Volt

The volt (symbol: V) is the SI derived unit of electromotive force, commonly called "voltage".[1] 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 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.[2] It can be written in terms of SI base units as: m2 • kg • s−3 • A−1. It is also equal to one joule of energy per coulomb of charge, J/C.

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} = 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).[ The relationship KJ = 2e/h is apparently exact, with no correction terms required in a practical implementation.


Common voltages


• Nerve cell resting potential: around −75 mV[5]


• 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)


• Commercial and Military Jet aircraft: 400 V AC, 28 V DC[citation needed]


• 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[citation needed])


• 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.