Volt

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Volt

For other uses, see Volt (disambiguation).

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

Volt
Josephson junction array chip developed by theNational Bureau of Standards as a standard volt
Unit information
Unit system	SI derived unit
Unit of	Electric potential, electromotive force
Symbol	V
Named after	Alessandro Volta
In SI base units:	kg·m2·s−3·A−1

DefinitionEdit

One volt is defined as the difference in electric potential between two points of a conducting wire when an electric current of one amperedissipates one watt of power between those points.^[2] It is also equal to the potential difference between two parallel, infinite planes spaced 1 meter apart that create anelectric field of 1 newton per coulomb. Additionally, it is the potential difference between two points that will impart one jouleof 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

${\displaystyle {\text{V}}={\frac {\text{potential energy}}{\text{charge}}}={\frac {{\text{N}}{\cdot }{\text{m}}}{\text{C}}}={\frac {{\text{kg}}{\cdot }{\text{m}}^{2}}{{\text{A}}{\cdot }{\text{s}}^{3}}}.}$

It can also be expressed as amperes timesohms (current times resistance, Ohm's law), watts per ampere (power per unit current,Joule's law), or joules per coulomb (energy per unit charge), which is also equivalent toelectronvolts per elementary charge:

${\displaystyle {\text{V}}={\text{A}}{\cdot }\Omega ={\frac {\text{W}}{\text{A}}}={\frac {\text{J}}{\text{C}}}={\frac {\text{eV}}{e}}.}$

Josephson junction definitionEdit

Main article: Josephson voltage standard

The "conventional" volt, V₉₀, defined in 1988 by the 18th General Conference on Weights and Measures 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 = 2e/h (where e is the elementary charge and his the Planck constant), the "conventional" value K_J-90 is used:

${\displaystyle K_{\text{J-90}}=0.4835979\,{\frac {\text{GHz}}{\mu {\text{V}}}}.}$

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).^[3] 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.^[4]

Water-flow analogyEdit

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. Current is proportional to the diameter of the pipe or the amount of water flowing at that pressure. A resistor would be a reduced diameter somewhere in the piping and a capacitor/inductor could be likened to a "U" shaped pipe where a higher water level on one side could store energy temporarily.

The relationship between voltage and current is defined (in ohmic devices like resistors) byOhm's Law. Ohm's Law is analogous to theHagen–Poiseuille equation, as both are linear models relating flux and potential in their respective systems.

Common voltagesEdit

A multimeter can be used to measure the voltage between two positions.

1.5 V C-cell batteries

The voltage produced by eachelectrochemical 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^[5]
• Single-cell, rechargeable NiMH^[6] or NiCdbattery: 1.2 V
• Single-cell, non-rechargeable (e.g., AAA, AA, C and D cells): alkaline battery: 1.5 V;^[7]zinc-carbon battery: 1.56 V if fresh and unused^{[citation needed]}
• LiFePO₄ rechargeable battery: 3.3 V
• Cobalt-based Lithium polymerrechargeable 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 12 or 24 volts, with some antique vehicles using 6 volts.
• Electric Car Battery: 400 V when fully charged^[8]
• 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 (seeList of current systems for electric rail traction)
• High-speed train overhead power lines:25 kV at 50 Hz, but see the list of current systems for electric rail traction and 25 kV at 60 Hz for exceptions.
• High-voltage electric power transmissionlines: 110 kV and up (1.15 MV was the record as of 2005^{[citation needed]})
• Lightning: Varies greatly, often around 100 million Volts.