The ampere (SI unit symbol: A), often shortened to "amp", is the SI unit of electric current (dimension symbol: I) and is one of the seven SI base units. It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.
SI defines 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 meter apart in vacuum, would produce between these conductors a force equal to 6993200000000000000♠2×10−7 newton per metre of length.
The old definition (valid until 1948) is:
The ampere is equivalent to one coulomb (roughly 7018624200000000000♠6.242×1018 times the elementary charge) per second. Amperes are used to express flow rate of electric charge. For any point experiencing a current, if the number of charged particles passing through it — or the charge on the particles passing through it — is increased, the amperes of current at that point will proportionately increase.
The ampere should not be confused with the coulomb (also called "ampere-second") or the ampere hour (A⋅h). The ampere is a unit of current, the amount of charge transiting per unit time, and the coulomb is a unit of charge. When SI units are used, 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 7004300000000000000♠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.
Ampère's force law 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 the ampere is the constant current that will produce an attractive force of 2 × 10−7 newtons per metre of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one metre apart in a vacuum.
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 = It.
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 6997111800000000000♠0.001118 grams of silver per second from a silver nitrate solution. Later, more accurate measurements revealed that this current is 6999999850000000000♠0.99985 A.
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 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.
At present, techniques to establish the realization of an ampere have a relative uncertainty of approximately a few parts in 107, and involve realizations of the watt, the ohm and the volt.
Rather than a definition in terms of the force between two current-carrying wires, it has been proposed that the ampere should be defined in terms of the rate of flow of elementary charges. Since a coulomb is approximately equal to 7018624150930000000♠6.2415093×1018 elementary charges (such as those carried by protons, or the negative of those carried by electrons), one ampere is approximately equivalent to 7018624150930000000♠6.2415093×1018 elementary charges moving past a boundary in one second. (7018624150930000000♠6.2415093×1018 is the reciprocal of the value of the elementary charge in coulombs.) 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 definition was discussed at the 25th General Conference on Weights and Measures (CGPM) in 2014 but for the time being was not adopted.
The current drawn by typical constant-voltage energy distribution systems is usually dictated by the power (watt) consumed by the system and the operating voltage. For this reason the examples given below are grouped by voltage level.Hearing aid (typically 1 mW at 1.4 V): 0.7 mA
USB charging cable (typically 10.5 W at 5.0 V): 2.1 A
A typical motor vehicle has a 12 V battery. The various accessories that are powered by the battery might include:Instrument panel light (typically 2 W): 166 mA
Headlight (each, typically 60 W): 5 A
Starter motor (typically 1–2 kW): 80–160 A
Most Canada, Mexico and United States domestic power suppliers run at 120 V.
Household circuit breakers typically provide a maximum of 15 A or 20 A of current to a given set of outlets.22-inch/56-centimeter portable television (35 W): 290 mA
Tungsten light bulb (60–100 W): 500–830 mA
Toaster, kettle (1.5 kW): 12.5 A
Hair dryer (1.8 kW): 15 A
USB cell phone charger (typically 10 W): 83 mA
Most European domestic power supplies run at 230 V, and most Commonwealth domestic power supplies run at 240 V. For the same amount of power (in watts), 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. Typical circuit breakers will provide 16 A.
The current drawn by a number of typical appliances are:22-inch/56-centimeter portable television (35 W): 145–150 mA
Tungsten light bulb (60–100 W): 240–450 mA
Compact fluorescent lamp (11–30 W): 56–112 mA
Toaster, kettle (2 kW): 9 A
Immersion heater (4.6 kW): 19–20 A