For other uses, see Coulomb (disambiguation).
Unit system: SI derived unit
Unit of... Electric charge
Symbol: C
Named after: Charles-Augustin de Coulomb
Unit conversions
1 C in... is equal to...
   SI base units    1 A s
   CGS units    2997924580 statC
   Atomic units    6.24150965(16)e×10^18[1]

The coulomb (unit symbol: C) is the SI derived unit of electric charge (symbol: Q or q). It is defined as the charge transported by a constant current of one ampere in one second:

1\ \mathrm{C} = 1\ \mathrm{A} \times 1\ \mathrm{s}

One coulomb is also the amount of excess charge on the positive side of a capacitor of one farad charged to a potential difference of one volt:

1\ \mathrm{C} = 1\ \mathrm{F} \times 1\ \mathrm{V}

Name and notation



In the SI system, the coulomb is defined in terms of the ampere and second: 1 C = 1 A × 1 s.[3] The second is defined in terms of a frequency which is naturally emitted by caesium atoms.[4] The ampere is defined using Ampère's force law;[5] the definition relies in part on the mass of the international prototype kilogram, a metal cylinder housed in France.[6] In practice, the watt balance is used to measure amperes with the highest possible accuracy.[6]

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 (or protons).

SI prefixes

SI multiples for coulomb (C)
Submultiples Multiples
Value Symbol Name Value 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
Common multiples are in bold face.

See also SI prefix.


Relation to elementary charge

The elementary charge, the charge of a proton (equivalently, the negative of the charge of an electron), is approximately 1.602176487(40)×10^−19 C.[1] 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.[7] For example, in conventional electrical units, the values of the Josephson constant KJ and von Klitzing constant RK are exact defined values (written KJ-90 and RK-90), and it follows that the elementary charge e =2/(KJRK) is also an exact defined value in this unit system.[7] Specifically, e90 = (2×10−9)/(25812.807 × 483597.9) C exactly.[7] SI itself may someday change its definitions in a similar way.[7] For example, one possible proposed redefinition is "the ampere...is [defined] such that the value of the elementary charge e (charge on a proton) is exactly 1.602176487×10^−19 coulombs"[8] This proposal is not yet accepted as part of the SI; the SI definitions are unlikely to change until at least 2015.[9]

In everyday terms

  • The charges in static electricity from rubbing materials together are typically a few microcoulombs.[10]
  • The amount of charge that travels through a lightning bolt is typically around 15 C, although large bolts can be up to 350 C.[11]
  • The amount of charge that travels through a typical alkaline AA battery is about 5 kC = 5000 C ≈ 1.4 A⋅h. After that charge has flowed, the battery must be discarded or recharged.[12]
  • 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 negative of) 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 produced by the flow of the lightning's electrons is 300 million times greater.

See also

Notes and references

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