|Unit system||SI derived unit|
|Unit of||Electric charge|
|Named after||Charles-Augustin de Coulomb|
|1 C in ...||... is equal to ...|
|SI base units||A⋅s|
|CGS units||2997924580 statC|
|Atomic units||6.24150934(14)e×10 18|
Name and notation
This SI unit is named after Charles-Augustin de Coulomb. As with every International System of Units (SI) unit named for a person, the first letter of its symbol is upper case (C). However, when an SI unit is spelled out in English, it is treated as a common noun and 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 material using title case.
The SI system defines the coulomb in terms of the ampere and second: 1 C = 1 A × 1 s. The second is defined in terms of a frequency naturally emitted by caesium atoms. The ampere is defined using Ampère's force law; the definition relies in part on the mass of the international prototype kilogram, a metal cylinder housed in France. In practice, the Kibble balance is used to measure amperes with the highest possible accuracy.
Since the charge of one electron is known to be about 1766208(98)×10−19 C1.602, 1 C can also be considered the charge of roughly 6.241509×10 18 electrons or +1 C the charge of that many positrons or protons, where the number is the reciprocal of 1.602177×10 −19.
By 1873, the British Association for the Advancement of Science had defined the volt, ohm and farad, but not the coulomb. 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 electic current and the coulomb as the unit of electric charge. 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.
The proposed redefinition of the ampere and other SI base units would have the effect of fixing the numerical value of the elementary charge to an explicit constant expressed in coulombs, and therefore it would implicitly fix the value of the coulomb when expressed as a multiple of the fundamental charge (the numerical values of those quantities are the multiplicative inverses of each other).
|Value||SI symbol||Name||Value||SI 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 Metric prefix.
- One coulomb is the magnitude (absolute value) of electrical charge in 6.24150934(14)×10 18 protons or electrons.
- The inverse of this number gives the elementary charge of 1766208(98)×10−19 C1.602.
- The magnitude of the electrical charge of one mole of elementary charges (approximately 6.022×1023, or Avogadro's number) is known as a faraday unit of charge (closely related to the Faraday constant). One faraday equals 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 ∴ 1 mA⋅h = 3.6 C.
- One statcoulomb (statC), the obsolete CGS electrostatic unit of charge (esu), is approximately 3.3356×10−10 C or about one-third of a nanocoulomb.
Relation to elementary charge
The elementary charge, the charge of a proton (equivalently, the negative of the charge of an electron), is approximately 1766208(98)×10−19 C1.602. 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. 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. Specifically, e90 = (×10−9)/( 2812.807 × 25597.9) C 483 exactly. SI itself may someday change its definitions in a similar way. 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 176487×10−19 coulombs", 1.602 (in which the numeric value is the 2006 CODATA recommended value, since superseded). This proposal is not yet accepted as part of the SI.
In everyday terms
- The charges in static electricity from rubbing materials together are typically a few microcoulombs.
- The amount of charge that travels through a lightning bolt is typically around 15 C, although large bolts can be up to 350 C.
- The amount of charge that travels through a typical alkaline AA battery from being fully charged to discharged is about 5 kC = 5000 C ≈ 1400 mA⋅h.
- 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 ×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 in the discharge of an AA battery, but the total energy transferred by the flow of the lightning's electrons is 300 million times greater. 6.24
Notes and references
- 6.24150934(14)×10 18 is the reciprocal of the 2010 CODATA recommended value 1.602176565(35)×10 −19 for the elementary charge in coulomb.
- "SI Brochure, Appendix 1," (PDF). BIPM. p. 144.
- "SI brochure, section 2.2.2". BIPM.
- "SI brochure, section 22.214.171.124". BIPM.
- "SI brochure, section 126.96.36.199". BIPM.
- "Watt Balance". BIPM.
- "CODATA Value: elementary charge". The NIST Reference on Constants, Units, and Uncertainty. US National Institute of Standards and Technology. June 2015. Retrieved 2015-09-22.
2014 CODATA recommended values
- 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."
- (Anon.) (September 24, 1881) "The Electrical Congress," The Electrician, 7 .
- Donald Fenna, A Dictionary of Weights, Measures, and Units, OUP (2002), 51f.
- Mills, I. M.; Mohr, P. J.; Quinn, T. J.; Taylor, B. N.; Williams, E. R. (2005). "Redefinition of the kilogram: a decision whose time has come". Metrologia. 42 (2): 71. Bibcode:2005Metro..42...71M. doi:10.1088/0026-1394/42/2/001.
- Report of the CCU to the 23rd CGPM
- Martin Karl W. Pohl. "Physics: Principles with Applications" (PDF). DESY. Archived from the original (PDF) on 2011-07-18.
- Hasbrouck, Richard. Mitigating Lightning Hazards, Science & Technology Review May 1996. Retrieved on 2009-04-26.
- How to do everything with digital photography – David Huss, p. 23, at Google Books, "The capacity range of an AA battery is typically from 1100–2200 mAh."