The volt (symbol: V), named after Alessandro Volta, is the unit of measurement of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI).
Definition
One volt is defined as the electric potential between two points of a conducting wire when an
electric current of one
ampere dissipates one
watt of power between those points.
[ BIPM SI Brochure: Appendix 1 , p. 144.] It can be expressed in terms of SI base units (
metre,
kilogram,
second, and
ampere) as
\text{V} = \frac{\text{power}}{\text{electric current}} = \frac{\text{W}}{\text{A}} = \frac{\text{kg}{\cdot}\text{m}^2{\cdot}\text{s}^{-3}}{\text{A}} = \text{kg}{\cdot}\text{m}^2{\cdot}\text{s}^{-3}{\cdot}{\text{A}^{-1}}.
Equivalently, 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 (metre, kilogram, second, and ampere) as
\text{V} = \frac{\text{potential energy}}{\text{charge}} = \frac{\text{J}}{\text{C}} = \frac{\text{kg}{\cdot}\text{m}^2{\cdot}\text{s}^{-2}}{\text{A}{\cdot}\text{s}} = \text{kg}{\cdot}\text{m}^2{\cdot}\text{s}^{-3}{\cdot}{\text{A}^{-1}}.
It can also be expressed as amperes times (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 to per elementary charge:
\text{V} = \text{A}{\cdot}\Omega = \frac{\text{Wb}}{\text{s}} = \frac{\text{W}}{\text{A}} = \frac{\text{J}}{\text{C}} = \frac{\text{eV}}{e}.
Josephson junction definition
Historically the "conventional" volt,
V90, defined in 1987 by the 18th General Conference on Weights and Measures
and in use from 1990 to 2019, was implemented using the
Josephson effect for exact frequency-to-voltage conversion, combined with the
Caesium standard. Though the Josephson effect is still used to realize a volt, the constant used has changed slightly.
For the Josephson constant, KJ = 2 e/ h (where e is the elementary charge and h is the Planck constant), a "conventional" value KJ-90 = was used for the purpose of defining the volt. As a consequence of the 2019 revision of the SI, as of 2019 the Josephson constant has an exact value of = , which replaced the conventional value KJ-90.
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).
Water-flow analogy
In the
water-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, while
electric current is proportional to the amount of water flowing. A
resistor 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) 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.
Common voltages
The voltage produced by each electrochemical cell in a
Electric battery is determined by the chemistry of that cell (see ). 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:
-
Neuron resting potential: ~ 75 mV
[Bullock, Orkand, and Grinnell, pp. 150–151; Junge, pp. 89–90; Schmidt-Nielsen, p. 484.]
-
Single-cell, rechargeable NiMH
-
Single-cell, non-rechargeable (e.g., Electric battery): alkaline battery: 1.5 V;
-
Logic voltage levels: 1.2 V, 1.5 V, 1.8 V, 2.5 V, 3.3 V, 5.0 V
-
LiFePO4 rechargeable battery: 3.3 V
-
Cobalt-based lithium polymer rechargeable battery: 3.75 V (see Comparison of commercial battery types)
-
Transistor–transistor logic/CMOS (TTL) power supply: 5 V
-
USB: 5 V DC
-
PP3 battery: 9 V
-
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.
-
Household mains electricity AC (see Mains electricity by country for a list of countries with mains power plugs, voltages and frequencies)
-
100 V in Japan
-
120 V in North America
-
230 V in Europe, Asia, Africa and Australia
-
Rapid transit third rail: 600–750 V (see List of railway electrification systems)
-
High-speed train overhead power lines: 25 kV at 50 Hz, but see the List of railway electrification systems and 25 kV at 60 Hz for exceptions.
-
High-voltage electric power transmission lines: 110 kV and up (1.15 MV is the record; the highest active voltage is 1.10 MV
-
Lightning: a maximum of around 150 MV.
History
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
Electric battery, which produced a steady electric
Electric current. Volta had determined that the most effective pair of dissimilar metals to produce electricity was
zinc and
silver. In 1861, Latimer Clark and Sir Charles Bright coined the name "volt" for the unit of resistance.
[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:
] By 1873, the British Association for the Advancement of Science had defined the volt, ohm, and farad.
[Sir W. Thomson, et al. (1873) "First report of the Committee for the Selection and Nomenclature of Dynamical and Electrical Units" , 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."] In 1881, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt as the unit for electromotive force.
[(Anon.) (24 September 1881) "The Electrical Congress" , The Electrician, 7: 297.] They made the volt equal to 10
8 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., across a conductor when a current of one
ampere dissipates one
watt of power.
The "international volt" was defined in 1893 as 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.
A 2019 revision of the SI, including defining the value of the elementary charge, took effect on 20 May 2019.
See also
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Orders of magnitude (voltage)
-
Rail traction voltage
-
SI electromagnetism units
-
SI prefix for unit prefixes
-
Standardised railway voltages
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Voltmeter
External links
