The ampere (symbol: A) is the SI base unit of electric current. It is named after André-Marie Ampère, one of the main discoverers of electromagnetism.

Definition

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The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 newton per metre of length.

Electric current is the time rate of change or displacement of electric charge. One ampere represents the rate of 1 coulomb of charge per second.

$\backslash mathrm\{A=C/s\}\; \backslash ,$

The ampere is defined first (it is a base unit, along with the metre, the second, and the kilogram), without reference to the quantity of charge. The unit of charge, the coulomb, is defined to be the amount of charge displaced by a one ampere current in the time of one second.

Explanation

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Because it is a base unit, the definition of the ampere is not tied to any other electrical unit. The definition for the ampere is equivalent to fixing a value of the permeability of vacuum to μ₀ = 4π H/m. Prior to 1948, the so-called "international ampere" was used, defined in terms of the electrolytic deposition rate of silver. The older unit is equal to 0.999 85 A.

The ampere is most accurately realized using an ampere balance, but is in practice maintained via Ohm's Law from the units of voltage 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.

The unit of electric charge, the coulomb, is defined in terms of the ampere: one coulomb is the amount of electric charge (formerly quantity of electricity) carried in a current of one ampere flowing for one second. Current (electricity), then, is the rate at which charge flows through a wire or surface. One ampere of current (I) is equal to a flow of one coulomb of charge (Q) per second of time (t):

$\backslash mathrm\{I=Q/t\}\; \backslash ,$

Proposed future definition

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Since a coulomb is approximately equal to 6.24150948 elementary charges, one ampere is approximately equivalent to 6.24150948 elementary charges, such as electrons, moving through a surface in one second. More precisely, using the accepted 2002 CODATA value of the elementary charge (derived from the Josephson constant and von Klitzing constant, both measured values with non-zero uncertainty), the ampere can be thought of as an electric current approximately equal to 6.24150948 elementary charges per second.

As with other SI base units, there have been proposals to redefine the kilogram using conventional values (CIPM, 1988, Recommendations 1 and 2, PV, 56, 44-45 and Metrologia, 1989, 26, 69-70) of the von Klitzing constant (R_K-90 = 25 812.807 Ω exactly) and of the Josephson constant (K_J-90 = 483 597.9 GHz/V exactly) . This proposed definition of the kilogram is:

The kilogram is the mass which would be accelerated at precisely 2 m/s² if subjected to the per metre force between two straight parallel conductors of infinite length, of negligible circular cross section, placed 1 metre apart in vacuum, through which flow a constant current of exactly 6.241 509 629 152 65 elementary charges per second.

This redefinition of the kilogram has the effect of fixing the elementary charge to be e = 1.602176491612271 C and would result in a functionally equivalent definition for the coulomb as being the sum of exactly 6.241 509 629 152 65 elementary charges and the ampere as being the electrical current of exactly 6.241 509 629 152 65 elementary charges per second. This is slightly different from but within the standard uncertainty of the current 2002 CODATA value for the elementary charge which is 1.60217653 ± 0.00000014 C. This is because the 1988 CIPM conventional values for the Josephson and von Klitzing constants are not identical to the 2002 CODATA values.

See also

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• SI
• Ohm's Law
• Electric shock
• Ampere's law

External links

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• The NIST Reference on Constants, Units, and Uncertainty
• A short history of the SI units in electricity
• @.ampere : Ampère and history of electricity Correspondence, Main books, Movies, 3D VRML - web site create by the CNRS, edited by the Centre de Recherche en Histoire des Sciences et des Techniques : Mme Christine Blondel - CNRS and M. Stéphane Pouyllau - CNRS

SI base units | Units of electrical current

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