The volt (symbol: V) is the unit of
electric potential
The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
,
electric potential difference (
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge t ...
), and
electromotive force
In electromagnetism and electronics, electromotive force (also electromotance, abbreviated emf, denoted \mathcal or ) is an energy transfer to an electric circuit per unit of electric charge, measured in volts. Devices called electrical '' tr ...
in the
International System of Units (SI). It is named after the Italian physicist
Alessandro Volta (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 defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
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 movin ...
of one
ampere
The ampere (, ; symbol: A), often Clipping (morphology), shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One amp ...
dissipates one
watt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...
of
power between those points. Equivalently, it is the potential difference between two points that will impart one
joule
The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applie ...
of
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
per
coulomb
The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI).
In the present version of the SI it is equal to the electric charge delivered by a 1 ampere constant current in 1 second and to elementary ch ...
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
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 equatio ...
), 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
electronvolt
In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacu ...
s per
elementary charge
The elementary charge, usually denoted by 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 charge is a funda ...
:
:
Josephson junction definition
Historically, the "
conventional" volt, ''V''
90, defined in 1987 by the 18th
General Conference on Weights and Measures
The General Conference on Weights and Measures (GCWM; french: Conférence générale des poids et mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established ...
and in use from 1990 to 2019, was implemented using the
Josephson effect
In physics, the Josephson effect is a phenomenon that occurs when two superconductors are placed in proximity, with some barrier or restriction between them. It is an example of a macroscopic quantum phenomenon, where the effects of quantum me ...
for exact frequency-to-voltage conversion, combined with the
caesium frequency standard.
For the
Josephson constant, ''K''
J = 2''e''/''h'' (where ''e'' is the
elementary charge
The elementary charge, usually denoted by 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 charge is a funda ...
and ''h'' is the
Planck constant
The Planck constant, or Planck's constant, is a fundamental physical constant of foundational importance in quantum mechanics. The constant gives the relationship between the energy of a photon and its frequency, and by the mass-energy equivalen ...
), 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
In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artifacts such as the standard kilogram.
Effective 20 May 2019, the 144th ...
, as of 2019 the Josephson constant has an exact value of = , which replaced the conventional value ''K''
J-90.
This standard is typically realized using a series-connected array of several thousand or tens of thousands of
junctions
Junction may refer to:
Arts and entertainment
* ''Junction'' (film), a 2012 American film
* Jjunction, a 2002 Indian film
* Junction (album), a 1976 album by Andrew Cyrille
* Junction (EP), by Basement Jaxx, 2002
* Junction (manga), or ''Hot ...
, 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, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge t ...
(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 a ...
, while
current 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 activ ...
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
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 activ ...
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 equatio ...
. 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 flo ...
, as both are linear models relating
flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ...
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 r ...
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 o ...
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, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. ...
resting potential A 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 opp ...
: ~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 where the electrolyte (most commonly potassium hydroxide) has a pH value above 7. Typically these batteries derive energy from the reaction between zinc metal and manganese dioxid ...
: 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. As with 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, ...
-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 electroly ...
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
*
Automobile battery systems are 2.1 volts per cell; a "12 V" battery is 6 cells, or 12.6 V; a "24 V" battery is 12 cells, or 25.2 V. Some antique vehicles use "6 V" 3-cell batteries, or 6.3 volts.
* Household
mains electricity
Mains electricity or utility power, power grid, domestic power, and wall power, or in some parts of Canada as hydro, is a general-purpose Alternating current, alternating-current (AC) electric power supply. It is the form of electrical power t ...
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, Africa and Australia
*
Rapid transit
Rapid transit or mass rapid transit (MRT), also known as heavy rail or metro, is a type of high-capacity public transport generally found in urban areas. A rapid transit system that primarily or traditionally runs below the surface may be c ...
third rail
A third rail, also known as a live rail, electric rail or conductor rail, is a method of providing electric power to a railway locomotive or train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway ...
: 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
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 that facilitate this movement form a ''transmission network''. This i ...
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, both in the atmosphere or with one on the ground, temporarily neutralize themselves, causing the instantaneous release of an average ...
: a maximum of around 150 MV.
History
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 studied animal electricity. In 1780, he discovered that the muscles of dead frogs' legs t ...
,
Alessandro Volta developed the so-called
voltaic pile
upright=1.2, Schematic diagram of a copper–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, ...
, 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
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 shiny-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 (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
. In 1861,
Latimer Clark 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 ...
(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 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
.e., what is nowadays called the "voltage (difference)"across a conductor when a current of one
ampere
The ampere (, ; symbol: A), often Clipping (morphology), shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One amp ...
dissipates one
watt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...
of power.
The "international volt" was defined in 1893 as 1/1.434 of the
emf of a
Clark cell
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 International ...
. 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
redefinition of SI base units
In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artifacts such as the standard kilogram.
Effective 20 May 2019, the 144th ...
, including defining the value of the
elementary charge
The elementary charge, usually denoted by 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 charge is a funda ...
, took effect on 20 May 2019.
See also
*
Orders of magnitude (voltage)
*
Rail traction voltage
*
SI electromagnetism units
*
SI prefix
The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. ...
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.
An ...
References
External links
History of the electrical units.
{{SI units
SI derived units
Units of electrical potential
Alessandro Volta