Volt | |
---|---|

General information | |

Unit system | SI derived unit |

Unit of | Electric potential, electromotive force |

Symbol | V |

Named after | Alessandro Volta |

In SI base units: | kg·m ^{2}·s ^{−3}·A ^{−1} |

The **volt** (symbol: **V**) is the derived unit for electric potential, electric potential difference (voltage), and electromotive force.^{ [1] } It is named after the Italian physicist Alessandro Volta (1745–1827).

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.^{ [2] } 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 (m, kg, s, and A) as

It can also be expressed as amperes times ohms (current times resistance, Ohm's law), 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 electronvolts per elementary charge:

The "conventional" volt, V_{90}, defined in 1987 by the 18th General Conference on Weights and Measures ^{ [3] } and in use from 1990, is implemented using the Josephson effect 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 and *h* is the Planck constant), 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, the Josephson constant was redefined in 2019 to have an exact value of *K*_{J} = 483597.84841698... GHz/V,^{ [4] } 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, excited by microwave signals between 10 and 80 GHz (depending on the array design).^{ [5] } 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.^{ [6] }

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 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. Perhaps a 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 could be likened to a fly wheel apparatus.

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.

The voltage produced by each electrochemical cell in a battery is determined by the chemistry of that cell (see Galvanic 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.

Nominal voltages of familiar sources:

- Nerve cell resting potential: ~75 mV
^{ [7] } - Single-cell, rechargeable NiMH
^{ [8] }or NiCd battery: 1.2 V - Single-cell, non-rechargeable (e.g., AAA, AA, C and D cells): alkaline battery: 1.5 V;
^{ [9] }zinc–carbon battery: 1.56 V if fresh and unused - LiFePO
_{4}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
- 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 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 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
^{ [10] }) - Lightning: a maximum of around 150 MV.
^{ [11] }

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 1861, Latimer Clark and Sir Charles Bright coined the name "volt" for the unit of resistance.^{ [12] } By 1873, the British Association for the Advancement of Science had defined the volt, ohm, and farad.^{ [13] } In 1881, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt as the unit for electromotive force.^{ [14] } 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.^{ [15] } 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.^{ [16] }

A redefinition of SI base units, including defining the value of the elementary charge, took effect on 20 May 2019.^{ [17] }

The **ampere**, often shortened to **amp**, is the base unit of electric current in the International System of Units (SI). It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electromagnetism.

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 particles are called charge carriers, which may be one of several types of particles, depending on the conductor. In electric circuits the charge carriers are often electrons moving through a wire. In semiconductors they can be electrons or holes. In a electrolyte the charge carriers are ions, while in plasma, an ionized gas, they are ions and electrons.

The **centimetre–gram–second system of units** is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways in which the CGS system was extended to cover electromagnetism.

**Voltage**, **electric potential difference**, **electric pressure** or **electric tension** is the difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points. In the International System of Units, the derived unit for voltage is named *volt*. In SI units, work per unit charge is expressed as joules per coulomb, where 1 volt = 1 joule per 1 coulomb. The old SI definition for *volt* used power and current; starting in 1990, the quantum Hall and Josephson effect were used, and recently (2019) fundamental physical constants have been introduced for the definition of all SI units and derived units. Voltage or electric potential difference is denoted symbolically by ∆*V*, simplified *V*, or *U*, for instance in the context of Ohm's or Kirchhoff's circuit laws.

The **coulomb** 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/(1.602176634×10^{−19}) elementary charges. The same number of electrons has the same magnitude but opposite sign of charge, that is, a charge of −1 C.

**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 that describes this relationship:

In electromagnetism and electronics, **electromotive force** is the electrical action produced by a non-electrical source. Devices provide an emf by converting other forms of energy into electrical energy, such as batteries or generators. Sometimes an analogy to water pressure is used to describe electromotive force..

The **henry** is the SI derived unit of electrical inductance. If a current of 1 ampere flowing through a coil produces flux linkage of 1 weber turn, that coil has a self inductance of 1 henry. The unit is named after Joseph Henry (1797–1878), the American scientist who discovered electromagnetic induction independently of and at about the same time as Michael Faraday (1791–1867) in England.

The **farad** (symbol: **F**) is the SI derived unit of electrical capacitance, the ability of a body to store an electrical charge. It is named after the English physicist Michael Faraday (1791-1867). In SI base units 1 F = 1 kg^{−1}⋅m^{−2}⋅s^{4}⋅A^{2}.

Two-terminal components and electrical networks can be connected in series or parallel. The resulting electrical network will have two terminals, and itself can participate in a series or parallel topology. Whether a two-terminal "object" is an electrical component or an electrical network is a matter of perspective. This article will use "component" to refer to a two-terminal "object" that participate in the series/parallel networks.

The magnetic flux, represented by the symbol **Φ**, threading some contour or loop is defined as the magnetic field **B** multiplied by the loop area **S**, i.e. **Φ** = **B** ⋅ **S**. Both **B** and **S** can be arbitrary, meaning **Φ** can be as well. However, if one deals with the superconducting loop or a hole in a bulk superconductor, the magnetic flux threading such a hole/loop is actually quantized. The (superconducting) **magnetic flux quantum**Φ_{0} = *h*/(2*e*) ≈ 2.067833848...×10^{−15} Wb is a combination of fundamental physical constants: the Planck constant *h* and the electron charge *e*. Its value is, therefore, the same for any superconductor. The phenomenon of flux quantization was discovered experimentally by B. S. Deaver and W. M. Fairbank and, independently, by R. Doll and M. Näbauer, in 1961. The quantization of magnetic flux is closely related to the Little–Parks effect, but was predicted earlier by Fritz London in 1948 using a phenomenological model.

