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Unit system  SI derived unit 
Unit of  Electric charge 
Symbol  C 
Named after  CharlesAugustin de Coulomb 
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]} 
Unit system  SI derived unit 
Unit of  Electric charge 
Symbol  C 
Named after  CharlesAugustin de Coulomb 
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 (named after CharlesAugustin de Coulomb, unit symbol: C) is a fundamental unit of electrical charge, and is also the SI derived unit of electric charge (symbol: Q or q). It is equal to the charge of approximately 6.241×10^{18} 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 the positive side of a capacitor of one farad charged to a potential difference of one volt:
This SI unit is named after CharlesAugustin de Coulomb. As with every International System of Units (SI) unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (C). However, when an SI unit is spelled out in English, it should always begin with a lower case letter (coulomb), 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.^{[2]}
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), the reciprocal of 1.60217657×10 ^{−19}.
Submultiples  Multiples  

Value  Symbol  Name  Value  Symbol  Name  
10^{−1} C  dC  decicoulomb  10^{1} C  daC  decacoulomb  
10^{−2} C  cC  centicoulomb  10^{2} C  hC  hectocoulomb  
10^{−3} C  mC  millicoulomb  10^{3} C  kC  kilocoulomb  
10^{−6} C  µC  microcoulomb  10^{6} C  MC  megacoulomb  
10^{−9} C  nC  nanocoulomb  10^{9} C  GC  gigacoulomb  
10^{−12} C  pC  picocoulomb  10^{12} C  TC  teracoulomb  
10^{−15} C  fC  femtocoulomb  10^{15} C  PC  petacoulomb  
10^{−18} C  aC  attocoulomb  10^{18} C  EC  exacoulomb  
10^{−21} C  zC  zeptocoulomb  10^{21} C  ZC  zettacoulomb  
10^{−24} C  yC  yoctocoulomb  10^{24} C  YC  yottacoulomb  
Common multiples are in bold face. 
See also SI prefix.
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 K_{J} and von Klitzing constant R_{K} are exact defined values (written K_{J90} and R_{K90}), and it follows that the elementary charge e =2/(K_{J}R_{K}) is also an exact defined value in this unit system.^{[7]} Specifically, e_{90} = (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]}
