One volt is defined as the electric potential between two points of aconducting wire when anelectric current of oneampere dissipates onewatt ofpower between those points.[2] It can be expressed in terms of SI base units (m,kg,s, andA) as
Equivalently, it is the potential difference between two points that will impart onejoule ofenergy percoulomb of charge that passes through it. It can be expressed in terms of SI base units (m,kg,s, andA) as
It can also be expressed as amperes timesohms (current times resistance,Ohm's law),webers per second (magnetic flux per time), watts per ampere (power per current), or joules per coulomb (energy per charge), which is also equivalent toelectronvolts perelementary charge:
The volt is named afterAlessandro Volta. As with everySI unit named after a person, its symbol starts with anupper case letter (V), but when written in full, it follows the rules for capitalisation of acommon noun; i.e.,volt becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.
For theJosephson constant,KJ = 2e/h (wheree is theelementary charge andh is thePlanck constant), a "conventional" valueKJ-90 =0.4835979 GHz/μV was used for the purpose of defining the volt. As a consequence of the2019 revision of the SI, as of 2019 the Josephson constant has an exact value ofKJ =483597.84841698... GHz/V, which replaced the conventional valueKJ-90.
This standard is typically realized using a series-connected array of several thousand or tens of thousands ofjunctions, excited by microwave signals between 10 and 80 GHz (depending on the array design).[4] 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.[5]
In thewater-flow analogy, sometimes used to explain electric circuits by comparing them with water-filled pipes,voltage (difference in electric potential) is likened to difference in water pressure, whilecurrent is proportional to the amount of water flowing. Aresistor would be a reduced diameter somewhere in the piping or something akin to a radiator offering resistance to flow.
The relationship between voltage and current is defined (in ohmic devices like resistors) byOhm's law. Ohm's Law is analogous to theHagen–Poiseuille equation, as both are linear models relatingflux andpotential in their respective systems.
Amultimeter can be used to measure the voltage between two positions.1.5 V C-cell batteries
The voltage produced by eachelectrochemical cell in abattery is determined by the chemistry of that cell (seeGalvanic cell § Cell voltage). 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.
Automotive battery systems use cells with 2.1 volts per cell; a "12 V" battery has six cells connected in series, which produces 12.6 V; a "24 V" battery has 12 cells connected in series, producing 25.2 V. Some antique vehicles use "6 V" 3-cell batteries, or 6.3 volts.
In 1800, as the result of a professional disagreement over the galvanic response advocated byLuigi Galvani,Alessandro Volta developed the so-calledvoltaic pile, a forerunner of thebattery, which produced a steady electriccurrent. Volta had determined that the most effective pair of dissimilar metals to produce electricity waszinc andsilver. In 1861,Latimer Clark and SirCharles Bright coined the name "volt" for the unit of resistance.[11] By 1873, the British Association for the Advancement of Science had defined the volt, ohm, and farad.[12] In 1881, the International Electrical Congress, now theInternational Electrotechnical Commission (IEC), approved the volt as the unit for electromotive force.[13] They made the volt equal to 108cgs 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 aDaniell cell, the standard source of voltage in the telegraph systems of the day.[14] 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 oneampere dissipates onewatt of power.
The "international volt" was defined in 1893 as1⁄1.434 of theemf of aClark cell. This definition was abandoned in 1908 in favor of a definition based on the internationalohm and international ampere until the entire set of "reproducible units" was abandoned in 1948.[15]
^Keller, Mark W. (18 January 2008),"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,ISSN0026-1394,S2CID122008182, archived fromthe original(PDF) on 27 May 2010, retrieved11 April 2010,Theoretically, there are no current predictions for any correction terms. Empirically, several experiments have shown thatKJ andRK 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.
^Horowitz, Paul; Winfield, Hill (2015).The Art of Electronics (3. ed.). Cambridge [u.a.]: Cambridge Univ. Press. p. 689.ISBN978-0-521-809269.
^As names for units of various electrical quantities, Bright and Clark suggested "ohma" for voltage, "farad" for charge, "galvat" for current, and "volt" for resistance. See:
^Sir W. Thomson, et al. (1873)"First report of the Committee for the Selection and Nomenclature of Dynamical and Electrical Units"Archived 23 April 2017 at theWayback Machine,Report of the 43rd Meeting of the British Association for the Advancement of Science (Bradford, September 1873), pp. 222–225. From p. 223: "The 'ohm', as represented by the original standard coil, is approximately 109 C.G.S. units of resistance; the 'volt' is approximately 108 C.G.S. units of electromotive force; and the 'farad' is approximately 1/109 of the C.G.S. unit of capacity."
