Volt

For other uses, see Volt (disambiguation).

Volt
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
Symbol	V
Named after	Alessandro Volta
In SI base units:	kg·m2·s−3·A−1

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

Definition of Volt

One volt is defined as the difference in electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points. It is also equal to the potential difference between two parallel, infinite planes spaced 1 meter apart that create an electric field of 1 newton per coulomb. Additionally, 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

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

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

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

Josephson junction definition

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 h is the Planck constant), the "conventional" value K_J-90 is used:

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

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). 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 (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) 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.

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:

• Nerve cell resting potential: ~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: 1.5 V; zinc-carbon battery: 1.56 V if fresh and unused
• LiFePO₄ 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 "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.
• Electric vehicle battery: 400 V when fully charged
• 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 was the record as of 2005)
• Lightning: Varies greatly, often around 100 MV.

History

Alessandro Volta

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. 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 (IEC), approved the volt as the unit for electromotive force. 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. 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.

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

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.

Prior to the development of the Josephson junction voltage standard, the volt was maintained in national laboratories using specially constructed batteries called "standard cells". The United States used a design called the Weston cell from 1905 to 1972.

A proposed redefinition of SI base units, including defining the value of the elementary charge, is expected to take effect in 2019.

See also

• Orders of magnitude (voltage)
• Rail traction voltage
• SI electromagnetism units
• SI prefix for unit prefixes
• Standardised railway voltages
• Voltmeter

External links

• History of the electrical units.