Image:Galvanometer.png
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, 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.[1][2] For a description of this force law, see Serway.[3] See also Ampère's force law. The ampere is a base unit, along with the metre, kelvin, second, 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.[4] That is, in general, charge Q is determined by steady current I flowing for time t as:
- Failed to parse (Missing texvc executable; please see math/README to configure.): Q=I\ t \ .
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 magnetic constant at μ0 Failed to parse (Missing texvc executable; please see math/README to configure.): \ \overset{\underset{\mathrm{def}}{}}{=}\
4π×10−7 H/m.[5][6] Prior to 1948, the so-called "international ampere" was used, defined in terms of the electrolytic deposition rate of silver.[7] The older unit is equal to 0.999 85 A.
Realization
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 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 official realization of a standard ampere is discussed in NIST Special publication 330 Barry N Taylor (editor) Appendix 2, p. 56.
Proposed future definition
Since a coulomb is approximately equal to 6.24150948×1018 elementary charges, one ampere is approximately equivalent to 6.24150948×1018 elementary charges, such as electrons, 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:
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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. |
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This redefinition of the kilogram has the effect of fixing the elementary charge to be e = 1.60217653×10-19 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 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 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 CGPM in 2007 to alert Member States to these activities;
| This SI unit is named after André-Marie Ampère. As with all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (A). When an SI unit is spelled out in English, it should always begin with a lowercase letter (ampere), except for at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because of the "d".
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See also
References
- ^ BIPM official definition
- ^ Paul M. S. Monk, Physical Chemistry: Understanding our Chemical World, John Wiley and Sons, 2004 online.
- ^ Raymond A Serway & Jewett JW (2006). Serway's principles of physics: a calculus based text, Fourth Edition, Belmont, CA: Thompson Brooks/Cole, p. 746. ISBN 053449143X.
- ^ BIPM Table 3
- ^ NIST value for μ0.
- ^ NIST definition of μ0; see last sentence.
- ^ Robert B. Northrop, Introduction to Instrumentation and Measurements, CRC Press, 1997 online
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