ampere

1. A unit of electric current in the meter-kilogram-second system. It is the steady current that when flowing in straight parallel wires of infinite length and negligible cross section, separated by a distance of one meter in free space, produces a force between the wires of 2 × 10 ^-7 newtons per meter of length.
2. A unit in the International System specified as one International coulomb per second and equal to 0.999835 ampere.

[After André Marie AMPÈRE.]

Symbol A. The SI unit of electric current. The constant current that, maintained in two straight parallel infinite conductors of negligible cross section placed one metre apart in a vacuum, would produce a force between the conductors of 2 × 10⁻⁷ N m⁻¹. This definition replaced the earlier international ampere defined as the current required to deposit 0.001 118 00 gram of silver from a solution of silver nitrate in one second. The unit is named after the French physicist André Marie Ampère (1775–1836).

(Amp), a unit of measurement of the quantity of electric current, equal to a flow of 1 coulomb per second or 6.25 time 10¹⁸ electrons per second. The current produced by 1 volt acting through a resistance of 1 ohm.

[Etymology: A. M. Ampère; France 1775-1836] electric current strength. Symbol A. The amperes of steady current crossing any cross-section of a circuit equals the ratio of the charge in coulombs to the time in seconds, identically the amperes of steady current produced between two points of a conductor equals the ratio of the potential difference in volts across these points to the intervening resistance in ohms (the conductor not being the seat of any electromotive force). However, this unit is defined as a base unit in any m.k.s. A. system (including the SI), and was for the e.m.u. system.

SI, Metric-m.k.s. A. 1948 the base unit for all electromagnetic units, defined as the constant current which, if maintained in two straight parallel conductors of infinite length and of negligible cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10^-7 newton per metre of length. (This number effectively set the magnetic permeability of vacuum at 4π × 10^-7 H·m^-1; see permittivity.) The following are among the coherent derived units:

• A·m^-1 for magnetic field strength, magnetization;
• A·m^-2 for current density;
• A·s = coulomb for quantity of electricity;
• A·s·V^-1 = farad for electric capacitance;
• A·V^-1 = siemens for electric conductance;
• A·H = weber for magnetic flux;
• A·turn for magnetomotive force.

This ampere is equatable with 6.241 45~ × 10¹⁸ electronic charges per second.

Metric-c.g.s. See abampere and statampere. See also practical unit.

History

The name ‘ampère’ was agreed, along with related units and the use of the c.g.s. system, in 1881 at the first International Electrical Conference,
[Nature Vol. 24, 512 (1881)] as the ‘current produced by a volt in an ohm’, with the implication that there should be both an absolute form and a corresponding practical unit. The former, later discriminated as the abampere, falls within the e.m.u. system, and is fundamentally definable in terms of purely mechanical units. The practical ampere = 10^-1 abampere.

To make it a base unit instead of a derived unit, a specification for a laboratory realization of the ampere was established. This was expressed in terms of the rate of electrolytic deposition of silver, so has often been called the silver ampere or Ag ampere; the definition was ‘the unvarying current which deposits 1.118 mg of silver by electrolysis from a silver nitrate solution in one second’. The specification was subsequently shown to have made the ampere slightly smaller than intended,
[Nature Vol. 78, 678-81 (1908)] prompting the adoption by the IEC of 1908 of the distinct name international ampere, with no reference to it being either absolute or practical (though it was the latter). Because of experimental vagaries, the value for conversions is normally referred to as the mean international ampere = 0.999 85~ A. There is also the US international ampere = 0.999 835~ A.

At the implementation of the Metric-m.k.s. A. system in 1948, with the ampere as the base electrical unit but its definition made compatible with the original absolute units, the modern ampere became essentially the old practical ampere; this became identically the ampere of the SI (again the base electric unit).

The calibration of reference electrical instruments from the fundamental definition presents obvious practical problems with accuracy, as well as the impossibility of literally infinite length. Until the 1980s the method involved weighing on a balance the magnetic force between two coils of carefully measured copper wire; this gave an accuracy of barely 1 in 10⁵. Discovery of the Josephson effect, then of the quantum Hall effect, applying at very low temperatures with superconductors, together with subsequent development of the moving-coil balance and related work with the volt, improved accuracies about a thousandfold for the ampere, volt, ohm, etc.
[Hartland A. Contemp. Phys. Vol. 29, 477 (1988) http://www.npl.co.uk/npl/publications/electricity/] For maximum accuracy, the ampere has been realized via the watt, by comparison of electrical power and mechanical power.
[Taylor B. N. Metrologia Vol. 21, 37-9 (1985)]

Previous to adoption of the ampere, the names weber, oersted, and oerstedt were applied to units of electric current strength.

