Coulomb

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.^[1] It is named after Charles-Augustin de Coulomb.

Definition

One coulomb is the amount of electric charge transported in one second by a steady current of one ampere.^[2][3][4]

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:

Explanation

In principle, the coulomb could be defined in terms of the charge of a proton or elementary charge. Since the values of the Josephson ^[5] constants have been given conventional values (K_J ≡ 4.835 979 × 10¹⁴ Hz/V and R_K ≡ 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.

SI prefixes

See also SI prefix.

Conversions

• 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 (N_A), one coulomb is equal to approximately 1.036 × N_A  × 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.

This SI unit is named after Charles-Augustin de Coulomb. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (C). When an SI unit is spelled out in English, it should always begin with a lowercase letter (coulomb), except where any word 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.

See also

• Abcoulomb, a cgs unit of charge
• Ampère's circuital law
• Coulomb's law
• Electrostatics
• Elementary charge
• Faraday (unit), an obsolete unit
• Quantity of electricity

References

1. ↑ [1] Electric Charge Prof. Joseph F. Becker, San Jose State University
2. ↑ BIPM Table 3
3. ↑ NIST: Table 3. SI derived units with special names
4. ↑ BIPM SI Brochure, Appendix 1, p. 144
5. ↑ CIPM (1988) Recommendation 1, PV 56; 19) and von Klitzing (CIPM (1988), Recommendation 2, PV 56; 20

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