The coulomb (symbol: C) is the SI derived unit of electric charge, and is approximately equal to the charge of 6.24151×1018 protons or -6.24151×1018 electrons. It is named after Charles-Augustin de Coulomb.
One coulomb is the amount of electric charge transported in one second by a constant current of one ampere.
One coulomb is also the amount of excess charge on the positive side of a capacitance of one farad charged to a potential difference of one volt:
In principle, the coulomb could be defined in terms of the charge of a proton or elementary charge. Since the values of the Josephson  constants have been given conventional values (KJ ≡ 4.835 979×1014 Hz/V and RK ≡ 2.581 280 7×104 Ω), it is possible to combine these values to form an alternative (not yet official) definition of the coulomb. A coulomb is then equal to exactly 6.241 509 629 152 65×1018 positive elementary charges. Combined with the present definition of the ampere, this proposed definition would make the kilogram a derived unit.
In everyday situations, positive and negative charges are usually balanced out. According to Coulomb's Law, two point charges of +1 C, one meter apart, would experience a repulsive force of 9×109 N, a force roughly equal to the weight of 900,000 metric tons of mass.
See also SI prefix.
- The magnitude of the electrical charge of one mole of protons (approximately 6.022×1023, or Avogadro's number) is known as the Faraday constant or a faraday. One faraday is equal to 96485.3399 coulombs. In terms of Avogadro's number (NA), one coulomb is equal to approximately 1.036 × NA ×10−5 elementary charges.
- one ampere-hour = 3600 C, one mAh = 3.6 C
- The elementary charge is 1.602176487×10−19 C
- One statcoulomb (statC), the CGS electrostatic unit of charge (esu), is approximately 3.3356×10−10 C or about 1/3 nC.
- One coulomb is the amount of electrical charge in 6.241506×1018 protons, or equivalently, negative one (-1) coulomb is the electrical charge of 6.241506×1018 electrons.
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