Demonstration model of a moving iron ammeter. As the current through the coil increases, the plunger is drawn further into the coil and the pointer deflects to the right.
|Unit system||SI base unit|
|Unit of||Electric current|
|Named after||André-Marie Ampère|
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.
In practical terms, the ampere is a measure of the amount of electric charge passing a point in an electric circuit per unit time, with an equivalent charge to 6.241×1018 charge carriers (or one coulomb) per second constituting one ampere. Amperes are a measure of flow rate of electric charge. For any point, if we increase the number of charged particles passing, the charge on the particles, or the speed at which charged particles are moving past it, the amperes at that point will also increase.
The practical definition may lead to confusion with the definition of the coulomb (i.e., 1 ampere-second) and the ampere-hour (A·h), but amperes measure charge flow, the amount of charge transiting per unit time, and coulombs are a sum count of charge. This means that measures of a constant or instantaneous current (e.g., the nominal flow of charge per second through a simple circuit) are noted in amperes (e.g., "a 20 milliamp circuit" or the momentary current of a lightning bolt) and the flow of charge through a circuit over a period of time is noted in coulombs (e.g., "a variable-current circuit that flowed a total of 10 coulombs over 2 seconds" or the total charge transfer of a lightning bolt).
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 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 0.001118000 grams of silver per second from a silver nitrate solution. Later, more accurate measurements revealed that this current is 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.
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 6.2415093×1018 elementary charges (such as electrons), one ampere is approximately equivalent to 6.2415093×1018 elementary charges moving past a boundary in one second, or the reciprocal of the value of the elementary charges 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 (watts) 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
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
North American domestic supply – 120 V AC
Most United States, Canada and Mexico 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
European & Commonwealth domestic supply – 230-240 V AC
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.[Note 1] 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
- Ampacity (current-carrying capacity)
- Electric current
- Electric shock
- Hydraulic analogy
- Magnetic constant
- Orders of magnitude (current)
- The formula for power is given by
- 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. Retrieved 21 November 2011.
- "2.1. Unit of electric current (ampere)", SI brochure (8th ed.), BIPM, retrieved 19 November 2011
- Base unit definitions: Ampere. Physics.nist.gov. Retrieved on 2010-09-28.
- "2. SI base units", SI brochure (8th ed.), BIPM, retrieved 19 November 2011
- The other six are the metre, kelvin, second, mole, candela, and kilogram
- Bodanis, David (2005), Electric Universe, New York: Three Rivers Press, ISBN 978-0-307-33598-2
- 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.
- Beyond the Kilogram: Redefining the International System of Units, USA: National Institute of Standards and Technology, 2006, archived from the original on 21 March 2008, retrieved March 2008.
- Monk, Paul MS (2004), Physical Chemistry: Understanding our Chemical World, John Wiley & Sons, ISBN 0-471-49180-2.
- The International System of Units (SI) (PDF) (8th ed.), Bureau International des Poids et Mesures, 2006, p. 144.
- Kowalski, L, A short history of the SI units in electricity, Montclair.
- History of the ampere, Sizes
- "Appendix 2: Practical realization of unit definitions: Electrical quantities", SI brochure, BIPM.
- "Value", Physics, USA: NIST.
- The NIST Reference on Constants, Units, and Uncertainty
- NIST Definition of ampere and μ0
- Tutorial video explaining amperes and current