Coulomb Explained

Coulomb
Symbol:	C
Units1:	SI base units
Units2:	CGS units
Inunits2:	≘
Inunits3:	≈

The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI).^{[1] [2]} It is equal to the electric charge delivered by a 1 ampere current in 1 second and is defined in terms of the elementary charge e, at about .

Definition

The SI defines the coulomb by taking the value of the elementary charge e to be, but was previously defined in terms of the force between two wires. The coulomb was originally defined, using the latter definition of the ampere, as .^[3] The 2019 redefinition of the ampere and other SI base units fixed the numerical value of the elementary charge when expressed in coulombs and therefore fixed the value of the coulomb when expressed as a multiple of the fundamental charge.

One coulomb is approximately (and is thus not an integer multiple of the elementary charge), where the number is the reciprocal of The coulomb is exactly$$1\; ~\; \backslash mathrm\; =\; \backslash frac\; ~\; e\; .$$

SI prefixes

See main article: Orders of magnitude (charge). Like other SI units, the coulomb can be modified by adding a prefix that multiplies it by a power of 10.

Conversions

• The magnitude of the electrical charge of one mole of elementary charges (approximately, the Avogadro number) is known as a faraday unit of charge (closely related to the Faraday constant). One faraday equals In terms of the Avogadro constant (N_A), one coulomb is equal to approximately  × N_A elementary charges.
• Every farad of capacitance can hold one coulomb per volt across the capacitor.
• One ampere hour equals, hence = .
• One statcoulomb (statC), the obsolete CGS electrostatic unit of charge (esu), is approximately or about one-third of a nanocoulomb.

In everyday terms

• The charges in static electricity from rubbing materials together are typically a few microcoulombs.^[4]
• The amount of charge that travels through a lightning bolt is typically around 15 C, although for large bolts this can be up to 350 C.^[5]
• The amount of charge that travels through a typical alkaline AA battery from being fully charged to discharged is about  =  ≈ .^[6]
• A typical smartphone battery can hold  ≈ .

Name and history

^[7]

By 1878, the British Association for the Advancement of Science had defined the volt, ohm, and farad, but not the coulomb.^[8] In 1881, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt as the unit for electromotive force, the ampere as the unit for electric current, and the coulomb as the unit of electric charge.^[9] At that time, the volt was defined as the potential difference [i.e., what is nowadays called the "voltage (difference)"] across a conductor when a current of one ampere dissipates one watt of power.The coulomb (later "absolute coulomb" or "abcoulomb" for disambiguation) was part of the EMU system of units. The "international coulomb" based on laboratory specifications for its measurement was introduced by the IEC in 1908. The entire set of "reproducible units" was abandoned in 1948 and the "international coulomb" became the modern coulomb.^[10]

See also

• Abcoulomb, a cgs unit of charge
• Ampère's circuital law
• Coulomb's law
• Electrostatics
• Elementary charge
• Faraday constant, the number of coulombs per mole of elementary charges

Notes and References

1. Web site: SI Brochure (2019) . SI Brochure . 127 . . May 23, 2019.
2. Web site: BIPM . 20 May 2019 . Mise en pratique for the definition of the ampere in the SI . 2022-02-18 . BIPM.
3. Web site: The NIST Reference on Units, Constants, and Uncertainty .
4. Web site: Physics: Principles with Applications . . Martin Karl W. Pohl . https://web.archive.org/web/20110718230251/http://www-zeuthen.desy.de/~pohlmadq/teach/112/ch16.pdf . 2011-07-18.
5. Hasbrouck, Richard. Mitigating Lightning Hazards, Science & Technology Review May 1996. Retrieved on 2009-04-26.
6. , "The capacity range of an AA battery is typically from 1100–2200 mAh."
7. Web site: SI Brochure, Appendix 1 . https://web.archive.org/web/20060618214631/http://www1.bipm.org/utils/common/pdf/si_brochure_8_en.pdf . 2006-06-18 . live . BIPM . 144.
8. W. Thomson, et al. (1873) "First report of the Committee for the Selection and Nomenclature of Dynamical and Electrical Units," Report of the 43rd Meeting of the British Association for the Advancement of Science (Bradford, September 1873), pp. 222–225. From p. 223: "The 'ohm', as represented by the original standard coil, is approximately 10⁹ C.G.S. units of resistance; the 'volt' is approximately 10⁸ C.G.S. units of electromotive force; and the 'farad' is approximately 1/10⁹ of the C.G.S. unit of capacity."
9. (Anon.) (September 24, 1881) "The Electrical Congress", The Electrician, 7.
10. Donald Fenna, A Dictionary of Weights, Measures, and Units, OUP (2002), 51f.