|Unit system||SI derived unit|
|Unit of||Electric potential, electromotive force|
|Named after||Alessandro Volta|
|In SI base units:||kg⋅m2⋅s-3⋅A-1|
The volt (symbol: V) is the derived unit for electric potential, electric potential difference (voltage), and electromotive force. The volt is named in honour of the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, possibly the first chemical battery.
The volt is a measure of electric potential. Electrical potential is a type of potential energy, and refers to the energy that could be released if electric current is allowed to flow. An analogy is that a suspended object (a mass) that is said to have gravitational potential energy (as a result of a gravitational field), which is the amount of energy that would be released if the object was allowed to fall.
In alternating current, the voltages increase, decrease and change direction at regular intervals. As a result, voltage for alternating current almost never refers to the voltage at a particular instant, but instead is the root mean square (RMS) voltage, which is a way of defining an effective voltage when calculating power (RMS voltage × RMS current). In most cases, the fact that a voltage is an RMS voltage is not explicitly stated, but assumed.
One volt is defined as the difference in electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points. It is also equal to the potential difference between two parallel, infinite planes spaced 1 meter apart that create an electric field of 1 newton per coulomb. Additionally, it is the potential difference between two points that will impart one joule of energy per coulomb of charge that passes through it. It can be expressed in terms of SI base units (m, kg, s, and A) as:
It can also be expressed as amperes times ohms (current times resistance, Ohm's law), watts per ampere (power per unit current, Joule's law), or joules per coulomb (energy per unit charge) which is also equivalent to electron volts per elementary charge:
Josephson junction definition
The "conventional" volt, V90, defined in 1988 by the 18th General Conference on Weights and Measures and in use from 1990, is implemented using the Josephson effect for exact frequency-to-voltage conversion, combined with the cesium frequency standard. For the Josephson constant, KJ = 2e/h (where e is the elementary charge and h is the Planck constant), the "conventional" value KJ-90 is used:
This standard is typically realized using a series connected array of several thousand or tens of thousands of junctions, excited by microwave signals between 10 and 80 GHz (depending on the array design). Empirically, several experiments have shown that the method is independent of device design, material, measurement setup, etc., and no correction terms are required in a practical implementation.
Water flow analogy
The relationship between voltage and current is defined (in ohmic devices like resistors) by Ohm's Law. Ohm's Law is analogous to the Hagen–Poiseuille equation, as both are linear models relating flux and potential in their respective systems.
|This section needs additional citations for verification. (June 2014)|
The voltage produced by each electrochemical cell in a battery is determined by the chemistry of that cell. Cells can be combined in series for multiples of that voltage, or additional circuitry added to adjust the voltage to a different level. Mechanical generators can usually be constructed to any voltage in a range of feasibility.
Nominal voltages of familiar sources:
- Nerve cell resting potential: around −75 mV
- Single-cell, rechargeable NiMH or NiCd battery: 1.2 V
- Single-cell, non-rechargeable (e.g., AAA, AA, C and D cells): alkaline battery: 1.5 V; zinc-carbon battery: 1.56 V if fresh and unused
- LiFePO4 rechargeable battery: 3.3 V
- Lithium polymer rechargeable battery: 3.75 V (see Rechargeable battery#Table of rechargeable battery technologies)
- Transistor-transistor logic/CMOS (TTL) power supply: 5 V
- USB: 5 V DC
- PP3 battery: 9 V
- Automobile electrical system: nominal 12 V, about 11.8 V discharged, 12.8 V charged, and 13.8–14.4 V while charging (vehicle running).
- Trucks/lorries: 24 V DC (Europe), 12 V DC (North America)
- Household mains electricity AC: (see List of countries with mains power plugs, voltages and frequencies)
- 100 V in Japan
- 120 V in North America,
- 230 V in Europe, Asia and Africa,
- Rapid transit third rail: 600–750 V (see List of current systems for electric rail traction)
- High-speed train overhead power lines: 25 kV at 50 Hz, but see the list of current systems for electric rail traction and 25 kV at 60 Hz for exceptions.
- High-voltage electric power transmission lines: 110 kV and up (1.15 MV was the record as of 2005)
- Lightning: Varies greatly, often around 100 MV.
|This section needs additional citations for verification. (June 2014)|
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 as the unit for electromotive force. They made the volt equal to 108 cgs units of voltage, the cgs system at the time being the customary system of units in science. They chose such a ratio because the cgs unit of voltage is inconveniently small and one volt in this definition is approximately the emf of a Daniell cell, the standard source of voltage in the telegraph systems of the day. At that time, the volt was defined as the potential difference [i.e., what is nowadays called the "voltage (difference)"] across a conductor when a current of one ampere dissipates one watt of power.
The international volt was defined in 1893 as 1/1.434 of the emf of a Clark cell. This definition was abandoned in 1908 in favor of a definition based on the international ohm and international ampere until the entire set of "reproducible units" was abandoned in 1948.
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.
This SI unit is named after Alessandro Volta. As with every International System of Units (SI) unit named for a person, the first letter of its symbol is upper case (V). However, when an SI unit is spelled out in English, it should always begin with a lower case letter (volt)—except in a situation where any word in that position would be capitalized, such as at the beginning of a sentence or in material using title case. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.— Based on The International System of Units, section 5.2.
- "SI Brochure, Table 3 (Section 2.2.2)". BIPM. 2006. Retrieved 2007-07-29.
- BIPM SI Brochure: Appendix 1, p. 144
- Burroughs, Charles J.; Bent, Samuel P.; Harvey, Todd E.; Hamilton, Clark A. (1999-06-01), "1 Volt DC Programmable Josephson Voltage Standard", IEEE Transactions on Applied Superconductivity (Institute of Electrical and Electronics Engineers (IEEE)) 9 (3): 4145–4149, doi:10.1109/77.783938, ISSN 1051-8223, retrieved 2014-06-27
- Keller, Mark W (2008-01-18), "Current status of the quantum metrology triangle" (PDF), Metrologia 45 (1): 102–109, Bibcode:2008Metro..45..102K, doi:10.1088/0026-1394/45/1/014, ISSN 0026-1394,
Theoretically, there are no current predictions for any correction terms. Empirically, several experiments have shown that KJ and RK are independent of device design, material, measurement setup, etc. This demonstration of universality is consistent with the exactness of the relations, but does not prove it outright.
- Bullock, Orkand, and Grinnell, pp. 150–151; Junge, pp. 89–90; Schmidt-Nielsen, p. 484
- Hamer, Walter J. (January 15, 1965). Standard Cells: Their Construction, Maintenance, and Characteristics (PDF). National Bureau of Standards Monograph #84. US National Bureau of Standards.
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