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The volt (symbol: V) is the
derived unit SI derived units are units of measurement derived from the seven base units specified by the International System of Units (SI). They are either dimensionless or can be expressed as a product of one or more of the base units, possibly scaled by an ...
for
electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is the amount of work energy needed to move a unit of electric charge (for example, a Coulomb) from a reference point to the specif ...
,
electric potential difference Voltage, electric potential difference, electromotive force emf, electric pressure or electric tension is the difference in electric potential between two points, which (in a static electric field) is defined as the work needed per unit of ch ...
(
voltage Voltage, electric potential difference, electromotive force emf, electric pressure or electric tension is the difference in electric potential between two points, which (in a static electric field) is defined as the work needed per unit of ch ...
), and
electromotive force In electromagnetism and electronics, electromotive force (emf, denoted \mathcal and measured in volts) is the electrical action produced by a non-electrical source. Devices (known as ''transducers'') provide an emf by converting other forms of en ...
. It is named after the Italian physicist
Alessandro Volta Alessandro Giuseppe Antonio Anastasio Volta (, ; 18 February 1745 – 5 March 1827) was an Italian physicist, chemist, and pioneer of electricity and power who is credited as the inventor of the electric battery and the discoverer of methane. ...
(1745–1827).


Definition

One volt is defined as the
electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is the amount of work energy needed to move a unit of electric charge (for example, a Coulomb) from a reference point to the specif ...
between two points of a conducting wire when an
electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving part ...
of one
ampere The ampere (, ; symbol: A), often shortened to "amp",SI supports only the use of symbols and deprecates the use of abbreviations for units. is the base unit of electric current in the International System of Units (SI). It is named after André- ...
dissipates one
watt The watt (symbol: W) is a unit of power or radiant flux. In the International System of Units (SI), it is defined as a derived unit of (in SI base units) 1 kg⋅m2⋅s−3 or, equivalently, 1 joule per second. It is used to quantify the rate of ...
of
power Power typically refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power *** Solar power Power may also refer to: Mathematics, science and technology Computing * IBM Power (soft ...
between those points. Equivalently, it is the potential difference between two points that will impart one
joule The joule ( ; symbol: J) is a derived unit of energy in the International System of Units. It is equal to the energy transferred to (or work done on) an object when a force of one newton acts on that object in the direction of the force's motion ...
of
energy In physics, energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object.The second law of thermodynamics imposes limitations on the capacity of a system to transfer energy by p ...
per
coulomb The coulomb (symbol: C) is the International System of Units (SI) unit of electric charge. Under the 2019 redefinition of the SI base units, which took effect on 20 May 2019, the coulomb is exactly 1/() elementary charges. The same number of ele ...

coulomb
of charge that passes through it. It can be expressed in terms of SI base units ( m, kg, s, and A) as: : \text = \frac = \frac = \frac. It can also be expressed as amperes times
ohm The ohm (symbol: Ω) is the SI derived unit of electrical resistance, named after German physicist Georg Ohm. Various empirically derived standard units for electrical resistance were developed in connection with early telegraphy practice, and ...
s (current times resistance,
Ohm's law Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation ...
), webers per second (magnetic flux per time), watts per ampere (power per unit current, definition of electric power), or joules per coulomb (energy per unit charge), which is also equivalent to
electronvolt In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacuum. When used as a ...
s per
elementary charge The elementary charge, usually denoted by or sometimes e is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charg ...
: : \text = \text \Omega = \frac = \frac = \frac = \frac.


