The coulomb is named after Charles-Augustin de Coulomb. As with every SI unit named for a person, its symbol starts with an upper case letter (C), but when written in full, it follows the rules for capitalisation of a common noun; i.e., coulomb becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.[3]
By 1878, the British Association for the Advancement of Science had defined the volt, ohm, and farad, but not the coulomb.[4] In 1881, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt as the unit for electromotive force, the ampere as the unit for electric current, and the coulomb as the unit of electric charge.[5]
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 coulomb (later "absolute coulomb" or "abcoulomb" for disambiguation) was part of the EMU system of units. The "international coulomb" based on laboratory specifications for its measurement was introduced by the IEC in 1908. The entire set of "reproducible units" was abandoned in 1948 and the "international coulomb" became the modern coulomb.[6]
Definition
The SI defines the coulomb by taking the value of the elementary charge e to be 1.602176634×10−19 C,[7] but was previously defined in terms of the force between two wires. The coulomb was originally defined, using the latter definition of the ampere, as 1 A × 1 s.[8]
The 2019 redefinition of the ampere and other SI base units fixed the numerical value of the elementary charge when expressed in coulombs and therefore fixed the value of the coulomb when expressed as a multiple of the fundamental charge.
One coulomb is approximately 6241509074460762607.776 e (and is thus not an integer multiple of the elementary charge), where the number is the reciprocal of 1.602176634×10−19 C.[9] The coulomb is exactly
Like other SI units, the coulomb can be modified by adding a prefix that multiplies it by a power of 10.
SI multiples of coulomb (C)
Submultiples
Multiples
Value
SI symbol
Name
Value
SI symbol
Name
10−1 C
dC
decicoulomb
101 C
daC
decacoulomb
10−2 C
cC
centicoulomb
102 C
hC
hectocoulomb
10−3 C
mC
millicoulomb
103 C
kC
kilocoulomb
10−6 C
μC
microcoulomb
106 C
MC
megacoulomb
10−9 C
nC
nanocoulomb
109 C
GC
gigacoulomb
10−12 C
pC
picocoulomb
1012 C
TC
teracoulomb
10−15 C
fC
femtocoulomb
1015 C
PC
petacoulomb
10−18 C
aC
attocoulomb
1018 C
EC
exacoulomb
10−21 C
zC
zeptocoulomb
1021 C
ZC
zettacoulomb
10−24 C
yC
yoctocoulomb
1024 C
YC
yottacoulomb
10−27 C
rC
rontocoulomb
1027 C
RC
ronnacoulomb
10−30 C
qC
quectocoulomb
1030 C
QC
quettacoulomb
Common multiples are in bold face.
Conversions
The magnitude of the electrical charge of one mole of elementary charges (approximately 6.022×1023, the Avogadro number) is known as a faraday unit of charge (closely related to the Faraday constant). One faraday equals 9.648533212...×104 coulombs.[10] In terms of the Avogadro constant (NA), one coulomb is equal to approximately 1.036×10−5 mol × NA elementary charges.
^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 109 C.G.S. units of resistance; the 'volt' is approximately 108 C.G.S. units of electromotive force; and the 'farad' is approximately 1/109 of the C.G.S. unit of capacity."