634×10−19 C (coulomb), which means an ampere is an electrical current equivalent to 1019 elementary charges passing every 1.602176634 seconds. Prior to the redefinition the ampere was defined as the current that would need to be passed through 2 parallel wires 1 metre apart to produce a magnetic force of 2×10−7 newtons per metre.
The earlier CGS system had two definitions of current, one essentially the same as the SI's and the other using electric charge as the base unit, with the unit of charge defined by measuring the force between two charged metal plates. The ampere was then defined as one coulomb of charge per second. In SI, the unit of charge, the coulomb, is defined as the charge carried by one ampere during one second.
The ampere is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10−19 when 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 the caesium-133 atom.
The SI unit of charge, the coulomb, "is the quantity of electricity carried in 1 second by a current of 1 ampere". 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 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. 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.
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.
Former definition in the SI
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.: 113 
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".: 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.
The standard ampere is most accurately realised using a Kibble balance, but is in practice maintained via Ohm's law from the units of electromotive force and resistance, the volt and the ohm, since the latter two can be tied to physical phenomena that are relatively easy to reproduce, the Josephson effect and the quantum Hall effect, respectively.
Units derived from the ampere
The international system of units (SI) is based on 7 SI base units the second, metre, kilogram, kelvin, Ampere, mole, and candela representing 7 fundamental types of physical quantity, or "dimensions", (time, length, mass, temperature, electrical current, amount of substance, and luminous intensity respectively) with all other SI units being defined using these 7 basic building blocks. These SI derived units can either be given special names e.g. watt, volt, lux, etc. or left as bare descriptions, e.g. metre per second. The units with special names derived from the ampere (or amp) are:
|Dimension||Unit||Symbol||Meaning||In SI base units|
|Electric charge||Coulomb||C||Amp second||A s|
|Electrical potential difference, Electromotive force, or Voltage||Volt||V||Joule per coulomb or watt per amp||kg m2 s−3 A−1|
|Electrical resistance||Ohm||Ω||Volt per amp or watt per amp2||kg m2 s−3 A−2|
|Electrical conductance||Siemens||S||Amp 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 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 not defined by the ampere, notably the hertz, joule, watt, candela, lumen, and lux.
|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|
- Ampacity (current-carrying capacity)
- Electric current
- Electric shock
- Hydraulic analogy
- Magnetic constant
- Orders of magnitude (current)
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