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 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 (see
Baghdad 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 can be written in
terms of SI base units as:
m^{2} ·
kg ·
s^{−3} ·
A^{−1}. It is also equal to one
joule of
energy per
coulomb of charge, J/C.
- \mbox{V} = \dfrac{\mbox{W}}{\mbox{A}} =
\dfrac{\mbox{J}}{\mbox{A} \cdot \mbox{s}} = \dfrac{\mbox{N} \cdot
\mbox{m} }{\mbox{A} \cdot \mbox{s}} = \dfrac{\mbox{kg} \cdot
\mbox{m}^2}{\mbox{A} \cdot \mbox{s}^{3}} = \dfrac{\mbox{kg} \cdot
\mbox{m}^2}{\mbox{C} \cdot \mbox{s}^2} = \dfrac{\mbox{N} \cdot
\mbox{m}} {\mbox{C}} = \dfrac{\mbox{J}}{\mbox{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}} = 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
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
- 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
- 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.
- Theoretical maximum capacity of the Large Hadron
Collider: 7 trillion volts.
Note: Where
RMS (
root mean
square) is stated above, the peak voltage is \sqrt{2} 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
References
- BIPM SI Brochure: Appendix 1, p. 144
- Bullock, Orkand, and Grinnell, pp. 150–151; Junge, pp. 89–90;
Schmidt-Nielsen, p. 484
- http://www.nytimes.com/info/large-hadron-collider/
External links