The International Standard unit for electrical current. A unit of the rate of flow of electric current; an electromotive force of 1 volt acting across a resistance of 1 ohm results in a current flow of 1 ampere.

A unit of electric current strength, the current yielded by one volt of electromotive force against one ohm of resistance.

To convert from ampere-hours to:

coulombs, multiply by 3600.
faradays, multiply by 0.03731.

For other uses, see Ampere (disambiguation).

Current can be measured by a galvanometer, via the deflection of a magnetic needle in the magnetic field created by the current.

The ampere (symbol: A) is the SI unit of electric current.^[1] The ampere, in practice often shortened to amp, is an SI base unit, and is named after André-Marie Ampère, one of the main discoverers of electromagnetism.

In practical terms, the ampere is a measure of the amount of electric charge passing a point per unit time. Around 6.242 × 10¹⁸ electrons passing a given point each second constitutes one ampere.^[2] (Since electrons have negative charge, they flow in the opposite direction to the conventional current.)

Contents 1 Definition 2 History 3 Realisation 4 Proposed future definition 5 See also 6 References 7 External links

Definition

Qualitatively, the ampere "is now defined in terms of a current that, if maintained in two straight parallel conductors of specific sizes and positions, would produce a certain amount of [magnetic] force between the conductors."^[3] Quantitatively, the ampere is defined to be the constant current which will produce an attractive force of 2 × 10^–7 newtons per metre of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one metre apart in a vacuum.^[1][4][5] The definition is based on Ampère's force law.^[6] 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.

In terms of Ampère's force law,

so

The SI unit of charge, the coulomb, "is the quantity of electricity carried in 1 second by a current of 1 ampere."^[7] 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.

History

The term honors André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.

The ampere was originally defined as one tenth of the CGS system electromagnetic unit of current (now known as the abampere), the amount of current which generates a force of two dynes per centimetre of length between two wires one centimetre apart.^[8] 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.^[9] Later, more accurate measurements revealed that this current is 0.99985 A.

Realisation

The 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.^[10]

At present, techniques to establish the realization of an ampere have a relative uncertainty of approximately a few parts in 10⁷, and involve realizations of the watt, the ohm and the volt.^[11]

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.^[12] 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. 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, and, depending on the outcome of experiments over the next few years, to formally propose the change at the 24th General Conference on Weights and Measures (CGPM) in 2011.^[13]

See also

• Ammeter
• Electric shock
• Hydraulic analogy
• Magnetic constant

References

1. ^ ^{a b} BIPM official definition
2. ^ Bodanis, David. (2005). Electric Universe. New York: Three Rivers Press
3. ^ Beyond the Kilogram: Redefining the International System of Units(2006). National Institute of Standards and Technology. Square brackets appear in original. Retrieved March 2008.
4. ^ The BIPM does not distinguish between quantum vacuum and free space.
5. ^ Paul M. S. Monk, Physical Chemistry: Understanding our Chemical World, John Wiley and Sons, 2004 online.
6. ^ Raymond A Serway & Jewett JW (2006). Serway's principles of physics: a calculus based text (Fourth Edition ed.). Belmont, CA: Thompson Brooks/Cole. p. 746. ISBN 053449143X. http://books.google.com/books?id=1DZz341Pp50C&pg=RA1-PA746&dq=wire+%22magnetic+force%22&lr=&as_brr=0&sig=4vMV_CH6Nm8ZkgjtDJFlupekYoA#PRA1-PA746,M1. 
7. ^ Bureau International des Poids et Mesures. (2006).The International System of Units (SI), 8th ed. p. 144.
8. ^ A short history of the SI units in electricity
9. ^ History of the ampere
10. ^ Practical realization of unit definitions: Electrical quantities
11. ^ BIPM SI brochure; Appendix 2
12. ^ Beyond the Kilogram: Redefining the International System of Units
13. ^ 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

External links

• The NIST Reference on Constants, Units, and Uncertainty
• A short history of the SI units in electricity
• NIST Definition of ampere and μ₀

v • d • e SI base units Metre · Kilogram · Second · Ampere · Kelvin · Mole · Candela SI derived units

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n. - ampere

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n. - ampère

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n. - Ampere

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n. - αμπέρ

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n. - ampère (m) (Eletr.)

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ампер

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n. - amperio

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n. - ampere

中文（简体）(Chinese (Simplified))
安培

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n. - 安培

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n. - 암페어(전류의 실용 단위)

日本語 (Japanese)
n. - アンペア

العربيه (Arabic)
‏(الاسم) الأمبير : وحدة لقياس, قوة التيار الكهربائي‏

עברית (Hebrew)
n. - ‮אמפר (יחידת-זרם)‬

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