The coulomb is a fundamental unit of electrical charge, and is also the SI derived unit of
electric charge. It is equal to the charge of approximately 6.241×1018 electrons.
Its SI definition is the charge transported by a constant current of one ampere in one
second:
One coulomb is also the amount of excess charge on a capacitor of one farad charged to a potential
difference of one volt:
Name and notation This SI unit is named after Charles-Augustin
de Coulomb. As with every International System of Units unit whose name is derived from the
proper name of a person, the first letter of its symbol is upper case. However, when
an SI unit is spelled out in English, it should always begin with a lower case letter, except
in a situation where any word in that position would be capitalized, such as at the beginning
of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms
to this rule because the "d" is lowercase.— Based on The International System of Units, section
5.2. Definition
In the SI system, the coulomb is defined in terms of the ampere and second: 1 C = 1 A ×
1 s. The second is defined in terms of a frequency which is naturally emitted by caesium
atoms. The ampere is defined using Ampère's force law; the definition relies in part on
the mass of the international prototype kilogram, a metal cylinder housed in France. In practice,
the watt balance is used to measure amperes with the highest possible accuracy.
Since the charge of one electron is known to be about 1.60217657×10^−19 coulombs,
a coulomb can also be considered to be the charge of roughly 6.241509324×10^18 electrons,
the reciprocal of 1.60217657×10^−19. SI prefixes
See also SI prefix. Conversions
The magnitude of the electrical charge of one mole of elementary charges is known as
a faraday unit of charge. One faraday equals 96485.3399 coulombs. In terms of Avogadro's
number, one coulomb is equal to approximately 1.036 × NA×10−5 elementary charges.
One ampere-hour = 3600 C, 1 mA⋅h = 3.6 C. The elementary charge is 1.602176487×10^−19 C.
One statcoulomb, the obsolete CGS electrostatic unit of charge, is approximately 3.3356×10−10 C
or about one-third of a nanocoulomb. One coulomb is the magnitude of electrical
charge in 6.24150965(16)×10^18 protons or electrons.
Relation to elementary charge The elementary charge, the charge of a proton,
is approximately 1.602176487(40)×10^−19 C. In SI, the elementary charge in coulombs is
an approximate value: no experiment can be infinitely accurate. However, in other unit
systems, the elementary charge has an exact value by definition, and other charges are
ultimately measured relative to the elementary charge. For example, in conventional electrical
units, the values of the Josephson constant KJ and von Klitzing constant RK are exact
defined values, and it follows that the elementary charge e =2/(KJRK) is also an exact defined
value in this unit system. Specifically, e90 =/(25812.807 × 483597.9) C exactly. SI itself may someday
change its definitions in a similar way. For example, one possible proposed redefinition
is "the ampere...is [defined] such that the value of the elementary charge e is exactly
1.602176487×10^−19 coulombs" This proposal is not yet accepted as part of the SI; the
SI definitions are unlikely to change until at least 2015.
In everyday terms The charges in static electricity from rubbing
materials together are typically a few microcoulombs. The amount of charge that travels through
a lightning bolt is typically around 15 C, although large bolts can be up to 350 C.
The amount of charge that travels through a typical alkaline AA battery from being fully
charged to discharged is about 5 kC = 5000 C ≈ 1.4 A⋅h.
According to Coulomb's law, two negative point charges of 1 C, placed one meter apart, would
experience a repulsive force of 9×109 N, a force roughly equal to the weight of 920000 metric
tons of mass on the surface of the Earth. The hydraulic analogy uses everyday terms
to illustrate movement of charge and the transfer of energy. The analogy equates charge to a
volume of water, and voltage to pressure. One coulomb equals the charge of 6.24×1018 electrons.
The amount of energy transferred by the flow of 1 coulomb can vary; for example, 300 times
fewer electrons flow through a lightning bolt than through an AA battery, but the total
energy transferred by the flow of the lightning's electrons is 300 million times greater.
See also Abcoulomb, a cgs unit of charge
Ampère's circuital law Coulomb's law
Electrostatics Elementary charge
Faraday, an obsolete unit Quantity of electricity
Notes and references