# Coulomb

{{#invoke:Hatnote|hatnote}} Template:Infobox Unit The coulomb (named after Charles-Augustin de Coulomb, unit symbol: C) is a fundamental unit of electrical charge, and is also the SI derived unit of electric charge (symbol: Q or q). It is equal in magnitude (absolute value) to the charge of approximately 6.241Template:E electrons, but has the opposite sign.

Its SI definition is the charge transported by a constant current of one ampere in one second:

$1\ \mathrm {C} =1\ \mathrm {A} \cdot 1\ \mathrm {s}$ One coulomb is also the amount of excess charge on a capacitor of one farad charged to a potential difference of one volt:

$1\ \mathrm {C} =1\ \mathrm {F} \cdot 1\ \mathrm {V}$ ## Definition

In the SI system, the coulomb is defined in terms of the ampere and second: 1 C = 1 A × 1 s. The second is defined in terms of a frequency which is 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 watt balance is used to measure amperes with the highest possible accuracy.

Since the charge of one electron is known to be about −Template:Physconst −1 C can also be considered to be the charge of roughly Template:Gaps (or +1 C the charge of that many positrons or protons), where the number is the reciprocal of Template:Gaps.

## Relation to elementary charge

The elementary charge, the charge of a proton (equivalently, the negative of the charge of an electron), is approximately Template:Physconst. 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 = (2Template:E)/(Template:Gaps × Template:Gaps) C 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 Template:Gaps coulombs", (in which the numeric value is the 2006 CODATA recommended value, since superseded). This proposal is not yet accepted as part of the SI; the SI definitions are unlikely to change until at least 2015.

## 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 ≈ 1.4 A⋅h.
• According to Coulomb's law, two negative point charges of Template:Val, placed one meter apart, would experience a repulsive force of Template:Val, a force roughly equal to the weight of Template:Gaps metric tons of mass on the surface of the Earth.
• 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 Template:Val. 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 through an AA battery, but the total energy transferred by the flow of the lightning's electrons is 300 million times greater.

## Notes and references

1. Template:Cite web
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3. Template:Cite web
4. Template:Cite web
5. Template:Cite web
6. Template:Gaps is the reciprocal of the 2010 CODATA recommended value Template:Gaps for the elementary charge in coulomb.
7. Template:Cite doi
8. Report of the CCU to the 23rd CGPM
9. Template:Cite web
10. Template:Cite web
11. Hasbrouck, Richard. Mitigating Lightning Hazards, Science & Technology Review May 1996. Retrieved on 2009-04-26.
12. Template:Google books, "The capacity range of an AA battery is typically from 1100–2200 mAh."