Ampere

From Wikipedia, the free encyclopedia

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 base unit of electric current. It is named after André-Marie Ampère, one of the main discoverers of electromagnetism.

[edit] Definition

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 2×10^–7 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.

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.

[edit] 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 of vacuum to μ₀ = 4π×10⁻⁷ 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):

[edit] 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:

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 = 1.60217653×10^-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.

[edit] 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 for all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (A). But when an SI unit is spelled out, it should always be written in lowercase (ampere), unless it begins a sentence or is the name "degree Celsius". — Based on The International System of Units, section 5.2.

[edit] See also

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

[edit] External links

• 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