thumb|Josephson junction array chip developed by NIST as a standard volt. The volt (symbol: V) is the SI derived unit of electromotive force, commonly called "voltage (difference)". It is also the unit for the related but slightly different quantity electric potential difference (also called "electrostatic potential difference"). It is named in honor of the Lombard physicist Alessandro Volta (1745–1827), who invented the voltaic pile, possibly the first chemical battery (see Baghdad Battery).

Definition

The volt is defined as the value of the voltage (difference) across a conductor when a current of one ampere dissipates one watt of power in the conductor. It can be written in terms of SI base units as: m² · kg · s^-2 · C^-1. It is also equal to one joule of energy per coulomb of charge, J/C.

$\backslash mbox\{V\}\; =\; \backslash dfrac\{\backslash mbox\{W\}\}\{\backslash mbox\{A\}\}\; =\; \backslash dfrac\{\backslash mbox\{J\}\}\{\backslash mbox\{C\}\}\; =\; \backslash dfrac\{\backslash mbox\{N\}\; \backslash cdot\; \backslash mbox\{m\}\; \}\{\backslash mbox\{A\}\; \backslash cdot\; \backslash mbox\{s\}\}\; =\; \backslash dfrac\{\backslash mbox\{kg\}\; \backslash cdot\; \backslash mbox\{m\}^2\}\{\backslash mbox\{A\}\; \backslash cdot\; \backslash mbox\{s\}^\{3\}\}\; =\; \backslash dfrac\{\backslash mbox\{kg\}\; \backslash cdot\; \backslash mbox\{m\}^2\}\{\backslash mbox\{C\}\; \backslash cdot\; \backslash mbox\{s\}^2\}$

Josephson junction definition

Since 1990 the volt has been maintained internationally for practical measurement using the Josephson effect, where a conventional value is used for the Josephson constant, fixed by the 18th General Conference on Weights and Measures as:

K_{J-90} = 0.4835979 GHz/µV.

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 resistance), and the pressure difference between the front end of the pipe and the exit is 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 (amperes x volts = watts).

The relationship between voltage and current (in ohmic devices) is defined by Ohm's Law.

Common voltages

thumb|250px| A multimeter can be used to measure the voltage between two positions.

1.5 V C-cell batteries

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
• Lithium polymer rechargeable battery: 3.75 V
• Transistor-transistor logic/CMOS (TTL) power supply: 5 V
• PP3 battery: 9 V
• Automobile electrical system: "12 V", about 11.8 V discharged, 12.8 V charged, and 13.8-14.4 V while charging (vehicle running).
• Household mains electricity: 240 V RMS in Australia, 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)
• Commercial and Military Jet aircraft: 400 V AC, 28 V DC
• Trucks/lorries: 24 V DC
• Rapid transit third rail: 600 to 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 for exceptions.
• High voltage electric power transmission lines: 110 kV RMS and up (1150 kV RMS was the record as of 2005)
• Lightning: Varies greatly, often around 100 MV.