The

**ampere** often

shortenedIn linguistics, clipping is the word formation process which consists in the reduction of a word to one of its parts . Clipping is also known as "truncation" or "shortening."...

to

**amp**, is the

SIThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

unit of

electric currentElectric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

(quantity symbol: I, i) and is one of the seven

SI base unitThe International System of Units defines seven units of measure as a basic set from which all other SI units are derived. These SI base units and their physical quantities are:* metre for length...

s. It is named after

André-Marie AmpèreAndré-Marie Ampère was a French physicist and mathematician who is generally regarded as one of the main discoverers of electromagnetism. The SI unit of measurement of electric current, the ampere, is named after him....

(1775–1836), French mathematician and physicist, considered the father of electrodynamics. Note that SI supports only the use of symbols and deprecates the use of any abbreviations for units.

In practical terms, the ampere is a measure of the amount of

electric chargeElectric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

passing a point in an electric circuit per unit time with 6.241 × 10

^{18} electronThe electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s, or one

coulomb per second constituting one ampere.

The practical definition may lead to confusion with the definition of a coulomb (i.e., 1 amp-second), but in practical terms this means that measures of a constant current (e.g., the nominal flow of charge per second through a simple circuit) will be defined in amps (e.g., "a 20 mA circuit") and the flow of charge through a circuit over a period of time will be defined in coulombs (e.g., "a variable-current circuit that flows a total of 10 coulombs over 5 seconds"). In this way, amperes can be viewed as a flow rate, i.e. number of particles (charged) transiting per unit time, and coulombs simply as the number of particles.

## Definition

Ampère's force lawIn magnetostatics, the force of attraction or repulsion between two current-carrying wires is often called 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 it is "the constant current that will produce an attractive force of 2 × 10

^{–7} newton per metre of length between two straight, parallel conductors of infinite length and negligible circular

cross sectionIn geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...

placed one

metreThe metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

apart in a

vacuumIn everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

".

The

SISi, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

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:

That is, in general, charge

*Q* is determined by steady current

*I* flowing for a time

*t* as

*Q* =

*It*.

In practice, its name is often shortened to

**amp**.

## History

The ampere was originally defined as one tenth of the

CGS systemThe centimetre–gram–second system is a metric system of physical units based on centimetre as the unit of length, gram as a unit of mass, and second as a unit of time...

electromagnetic unit of current (now known as the

abampereThe abampere , also called the biot after Jean-Baptiste Biot, is the basic electromagnetic unit of electric current in the emu-cgs system of units . One abampere is equal to ten amperes in the SI system of units...

), the amount of current that generates a force of two

dyneIn physics, the dyne is a unit of force specified in the centimetre-gram-second system of units, a predecessor of the modern SI. One dyne is equal to exactly 10 µN...

s 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 systemThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

would be conveniently sized.

The "international ampere" was an early realization of the ampere, defined as the current that would deposit grams of silver per second from a

silver nitrateSilver nitrate is an inorganic compound with chemical formula . This compound is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides...

solution. Later, more accurate measurements revealed that this current is 0.99985 A.

## Realization

The standard ampere is most accurately realized using a

watt balanceThe watt balance is an experimental electromechanical weight measuring instrument that measures the weight of a test object very precisely by the strength of an electric current and a voltage. It is being developed as a metrological instrument that may one day provide a definition of the kilogram...

, but is in practice maintained via

Ohm's LawOhm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...

from the units of

electromotive forceIn physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...

and

resistanceThe electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

, the

voltThe volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...

and the

ohmThe ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...

, 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 10

^{7}, and involve realizations of the watt, the ohm and the volt.

## Proposed future definition

Rather than a definition in terms of the force between two current-carrying wires, it has been proposed to define the ampere in terms of the rate of flow of elementary charges.

Since a

coulomb is approximately equal to

elementary chargeThe elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...

s, one ampere is approximately equivalent to elementary charges per second, such as

electronThe electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s, moving past a boundary in one second. 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 MeasuresThe Interglobal Committee for Weights and Measures is the English name of the Comité international des poids et mesures . It consists of eighteen persons from Member States of the Metre Convention...

(CIPM) agreed to study the proposed change. The

new definitionA committee of the International Committee for Weights and Measures has proposed revised formal definitions of the SI base units, which are being examined by the CIPM and which may be considered by the 25th CGPM, in 2014....

is expected to be formally proposed at the 25th

General Conference on Weights and MeasuresThe General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures . It is one of the three organizations established to maintain the International System of Units under the terms of the Convention du Mètre of 1875...

(CGPM) in 2015.

## Everyday examples

The current drawn by typical constant-voltage energy distribution systems is usually dictated by the power (

wattThe watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

s) consumed by the system and the operating voltage. For this reason the examples given below are grouped by voltage level.

### Motor vehicles – 12 V DC

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.
- Headlights (typically 60 W): 5 A each.
- Starter Motor (typically 1–2 kW): 80-160 A

### US & Canada domestic supply – 120 V AC

Most United States domestic power suppliers run at 120 V.

Household

circuit breakerA circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...

s 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 (2 kW): 16.6 A
- Immersion heater (4.6 kW): 38.3 A

### European domestic supply – 230 V AC

Most European domestic power supplies run at 230 V, so the current drawn for a particular appliance will be less than for an equivalent United States appliance. This means that lighter (and cheaper) cabling can be used.

The current drawn by a number of typical appliances are:

- 22-inch/56-centimeter Portable Television (35 W): 150 mA
- Tungsten light bulb (60–100 W): 250–450 mA
- Toaster, kettle (2 kW): 9 A
- Immersion heater (4.6 kW): 20 A

## See also

- Ammeter
An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes , hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters...

- Ampacity
Ampacity is the maximum amount of electrical current a conductor or device can carry before sustaining immediate or progressive deterioration.Also described as current rating or current-carrying capacity, ampacity is the RMS electric current which a device can continuously carry while remaining...

(current-carrying capacity)
- Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

- Electric shock
Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....

- Hydraulic analogy
The electronic–hydraulic analogy is the most widely used analogy for "electron fluid" in a metal conductor. Since electric current is invisible and the processes at play in electronics are often difficult to demonstrate, the various electronic components are represented by hydraulic...

- Magnetic constant

## External links