Volt
The volt (symbol: V) is the SI unit of electric potential difference and electromotive force. It is named after the Italian physicist Alessandro Volta (1745–1827).
A similar unit, used for practical measurements of potential difference, is the conventional volt (symbol: V90).[note 1] The 2006 CODATA recommended value for V90 is:[1]
- V90 = (KJ–90/KJ) V = [1 + 1.9(2.5) × 10−8] V
The international volt (symbol: Vint) is now obsolete.
Contents
Definition
The volt is defined as the potential difference between two points on a conductor when the current flowing is one ampere and the power dissipated is one watt:[2] from the usual laws of electromagnetism, this implies that the volt is one watt per ampere:
- 1 V = 1 W A−1
Several equivalent definitions can be constructed, for example one volt is one ampere ohm, a historically important definition which derives from Ohm's law. Equally, one volt is one joule per coulomb, which gives the electronvolt as a convenient unit of energy at the atomic scale.
- 1 V = 1 A Ω = 1 J C−1 = 1 kg m2 A−1 s−3
Conventional volt
The conventional volt is defined as the potential difference corresponding to the n = 1 step of the inverse AC Josephson effect when the external frequency is 483 597.9 GHz. This defines a conventional value for the Josephson constant, denoted KJ–90, of 483 597.9 GHz V−1. This definition was approved by the International Committee for Weights and Measures (CIPM) in 1988, and has been in use since 1 January 1990 (hence the subscript "90").[3]
Realization
History
A unit of electromotive force (EMF) in the cgs system was first proposed in 1825 by Franz-Ernst Neumann (1798–1895): this cgs unit would be the EMF produced in a circuit cutting one magnetic line of force per second. Neumann's cgs unit proved far too small, and far too complicated to realize, for practical use, but the basic principle was retained by the First International Conference of Electricians (Paris, 1881), who defined a new unit, the volt, as 108 cgs units of EMF.
The factor of 108 resolved the problem of size, but not that of realization. The Fourth International Conference of Electricians (Chicago, 1893) attempted to resolve this with the institution of international electrical units, a practical system of units in which the "international volt" was defined so that the EMF of a Clark cell at 15 °C was exactly 1.434 Vint.
The 1893 system of electrical units was soon found to be overdefined, and the definition of the volt as a base unit was abandoned in 1908: instead, the international volt became a derived unit, defined in terms of the international ampere and the international ohm using Ohm's law.
Notes and references
Notes
- ↑ V90 is considered to be a physical constant, hence the use of italics for the symbol.
References
- ↑ Mohr, Peter J.; Taylor, Barry N.; Newell, David B. CODATA Recommended Values of the Fundamental Physical Constants: 2006. Rev. Mod. Phys. 2008, 80 (2), 633–730. doi:10.1103/RevModPhys.80.633, <http://physics.nist.gov/cuu/Constants/codata.pdf>.
- ↑ CIPM (1946), Resolution 2.
- ↑ CIPM (1988), Recommendation 1.
External links
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See also the corresponding article on Wikipedia. |
- volt at Sizes.com
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