volt: Definition and Much More from Answers.com

Dictionary:

volt¹

(vōlt)

[Etymology: A. G. A. Anastassio, Count Volta; Italy 1745-1827] voltage, electromotive force, potential difference. Symbol V. The electromotive force or potential difference in volts between two points of a steady current equals the ratio of power produced in watts to the current strength in amperes; equally the product of current strength in amperes and resistance in ohms. V = W·A^-1 = A·Ω.

SI, Metric-m.k.s.A. (= m²·kg·s^-3·A^-1 in base terms). The following are among the coherent derived units:

• V·m^-1 for electric field strength;
• V·s = weber for magnetic flux;
• V·A for apparent power;
• V·A^-1 = ohm for electric resistance.

Metric-c.g.s. See abvolt (a base unit for the e.m.u. system); statvolt. See also practical unit.

UK 1 MV = 1 crocodile.

The volt as used today was originally defined in 1881 at the first International Electrical Conference,

[Nature Vol. 24, 512 (1881)] among the practical units derived from the absolute units of the e.m.u. system. It was set at 10⁸ times the volt of that system i.e. 1 practical volt = 10⁸ abvolt. Like all the e.m. units, the abvolt was itself defined in terms of purely mechanical units; it had been initiated in the 1860s as the ohma.

Initially an explicit laboratory specification of the volt was established by the International Electrical Conference, making it a base unit instead of a derived unit. Expressed in terms of the potential of the common voltaic cell of the time, the specification was subsequently shown to have made the volt slightly larger than intended and not precisely consistent with the independently defined ampere and ohm.

[Nature Vol. 78, 678-81 (1908)] The IEC of 1908 took away the inconsistency by making the volt a derived unit from those others, but this still had a derived discrepancy from intent, so the Conference adopted the distinct name international volt, lacking reference to being absolute or practical (though it was the latter). Because of experimental vagaries, the value for conversions is normally referred to as the mean international volt, V_M = 1.000 34 V.

[Nature Vol. 163, 427-8 (1949)] There is also the US international volt, = 1.000 330~ V, defined by Congress in an Act of 1894.

With the implementation of the m.k.s.A. system in 1948, and its basing of electrical units on an ampere compatible with the original absolute units, the modern volt became essentially the old practical volt; this became identically the volt of the SI.

The calibration of reference electrical instruments from the fundamental definition presents obvious practical problems of accuracy. 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. For maximum accuracy, the volt is now realized using the Josephson effect that applies at very low temperatures with superconductors, via the Josephson constant, which relates volts to frequency.

[Hartland A. Contemp. Phys. Vol. 29, 477 (1988)

] Together with subsequent development of the moving-coil balance, this allows accuracy better than 1 in 10⁸.

Revised conventional values for the constants involved in laboratory realizations using these effects, adopted internationally beginning in 1990, resulted in reducing the resistance value of many laboratory standards by about 0.000 8%.

[Kibble B., Hartland A. New Scientist Vol. 1715, 48-51 (1990)]

To convert from volt inch to:

volt/cm, multiply by .3937.
statvolts, multiply by .003336.

The volt (symbol: V) is the SI derived unit of electric potential difference or electromotive force.^[1][2] It is named in honor of the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, the first modern chemical battery.

The volt is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power. Hence, it is the base SI representation m² · kg · s^-3 · A^-1, which can be equally represented as one joule of energy per coulomb of charge, J/C.

Failed to parse (unknown function\dfrac): \mbox{V} = \dfrac{\mbox{W}}{\mbox{A}} = \dfrac{\mbox{W} \cdot \mbox{s}}{\mbox{A} \cdot \mbox{s}} = \dfrac{\mbox{J}}{\mbox{C}} = \dfrac{\mbox{N} \cdot \mbox{m} }{\mbox{A} \cdot \mbox{s}} = \dfrac{\mbox{kg} \cdot \mbox{m}^2}{\mbox{A} \cdot \mbox{s}^{3}}

Josephson junction definition

Since 1990 the volt is maintained internationally for practical measurement using the Josephson effect, where a conventional value is used for the Josephson constant, fixed by the 18th General Conference on Weights and Measures as

K_{J-90} = 0.4835979 GHz/µV.

Hydraulic analogy

In the hydraulic analogy sometimes used to explain electric circuits by comparing them to water-filled pipes, voltage is likened to water pressure – it determines how fast the electrons will travel through the circuit. Current (in amperes), in the same analogy, is a measure of the volume of water that flows past a given point, the rate of which is determined by the voltage, and the total output measured in watts. The equation that brings all three components together is: volts × amperes = watts

Common voltages

Nominal voltages of familiar sources:

• Nerve cell action potential: around 75 mV
• Single-cell, rechargeable NiMH or NiCd battery: 1.2 V
• Mercury battery 1.355 V
• Single-cell, non-rechargeable alkaline battery (e.g. AAA, AA, C and D cells): 1.5 V
• Lithium polymer rechargeable battery: 3.75 V
• Transistor-transistor logic/CMOS (TTL) power supply: 5 V
• PP3 battery: 9 V
• Automobile electrical system: "12 V", about 11.8 V discharged, 12.8 V charged, and 13.8-14.4 V while charging (vehicle running).
• Household mains electricity: 240 V RMS in Australia, 230 V RMS in Europe, Asia and Africa, 120 V RMS in North America, 100 V RMS in Japan (see List of countries with mains power plugs, voltages and frequencies)
• Rapid transit third rail: 600 to 750 V (see List of current systems for electric rail traction)
• High speed train overhead power lines: 25 kV RMS at 50 Hz, but see List of current systems for electric rail traction for exceptions.
• High voltage electric power transmission lines: 110 kV RMS and up (1150 kV RMS is the record as of 2005)
• Lightning: Varies greatly, often around 100 MV.

Note: Where 'RMS' (root mean square) is stated above, the peak voltage is times greater than the RMS voltage for a sinusoidal signal centered around zero voltage.

History of the volt

In 1800, as the result of a professional disagreement over the galvanic response advocated by Luigi Galvani, Alessandro Volta developed the so-called Voltaic pile, a forerunner of the battery, which produced a steady electric current. Volta had determined that the most effective pair of dissimilar metals to produce electricity was zinc and silver. In the 1880s, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt for electromotive force. The volt was defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power.

Prior to the development of the Josephson junction voltage standard, the volt was maintained in national laboratories using specially constructed batteries called standard cells. The United States used a design called the Weston cell from 1905 to 1972.

See also

• SI for unit prefixes
• SI electromagnetism units
• Voltmeter
• Watt
• Ampere
• what is RMS?

References

External links

• Google Video Lecture about Electricity and Volt by Richard A. Muller