Coulomb

Coulomb
Unit system SI derived unit
Unit of Electric charge
Symbol C 
Named after Charles-Augustin de Coulomb
Unit conversions
1 C in ...... is equal to ...
   SI base units    A⋅s
   CGS units    2997924580 statC
   Atomic units    6.24150934(14)e×10^18[1]

The coulomb (symbol: C) is the International System of Units (SI) unit of electric charge. It is the charge (symbol: Q or q) transported by a constant current of one ampere in one second:

Thus, it is also the amount of excess charge on a capacitor of one farad charged to a potential difference of one volt:

It is equivalent to the charge of approximately 6.242×1018 (1.036×10−5 mol) protons, and −1 C is equivalent to the charge of approximately 6.242×1018 electrons.

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 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. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.— Based on The International System of Units, section 5.2.[2]

Definition

The SI system defines the coulomb in terms of the ampere and second: 1 C = 1 A × 1 s.[3] The second is defined in terms of a frequency 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 1.6021766208(98)×10−19 C,[7] 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.

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).

SI prefixes

SI multiples for coulomb (C)
Submultiples Multiples
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 SI prefix.

Conversions

Relation to elementary charge

The elementary charge, the charge of a proton (equivalently, the negative of the charge of an electron), is approximately 1.6021766208(98)×10−19 C[7]. 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.[8] 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.[8] Specifically, e90 = (2×10−9)/(25812.807 × 483597.9) C exactly.[8] SI itself may someday change its definitions in a similar way.[8] 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",[9] (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

See also

Notes and references

  1. 1 2 6.24150934(14)×1018 is the reciprocal of the 2010 CODATA recommended value 1.602176565(35)×10−19 for the elementary charge in coulomb.
  2. ↑ "SI Brochure, Appendix 1," (PDF). BIPM. p. 144.
  3. ↑ "SI brochure, section 2.2.2". BIPM.
  4. ↑ "SI brochure, section 2.2.1.3". BIPM.
  5. ↑ "SI brochure, section 2.2.1.4". BIPM.
  6. 1 2 "Watt Balance". BIPM.
  7. 1 2 3 "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
  8. 1 2 3 4 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.
  9. ↑ Report of the CCU to the 23rd CGPM
  10. ↑ Martin Karl W. Pohl. "Physics: Principles with Applications" (PDF). DESY.
  11. ↑ Hasbrouck, Richard. Mitigating Lightning Hazards, Science & Technology Review May 1996. Retrieved on 2009-04-26.
  12. ↑ 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."
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