Josephson junction definition

The " conventional" volt, V90, defined in 1987 by the 18th
General Conference on Weights and Measures The General Conference on Weights and Measures (French: ''Conférence Générale des Poids et Mesures'', abbreviated CGPM and sometimes referred to as the GCWM) is the supreme authority of the International Bureau of Weights and Measures, the inter ...
and in use from 1990, is implemented using the
Josephson effect The Josephson effect is the phenomenon of supercurrent, a current that flows continuously without any voltage applied, across a device known as a Josephson junction (JJ), which consists of two or more superconductors coupled by a weak link. The w ...
for exact frequency-to-voltage conversion, combined with the caesium frequency standard. For the
Josephson constant The magnetic flux, represented by the symbol , threading some contour or loop is defined as the magnetic field multiplied by the loop area , i.e. . Both and can be arbitrary, meaning can be as well. However, if one deals with the superconductin ...
, ''K''J = 2''e''/''h'' (where ''e'' is the
elementary charge The elementary charge, usually denoted by or sometimes e is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charg ...
and ''h'' is the
Planck constant The Planck constant, or Planck's constant, is the quantum of electromagnetic action that relates a photon's energy to its frequency. The Planck constant multiplied by a photon's frequency is equal to a photon's energy. The Planck constant is a ...
), a "conventional" value ''K''J-90 = 0.4835979 GHz/μV was used for the purpose of defining the volt. As a consequence of the
2019 redefinition of SI base units upright=1.35, The SI system after 1983, but before the 2019 redefinition: Dependence of base unit definitions on other base units (for example, the metre is defined as the distance travelled by light in a specific fraction of a second), with ...
, the Josephson constant was redefined in 2019 to have an exact value of = , which replaced the conventional value . 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

In the '' water-flow analogy'', sometimes used to explain electric circuits by comparing them with water-filled pipes,
voltage Voltage, electric potential difference, electromotive force emf, electric pressure or electric tension is the difference in electric potential between two points, which (in a static electric field) is defined as the work needed per unit of ch ...
(difference in electric potential) is likened to difference in water
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and ev ...
, while
current Currents or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stream), curre ...
is proportional to the amount of water flowing. A
resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active eleme ...
would be a reduced diameter somewhere in the piping or something akin to a radiator offering resistance to flow. Perhaps a
capacitor A capacitor is a device that stores electric charge in an electric field. It is a passive electronic component with two terminals. The effect of a capacitor is known as capacitance. While some capacitance exists between any two electrical co ...

capacitor
could be likened to a U bend where a higher water level can store energy and build up a head of pressure. Perhaps an
inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a coi ...

inductor
could be likened to a fly wheel apparatus. The relationship between voltage and current is defined (in ohmic devices like
resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active eleme ...
s) by
Ohm's law Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation ...
. Ohm's Law is analogous to the
Hagen–Poiseuille equation In nonideal fluid dynamics, the Hagen–Poiseuille equation, also known as the Hagen–Poiseuille law, Poiseuille law or Poiseuille equation, is a physical law that gives the pressure drop in an incompressible and Newtonian fluid in laminar flow fl ...
, as both are linear models relating
flux of \mathbf(\mathbf) with the unit normal vector \mathbf(\mathbf) ''(blue arrows)'' at the point \mathbf multiplied by the area dS. The sum of \mathbf\cdot\mathbf dS for each patch on the surface is the flux through the surface Flux describes ...
and
potential Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social sciences to indicate things that are in a state where they are able to change in ways ranging from the simple rel ...
in their respective systems.


Common voltages

The voltage produced by each
electrochemical cell An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic ce ...
in a 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: *
Nerve cell A neuron or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. It is the main component of nervous tissue in all animals except sponges and placozoa. Plants and fungi do ...
resting potentialA relatively static membrane potential which is usually referred to as the ground value for trans-membrane voltage. The relatively static membrane potential of quiescent cells is called the resting membrane potential (or resting voltage), as oppos ...
: ~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 An alkaline battery (IEC code: L) is a type of primary battery which derives its energy from the reaction between zinc metal and manganese dioxide. Compared with zinc-carbon batteries of the Leclanché cell or zinc chloride types, alkaline batter ...
: 1.5 V; zinc-carbon battery: 1.56 V if fresh and unused * LiFePO4 rechargeable battery: 3.3 V *
Cobalt Cobalt is a chemical element with the symbol Co and atomic number 27. Like nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, produc ...
-based
Lithium polymer A lithium polymer battery, or more correctly lithium-ion polymer battery (abbreviated as LiPo, LIP, Li-poly, lithium-poly and others), is a rechargeable battery of lithium-ion technology using a polymer electrolyte instead of a liquid electrolyte ...
rechargeable battery: 3.75 V (see
Comparison of commercial battery types This is a list of commercially-available battery types summarizing some of their characteristics for ready comparision. Common characteristics Cost in USD, adjusted for inflation. Typical. See for alternative electrode materials. Rechargeabl ...
) * Transistor-transistor logic/
CMOS#REDIRECT CMOS {{R from other capitalisation ...
(TTL) power supply: 5 V *
USB Universal Serial Bus (USB) is an industry standard that establishes specifications for cables and connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad ...

