Volt

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Volt
NISTvoltChip.jpg
Josephson voltage standard chip developed by the National Bureau of Standards as a standard volt
General information
Unit system SI derived unit
Unit of Electric potential, electromotive force
SymbolV
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).

Contents

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. [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:

Josephson junction definition

The "conventional" volt, V90, 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, KJ = 2e/h (where e is the elementary charge and h is the Planck constant), a "conventional" value KJ-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 KJ = 483597.84841698... GHz/V, [4] 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). [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]

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 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.

Common voltages

A multimeter can be used to measure the voltage between two positions. Electronic multi meter.jpg
A multimeter can be used to measure the voltage between two positions.
1.5 V C-cell batteries BateriaR14.jpg
1.5 V C-cell batteries

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:

History

Alessandro Volta Alessandro Volta.jpeg
Alessandro Volta
Group photograph of Hermann Helmholtz, his wife (seated) and academic friends Hugo Kronecker (left), Thomas Corwin Mendenhall (right), Henry Villard (center) during the International Electrical Congress PSM V85 D521 Group photograph of herman helmholtz and academic friends.png
Group photograph of Hermann Helmholtz, his wife (seated) and academic friends Hugo Kronecker (left), Thomas Corwin Mendenhall (right), Henry Villard (center) during the International Electrical Congress

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 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 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]

See also

Related Research Articles

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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.

Electric current Flow of electric charge

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 Difference in electric potential between two points in space

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Ohms law Law according to which the current through a conductor between two points is directly proportional to the voltage across the two points

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:

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Henry (unit) SI unit of inductance

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.

Farad SI unit of electric capacitance

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⋅s4⋅A2.

Series and parallel circuits

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Electric power the rate per unit of time at which electrical energy is transferred by an electric circuit

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.

Ohm SI derived unit of electrical resistance

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 ΔνCs 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 V90 – 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.

References

  1. "SI Brochure, Table 3 (Section 2.2.2)". BIPM. 2006. Archived from the original on 2007-06-18. Retrieved 2007-07-29.
  2. BIPM SI Brochure: Appendix 1, p. 144.
  3. "Resolutions of the CGPM: 18th meeting (12-15 October 1987)".
  4. "Mise en pratique for the definition of the ampere and other electric units in the SI" (PDF). BIPM.
  5. 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
  6. 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 KJ and RK 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.
  7. Bullock, Orkand, and Grinnell, pp. 150–151; Junge, pp. 89–90; Schmidt-Nielsen, p. 484.
  8. 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.
  9. SK Loo and Keith Keller (Aug 2004). "Single-cell Battery Discharge Characteristics Using the TPS61070 Boost Converter" (PDF). Texas Instruments.
  10. "World's Biggest Ultra-High Voltage Line Powers Up Across China". www.bloomberg.com. 1 January 2019. Retrieved 7 January 2020.
  11. Paul H. Risk (26 Jun 2013). "Lightning – High-Voltage Nature". RiskVA.
  12. 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:
  13. 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."
  14. (Anon.) (September 24, 1881) "The Electrical Congress," The Electrician, 7 : 297.
  15. 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.
  16. "Revised Values for Electrical Units" (PDF). Bell Laboratories Record. XXV (12): 441. December 1947.
  17. 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)