In physics, the **weber** is the SI derived unit of magnetic flux. A *flux density* of one Wb/m^{2} is one tesla.

A **magnetic circuit** is made up of one or more closed loop paths containing a magnetic flux. The flux is usually generated by permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials like iron, although there may be air gaps or other materials in the path. Magnetic circuits are employed to efficiently channel magnetic fields in many devices such as electric motors, generators, transformers, relays, lifting electromagnets, SQUIDs, galvanometers, and magnetic recording heads.

An **automotive battery** or **car battery** is a rechargeable battery that is used to start a motor vehicle. Its main purpose is to provide an electric current to the electric-powered starting motor, which in turn starts the chemically-powered internal combustion engine that actually propels the vehicle. Once the engine is running, power for the car's electrical systems is still supplied by the battery, with the alternator charging the battery as demands increase or decrease.

**Magnetic reluctance**, or **magnetic resistance**, is a concept used in the analysis of magnetic circuits. It is defined as the ratio of magnetomotive force (mmf) to magnetic flux. It represents the opposition to magnetic flux, and depends on the geometry and composition of an object.

**Electric power** is the rate, per unit time, at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second.

The **ohm** 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 the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time, and of a convenient scale for practical work as early as 1861. As of 2020, the definition of the ohm is expressed in terms of the quantum Hall effect.

A **conventional electrical unit** is a unit of measurement in the field of electricity which is based on the so-called "conventional values" of the Josephson constant, the von Klitzing constant agreed by the International Committee for Weights and Measures (CIPM) in 1988, as well as Δ*ν*_{Cs} used to define the second. These units are very similar in scale to their corresponding SI units, but are not identical because of the different values used for the constants. They are distinguished from the corresponding SI units by setting the symbol in italic typeface and adding a subscript "90" – e.g., the conventional volt has the symbol *V*_{90} – as they came into international use on 1 January 1990.

The **International System of Electrical and Magnetic Units** is an obsolete system of units used for measuring electrical and magnetic quantities. It was proposed as a system of practical international units by unanimous recommendation at the International Electrical Congress, discussed at other Congresses, and finally adopted at the International Conference on Electric Units and Standards in London in 1908. It was rendered obsolete by the inclusion of electromagnetic units in the International System of Units (SI) at the 9th General Conference on Weights and Measures in 1948.

The **siemens** is the derived unit of electric conductance, electric susceptance, and electric admittance in the International System of Units (SI). Conductance, susceptance, and admittance are the reciprocals of resistance, reactance, and impedance respectively; hence one siemens is redundantly equal to the reciprocal of one ohm and is also referred to as the *mho*. The 14th General Conference on Weights and Measures approved the addition of the siemens as a derived unit in 1971.

- ↑ "SI Brochure, Table 3 (Section 2.2.2)". BIPM. 2006. Archived from the original on 2007-06-18. Retrieved 2007-07-29.
- ↑ BIPM SI Brochure: Appendix 1, p. 144.
- ↑ "Resolutions of the CGPM: 18th meeting (12-15 October 1987)".
- ↑ "
*Mise en pratique*for the definition of the ampere and other electric units in the SI" (PDF). BIPM. - ↑ 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, Bibcode:1999ITAS....9.4145B, doi:10.1109/77.783938, ISSN 1051-8223, S2CID 12970127 - ↑ 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, archived from the original (PDF) on 2010-05-27, retrieved 2010-04-11,Theoretically, there are no current predictions for any correction terms. Empirically, several experiments have shown that

*K*_{J}and*R*_{K}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.
- ↑ Hill, Paul Horowitz; Winfield; Winfield, Hill (2015).
*The Art of Electronics*(3. ed.). Cambridge [u.a.]: Cambridge Univ. Press. p. 689. ISBN 978-0-521-809269. - ↑ SK Loo and Keith Keller (Aug 2004). "Single-cell Battery Discharge Characteristics Using the TPS61070 Boost Converter" (PDF). Texas Instruments.
- ↑ "World's Biggest Ultra-High Voltage Line Powers Up Across China".
*www.bloomberg.com*. 1 January 2019. Retrieved 7 January 2020. - ↑ Paul H. Risk (26 Jun 2013). "Lightning – High-Voltage Nature".
*RiskVA*. - ↑ 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:
- Latimer Clark and Sir Charles Bright (1861) "On the formation of standards of electrical quantity and resistance,"
*Report of the Thirty-first Meeting of the British Association for the Advancement of Science*(Manchester, England: September 1861), section: Mathematics and Physics, pp. 37-38. - Latimer Clark and Sir Charles Bright (November 9, 1861) "Measurement of electrical quantities and resistance,"
*The Electrician*,**1**(1) : 3–4.

- Latimer Clark and Sir Charles Bright (1861) "On the formation of standards of electrical quantity and resistance,"
- ↑ 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 10^{9}C.G.S. units of resistance ; the "volt" is approximately 10^{8}C.G.S. units of electromotive force ; and the "farad" is approximately 1/10^{9}of the C.G.S. unit of capacity." - ↑ (Anon.) (September 24, 1881) "The Electrical Congress,"
*The Electrician*,**7**: 297. - ↑ 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. - ↑ "Revised Values for Electrical Units" (PDF).
*Bell Laboratories Record*.**XXV**(12): 441. December 1947. - ↑
*Draft Resolution A "On the revision of the International System of units (SI)" to be submitted to the CGPM at its 26th meeting (2018)*(PDF)

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