USB
: 5 V DC *
PP3 battery Size comparison of batteries: D, C, AA, AAA, AAAA, PP3 (9-volt) The nine-volt battery, or 9-volt battery, is a common size of battery that was introduced for the early transistor radios. It has a rectangular prism shape with rounded edges and a p ...
: 9 V * Automobile battery systems are 2.1 volts per cell; a "12V" battery is 6 cells or 12.6V; a "24V" battery is 12 cells or 25.2V. Some antique vehicles use "6V" 3-cell batteries or 6.3 volts. * Household
mains electricity Mains electricity (Commonwealth English), also known by the American English terms utility power, power grid, domestic power, and wall power, or in some parts of Canada as hydro, is a general-purpose alternating-current (AC) electric power suppl ...
AC: (see
List of countries with mains power plugs, voltages and frequencies A ''list'' is any enumeration of a set of items. List or lists may also refer to: People * List (surname) Organizations * List College, an undergraduate division of the Jewish Theological Seminary of America * SC Germania List, German rugby un ...
) ** 100 V in Japan ** 120 V in North America, ** 230 V in Europe, Asia, Africa and Australia *
Rapid transit Rapid transit or mass rapid transit (MRT), also known as heavy rail, metro, subway, tube, U-Bahn, metropolitana or underground, is a type of high-capacity public transport generally found in urban areas. Unlike buses or trams, rapid transit s ...
third rail Shoe of NYC Subway car making contact with third rail. In the foreground is the third rail for the adjacent track. A third rail, also known as a live rail, electric rail or conductor rail, is a method of providing electric power to a railway ...
: 600–750 V (see
List of railway electrification systems This is a list of the power supply systems that are, or have been, used for tramway and railway electrification systems. Note that the voltages are nominal and vary depending on load and distance from the substation. Many modern trams and trains u ...
) * High-speed train overhead power lines: 25 kV at 50 Hz, but see the
List of railway electrification systems This is a list of the power supply systems that are, or have been, used for tramway and railway electrification systems. Note that the voltages are nominal and vary depending on load and distance from the substation. Many modern trams and trains u ...
and 25 kV at 60 Hz for exceptions. * High-voltage
electric power transmission Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines which facilitate this movement are known as a ''transmission network''. Thi ...
lines: 110 kV and up (1.15 MV is the record; the highest active voltage is 1.10 MV) *
Lightning Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions in the atmosphere or ground temporarily equalize themselves, causing the instantaneous release of as much as one gigajoule of energy. ...
:, often around 100 MV.


History

upleft, Alessandro Volta In 1800, as the result of a professional disagreement over the galvanic response advocated by
Luigi Galvani Luigi Galvani (, also , ; la, Aloysius Galvanus; 9 September 1737 – 4 December 1798) was an Italian physician, physicist, biologist and philosopher, who discovered animal electricity. He is recognized as the pioneer of bioelectromagnetics. In 17 ...
,
Alessandro Volta Alessandro Giuseppe Antonio Anastasio Volta (, ; 18 February 1745 – 5 March 1827) was an Italian physicist, chemist, and pioneer of electricity and power who is credited as the inventor of the electric battery and the discoverer of methane. ...
developed the so-called
voltaic pile –zinc voltaic pile. The copper and zinc discs were separated by cardboard or felt spacers soaked in salt water (the electrolyte). Volta's original piles contained an additional zinc disk at the bottom, and an additional copper disk at the top. The ...

