SI derived unit of voltage
The
volt
(symbol:
V
) is the unit of
electric potential
,
electric potential difference
(
voltage
), and
electromotive force
in the
International System of Units (SI)
.
[1]
Definition
[
edit
]
One volt is defined as the
electric potential
between two points of a
conducting wire
when an
electric current
of one
ampere
dissipates one
watt
of
power
between those points.
[2]
It can be expressed in terms of SI base units (
m
,
kg
,
s
, and
A
) as
Equivalently, 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
It can also be expressed as amperes times
ohms
(current times resistance,
Ohm's law
),
webers
per second (magnetic flux per time), watts per ampere (power per current), or joules per coulomb (energy per charge), which is also equivalent to
electronvolts
per
elementary charge
:
The volt is named after
Alessandro Volta
. As with every
SI
unit named for a person, its symbol starts with an
upper case
letter (V), but when written in full, it follows the rules for capitalisation of a
common noun
; i.e.,
volt
becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.
Josephson junction definition
[
edit
]
Historically the "
conventional
" volt,
V
90
, defined in 1987 by the 18th
General Conference on Weights and Measures
[3]
and in use from 1990 to 2019, was implemented using the
Josephson effect
for exact frequency-to-voltage conversion, combined with the
caesium frequency standard
. Though the Josephson effect is still used to realize a volt, the constant used has changed slightly.
For the
Josephson constant
,
K
J
= 2
e
/
h
(where
e
is the
elementary charge
and
h
is the
Planck constant
), a "conventional" value
K
J-90
= 0.4835979 GHz/μV was used for the purpose of defining the volt. As a consequence of the
2019 redefinition of SI base units
, as of 2019 the Josephson constant has an exact value of
K
J
=
483
597
.848
416
98
... GHz/V
, which replaced the conventional value
K
J-90
.
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).
[4]
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.
[5]
Water-flow analogy
[
edit
]
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
, while
current
is proportional to the amount of water flowing. A
resistor
would be a reduced diameter somewhere in the piping or something akin to a radiator offering resistance to flow.
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
[
edit
]
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
[6]
- Single-cell, rechargeable
NiMH
[7]
or
NiCd
battery: 1.2 V
- Single-cell, non-rechargeable (e.g.,
AAA, AA, C and D cells
):
alkaline battery
: 1.5 V;
[8]
zinc?carbon battery
: 1.56 V if fresh and unused
- Logic voltage levels
: 1.2 V, 1.5 V, 1.8 V, 2.5 V, 3.3 V, 5.0 V
- LiFePO
4
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 "12 V" battery is 6 cells, or 12.6 V; a "24 V" battery is 12 cells, or 25.2 V. Some antique vehicles use "6 V" 3-cell batteries, or 6.3 volts.
- 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 is the record; the highest active voltage is 1.10 MV
[9]
)
- Lightning
: a maximum of around 150 MV.
[10]
History
[
edit
]
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.
[11]
By 1873, the British Association for the Advancement of Science had defined the volt, ohm, and farad.
[12]
In 1881, the International Electrical Congress, now the
International Electrotechnical Commission
(IEC), approved the volt as the unit for electromotive force.
[13]
They made the volt equal to 10
8
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.
[14]
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.
[15]
A
redefinition of SI base units
, including defining the value of the
elementary charge
, took effect on 20 May 2019.
[16]
See also
[
edit
]
References
[
edit
]
- ^
"SI Brochure, Table 3 (Section 2.2.2)"
. BIPM. 2006. Archived from
the original
on 2007-06-18
. Retrieved
2007-07-29
.
- ^
BIPM SI Brochure: Appendix 1
, p. 144.
- ^
"Resolutions of the CGPM: 18th meeting (12?15 October 1987)"
.
- ^
Burroughs, Charles J.; Bent, Samuel P.; Harvey, Todd E.; Hamilton, Clark A. (1999-06-01),
"1 Volt DC Programmable Josephson Voltage Standard"
,
IEEE Transactions on Applied Superconductivity
,
9
(3),
Institute of Electrical and Electronics Engineers
(IEEE): 4145?4149,
Bibcode
:
1999ITAS....9.4145B
,
doi
:
10.1109/77.783938
,
ISSN
1051-8223
,
S2CID
12970127
- ^
Keller, Mark W. (2008-01-18),
"Current status of the quantum metrology triangle"
(PDF)
,
Metrologia
,
45
(1): 102?109,
Bibcode
:
2008Metro..45..102K
,
doi
:
10.1088/0026-1394/45/1/014
,
ISSN
0026-1394
,
S2CID
122008182
, archived from
the original
(PDF)
on 2010-05-27
, retrieved
2010-04-11
,
Theoretically, there are no current predictions for any correction terms. Empirically, several experiments have shown that
K
J
and
R
K
are independent of device design, material, measurement setup, etc. This demonstration of universality is consistent with the exactness of the relations, but does not prove it outright.
- ^
Bullock, Orkand, and Grinnell, pp. 150?151; Junge, pp. 89?90; Schmidt-Nielsen, p. 484.
- ^
Horowitz, Paul; Winfield, Hill (2015).
The Art of Electronics
(3. ed.). Cambridge [u.a.]: Cambridge Univ. Press. p. 689.
ISBN
978-0-521-809269
.
- ^
SK Loo; Keith Keller (Aug 2004).
"Single-cell Battery Discharge Characteristics Using the TPS61070 Boost Converter"
(PDF)
. Texas Instruments.
Archived
(PDF)
from the original on Oct 15, 2023.
- ^
"World's Biggest Ultra-High Voltage Line Powers Up Across China"
.
Bloomberg
. 1 January 2019
. Retrieved
7 January
2020
.
- ^
Paul H. Risk (26 Jun 2013).
"Lightning ? High-Voltage Nature"
.
RiskVA
.
- ^
As names for units of various electrical quantities, Bright and Clark suggested "ohma" for voltage, "farad" for charge, "galvat" for current, and "volt" for resistance. See:
- ^
Sir W. Thomson, et al. (1873)
"First report of the Committee for the Selection and Nomenclature of Dynamical and Electrical Units"
,
Report of the 43rd Meeting of the British Association for the Advancement of Science
(Bradford, September 1873), pp. 222-225. From p. 223: "The "ohm," as represented by the original standard coil, is approximately 10
9
C.G.S. units of resistance; the "volt" is approximately 10
8
C.G.S. units of electromotive force; and the "farad" is approximately 1/10
9
of the C.G.S. unit of capacity."
- ^
(Anon.) (September 24, 1881)
"The Electrical Congress"
,
The Electrician
,
7
: 297.
- ^
Hamer, Walter J. (January 15, 1965).
Standard Cells: Their Construction, Maintenance, and Characteristics
(PDF)
. National Bureau of Standards Monograph #84. US National Bureau of Standards.
- ^
"Revised Values for Electrical Units"
(PDF)
.
Bell Laboratories Record
.
XXV
(12): 441. December 1947.
- ^
Draft Resolution A "On the revision of the International System of units (SI)" to be submitted to the CGPM at its 26th meeting (2018)
(PDF)
, archived from
the original
(PDF)
on 2018-04-29
, retrieved
2018-11-02
External links
[
edit
]
Look up
volt
in Wiktionary, the free dictionary.