Ampere

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[[Image:Galvanometer.png|thumb|200px|Current can be measured by a galvanometer, via the deflection of a magnetic needle in the magnetic field created by the current.]]

The '''ampere''', in practice often shortened to '''amp''', (symbol: A) is a unit of [[electric current]], or amount of [[electric charge]] per second. The ampere is an [[SI base unit]], and is named after [[André-Marie Ampère]], one of the main discoverers of [[electromagnetism]].

== Definition ==
The ampere is a constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular [[cross section (geometry)|cross section]], and placed 1 [[metre]] apart in a vacuum, would produce between these conductors a force equal to 2×10–7 [[newton]] per metre of length.<ref>Paul M. S. Monk, ''Physical Chemistry: Understanding our Chemical World'', John Wiley and Sons, 2004 [http://books.google.com/books?vid=ISBN0471491802&id=LupAi35QjhoC&pg=PA16&lpg=PA16&ots=IMiGyIL-67&dq=ampere+definition+si&sig=9Y0k0wgvymmLNYFMcXodwJZwvAM online].</ref>
The ampere is a [[SI base unit|base unit]], along with the [[metre]], [[kelvin]], [[second]], [[Mole (unit)|mole]], [[candela]] and the [[kilogram]]: it is defined without reference to the quantity of [[electric charge]]. The unit of charge, the [[coulomb]], is defined, as a derived unit, to be the amount of charge displaced by a one ampere current in the time of one [[second]].

As a result, electric current is also the time rate of change or displacement of electric charge. One ampere represents the rate of 1 coulomb of charge per second.

:<math>\mathrm{1 \,A= 1 \frac{\,C}{s}} \,</math>

== Explanation ==
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 (electromagnetism)|permeability]] of vacuum to ''μ''0 = 4π×10−7 H/m. Prior to 1948, the so-called "international ampere" was used, defined in terms of the [[electrolysis|electrolytic]] deposition rate of [[silver]].<ref>Robert B. Northrop, ''Introduction to Instrumentation and Measurements'', CRC Press, 1997 [http://books.google.com/books?vid=ISBN0849378982&id=mcpcfpQfxB4C&pg=PA21&lpg=PA21&ots=ZTUUMNAShz&dq=ampere+1948+electrolytic++silver+international&sig=1Xv1-zsZ1pNuQuFjRzlhJaCWXiY online]</ref> The older unit is equal to 0.999 85 A.

The ampere is most accurately realized using a [[watt balance]], but is in practice maintained via [[Ohm's Law]] from the units of [[EMF]] and [[electrical resistance|resistance]], the [[volt]] and the [[Ohm (unit)|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]].<ref>Kuzman Ražnjević, ''Physical Quantities and the Units of the International System (Si)'', Begell House Publishers, 1995 [http://books.google.com/books?vid=ISBN1567000479&id=Dhd7jxDk0hIC&pg=PA64&lpg=PA64&ots=_A07F0tdP7&dq=ampere+%22International+Committee+for+Weights+and+Measures%22&sig=gPjXB1DK78FWSnTZVpwnifIktZY#PPA107,M1 online]</ref> [[Current (electricity)|Current]], 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):

:<math>\mathrm{I=Q/t} \,</math>

== Proposed future definition ==
Since a coulomb is approximately equal to 6.24150948×1018 [[elementary charge]]s, one ampere is approximately equivalent to 6.24150948×1018 elementary charges, such as [[electron]]s, moving past a boundary in one second.

As with other [[SI]] base units, there have been proposals to redefine the [[kilogram]] in such a way as to define some presently measured [[physical constants]] to fixed values. One proposed definition of the kilogram is:

{{cquote|The kilogram is the mass which would be accelerated at precisely'' 2×10-7 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'' 6 241 509 479 607 717 888 ''elementary charges per second.}}

This redefinition of the kilogram has the effect of fixing the [[elementary charge]] to be ''e'' = 1.60217653{{e|-19}} [[coulomb|C]] and would result in a functionally equivalent definition for the [[coulomb]] as being the sum of exactly 6 241 509 479 607 717 888 elementary charges and the ampere as being the electrical current of exactly 6 241 509 479 607 717 888 elementary charges per second. This is consistent with the current 2002 CODATA value for the elementary charge which is 1.60217653×10-19 ± 0.00000014×10-19 C.

===CIPM recommendation===
[[International Committee for Weights and Measures]] (CIPM) Recommendation 1 (CI-2005):
Preparative steps towards new definitions of the [[kilogram]], the ampere, the
[[kelvin]] and the [[mole (unit)|mole]] in terms of fundamental constants

The International Committee for Weights and Measures (CIPM),
* approve in principle the preparation of new definitions and mises en pratique of the kilogram, the ampere and the kelvin so that if the results of experimental measurements over the next few years are indeed acceptable, all having been agreed with the various Consultative Committees and other relevant bodies, the CIPM can prepare proposals to be put to Member States of the Metre Convention in time for possible adoption by the 24th [[General Conference on Weights and Measures|CGPM]] in 2011;
* give consideration to the possibility of redefining, at the same time, the mole in terms of a fixed value of the Avogadro constant;
* prepare a Draft Resolution that may be put to the 23rd [[General Conference on Weights and Measures|CGPM]] in 2007 to alert Member States to these activities;

{{SI unit lowercase|André-Marie Ampère|ampere|A}}

== See also ==
* [[SI]]
* [[Ohm's Law]]
* [[Hydraulic analogy]]
* [[Electric shock]]
* [[Ampère's law]]
* [[Ammeter]]

==References==

{{reflist}}

== External links ==
* [http://physics.nist.gov/cuu/ The NIST Reference on Constants, Units, and Uncertainty]
* [http://alpha.montclair.edu/~kowalskiL/SI/SI_PAGE.HTML A short history of the SI units in electricity]

[[Category:SI base units]]
[[Category:Units of electrical current]]

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