voltaic pile
, a forerunner of the battery, which produced a steady electric
current Currents or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stream), curre ...
. Volta had determined that the most effective pair of dissimilar metals to produce electricity was
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a silvery-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic t ...
and
silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical cond ...
. In 1861,
Latimer Clark Josiah Latimer Clark FRAS (10 March 1822 – 30 October 1898), was an English electrical engineer, born in Great Marlow, Buckinghamshire. Biography Josiah Latimer Clark was born in Great Marlow, Buckinghamshire, and was younger brother to Edwin ...
and Sir Charles Bright coined the name "volt" for the unit of resistance. By 1873, the British Association for the Advancement of Science had defined the volt, ohm, and farad. In 1881, the International Electrical Congress, now the
International Electrotechnical Commission The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and rela ...
(IEC), approved the volt as the unit for electromotive force.(Anon.) (September 24, 1881
"The Electrical Congress,"
''The Electrician'', 7 : 297.
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 Daniell cell is a type of electrochemical cell invented in 1836 by John Frederic Daniell, a British chemist and meteorologist, and consists of a copper pot filled with a copper (II) sulfate solution, in which is immersed an unglazed earthenware ...
, the standard source of voltage in the telegraph systems of the day. At that time, the volt was defined as the potential difference .e., what is nowadays called the "voltage (difference)"across a conductor when a current of one
ampere The ampere (, ; symbol: A), often shortened to "amp",SI supports only the use of symbols and deprecates the use of abbreviations for units. is the base unit of electric current in the International System of Units (SI). It is named after André- ...
dissipates one
watt The watt (symbol: W) is a unit of power or radiant flux. In the International System of Units (SI), it is defined as a derived unit of (in SI base units) 1 kg⋅m2⋅s−3 or, equivalently, 1 joule per second. It is used to quantify the rate of ...
of power. The "international volt" was defined in 1893 as 1/1.434 of the
emf EMF may refer to: Computing * Eclipse Modeling Framework, a modeling framework based on Eclipse software * Enhanced Metafile, a Microsoft Windows graphics file format * Electromagnetic Field (festival), a hacker camping festival Music * EMF (band ...
of a
Clark cellClark cell (1897) The Clark cell, invented by English engineer Josiah Latimer Clark in 1873, is a wet-chemical cell (colloquially: ''battery'') that produces a highly stable voltage. In 1893, the output of the Clark cell at 15 °C was defined by the ...
. This definition was abandoned in 1908 in favor of a definition based on the international
ohm The ohm (symbol: Ω) is the SI derived unit of electrical resistance, named after German physicist Georg Ohm. Various empirically derived standard units for electrical resistance were developed in connection with early telegraphy practice, and ...
and international ampere until the entire set of "reproducible units" was abandoned in 1948. A
redefinition of SI base units upright=1.35, The SI system after 1983, but before the 2019 redefinition: Dependence of base unit definitions on other base units (for example, the metre is defined as the distance travelled by light in a specific fraction of a second), with ...
, including defining the value of the
elementary charge The elementary charge, usually denoted by or sometimes e is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charg ...
, took effect on 20th May 2019.


See also

* Orders of magnitude (voltage) * Rail traction voltage *
SI electromagnetism units See also * SI * Second * Speed of light * List of electromagnetism equations References External links History of the electrical units. Electromagnetism {{Science-stub ...
*
SI prefix A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The prefix '' ...
for unit prefixes * Standardised railway voltages *
Voltmeter A voltmeter is an instrument used for measuring electric potential difference between two points in an electric circuit. It is connected in parallel. It usually has a high resistance so that it takes negligible current from the circuit. Analo ...


References


External links


History of the electrical units.
{{SI units SI derived units Units of electrical potential Alessandro Volta