array chip developed by
NIST as a standard volt.]]
The
volt (symbol:
V) is the
SI derived unit of
electromotive force, commonly called "
voltage". It is also the unit for the related but slightly different quantity
electric potential in a point (voltage as related to a
reference ground) and
electric potential difference (also called "electrostatic potential difference"). It is named in honor of the Italian physicist
Alessandro Volta (1745–1827), who invented the
voltaic pile, possibly the first chemical
battery.
Definition
The volt is defined as the value of the
voltage across a
conductor when a
current of one
ampere dissipates one
watt of
power in the conductor. It is also equal to one
joule of
energy per
coulomb of charge, J/C. It can be expressed in terms of SI units as follows:
:
It can also be written only using the SI base units m, kg, s and A as:
:
Josephson junction definition
Between 1990 and 1997 the volt was calibrated using the
Josephson effect for exact voltage-to-frequency conversion, combined with cesium-133 time reference, as decided by the 18th
General Conference on Weights and Measures. The following value for the
Josephson constant is used:
:K{J-90} = 2e/h = 0.4835979 GHz/µV.
This is typically used with an 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. However, as of July 2007, this is not the official BIPM definition of Volt.
Water flow analogy
In the
water flow analogy sometimes used to explain electric circuits by comparing them to water-filled pipes,
voltage difference is likened to water
pressure difference–the difference determines how quickly the electrons will travel through the circuit. Current (in amperes), in the same analogy, is a measure of the volume of water that flows past a given point per unit time (
volumetric flow rate). The flow rate is determined by the width of the pipe (analogous to
electrical conductivity), and the pressure difference between the front end of the pipe and the exit (analogous to
voltage). The analogy extends to power dissipation: the power given up by the water flow is equal to flow rate times pressure, just as the power dissipated in a resistor is equal to current times the voltage drop across the resistor (watts = amperes × volts).
The relationship between voltage and current (in ohmic devices) is defined by Ohm's Law.
Common voltages
can be used to measure the voltage between two positions.]]
Nominal voltages of familiar sources:
Nerve cell resting potential: around −75 mV
Single-cell, rechargeable NiMH or NiCd battery: 1.2 V
Mercury battery: 1.355 V
Single-cell, non-rechargeable alkaline battery (e.g., AAA, AA, C and D cells): 1.5 V
LiFePO4 rechargeable battery: 3.3 V
Lithium polymer rechargeable battery: 3.75 V (see Rechargeable battery#Table of rechargeable battery technologies)
Transistor-transistor logic/CMOS (TTL) power supply: 5 V
PP3 battery: 9 V
Automobile electrical system: nominal 12 V, about 11.8 V discharged, 12.8 V charged, and 13.8–14.4 V while charging (vehicle running).
Household mains electricity: 230 V RMS in Europe, Asia and Africa, 120 V RMS in North America, 100 V RMS in Japan (see List of countries with mains power plugs, voltages and frequencies)
Trucks/lorries: 24 V DC
Rapid transit third rail: 600–750 V (see List of current systems for electric rail traction)
High speed train overhead power lines: 25 kV RMS 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 transmission lines: 110 kV RMS and up (1.15 MV RMS was the record as of 2005)
Lightning: Varies greatly, often around 100 MV.
Note: Where RMS (root mean square) is stated above, the peak voltage is times greater than the RMS voltage for a sinusoidal signal centered around zero voltage.
History of the volt
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 the 1880s, the International Electrical Congress, now the
International Electrotechnical Commission (IEC), approved the volt as the unit for electromotive force. 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.
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.
See also
Ampere
Electric potential difference
Rail traction voltage
Ohm
SI electromagnetism units
SI prefix for unit prefixes
Voltage
Voltmeter
Watt
References
External links
Google Video Lecture about Electricity and Volt by Richard A. Muller
Category:SI derived units
Category:Units of electrical potential
![Final production vehicle, the 2011 Chevrolet Volt. The first pre-production test car based on the final Volt design was built in June 2009, in Warren, Michigan,[53][54] and by October 2009, 80 Volts had been built and are being tested under various conditions.[](http://web.archive.org./web/20110825012212im_/http://cdn6.wn.com/pd/27/40/47735c60733f10f7b2cee5fb0f10_grande.jpg "Final production vehicle, the 2011 Chevrolet Volt. The first pre-production test car based on the final Volt design was built in June 2009, in Warren, Michigan,[53][54] and by October 2009, 80 Volts had been built and are being tested under various conditions.[")
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![Final production vehicle, the 2011 Chevrolet Volt. The first pre-production test car based on the final Volt design was built in June 2009, in Warren, Michigan,[53][54] and by October 2009, 80 Volts had been built and are being tested under various conditions.[](http://web.archive.org./web/20110825012212im_/http://cdn5.wn.com/pd/27/40/47735c60733f10f7b2cee5fb0f10_thumb.jpg "Final production vehicle, the 2011 Chevrolet Volt. The first pre-production test car based on the final Volt design was built in June 2009, in Warren, Michigan,[53][54] and by October 2009, 80 Volts had been built and are being tested under various conditions.[")
