coulomb

Dictionary: cou·lomb   (kū'lŏm', -lōm')

The meter-kilogram-second unit of electrical charge equal to the quantity of charge transferred in one second by a steady current of one ampere.

Of or relating to the Coulomb force.

[After Charles Augustin de COULOMB.]

Symbol C. The SI unit of electric charge. It is equal to the charge transferred by a current of one ampere in one second. The unit is named after Charles de Coulomb.

[Etymology: C. A. de Coulomb; France 1736-1806] quantity of electricity, electric charge. Symbol C. The coulombs of electricity transported or of charge delivered by a steady current equal the product of current in amperes and time in seconds. But the Coulomb was defined as a base unit for the e.s.u. system.

SI, Metric-m.k.s. = s·A. The following are among the coherent derived units:

• C·m^-2 for electric flux density, surface density of charge, electric polarization;
• C·m^-3 for volumic charge, charge density;
• C·kg^-1 for exposure to X- or gamma-rays;
• C·V = joule for energy, work, quantity of heat;
• C·V^-1 = farad for electric capacitance.

This coulomb is equatable with 6.241 45~ × 10¹⁸ electrons.

Metric-c.g.s. See abcoulomb; statcoulomb. See also practical unit.

History

The name ‘coulomb’ was agreed, along with related units and the use of the c.g.s. system, in 1881 at the first International Electrical Conference,
[Nature Vol. 24, 512 (1881)] as the ‘quantity of electricity defined by the condition that an ampère gives one coulomb per second’, with the implication that there be both an absolute form and a corresponding practical unit. The former, later discriminated as the abcoulomb, falls within the e.m.u. system, and is fundamentally definable in terms of purely mechanical units. The practical coulomb = 10^-1 abcoulomb.

The creation of explicit laboratory specifications of the ampere, ohm, and volt, which were subsequently found to be slightly discrepant from what was intended, gave a slightly altered coulomb as a unit derived from them. The IEC of 1908 covered the discrepancy by adopting the distinct name international coulomb. Because of experimental vagaries, the value for conversions is normally referred to as the mean international coulomb, = 0.999 85~ C. There is also the US international coulomb, = 0.999 835~ C.

With the implementation of the Metric-m.k.s.A. system in 1948, and its basing of electrical units on an ampere compatible with the original absolute units, the modern coulomb became essentially the old practical coulomb. Sometimes called the absolute coulomb, this became identically the coulomb of the SI.

Since quantity of electricity or electric charge is inherently a count of electrons (or equivalently charged particles), the logical practice would be to define the coulomb from the electron, then the volt from coulomb, rather than the existing reverse practice. Were the electromagnetic units being created afresh today, the coulomb would likely be the charge of 10¹⁸ electrons, i.e. 0.160 2~ of the extant coulomb.

1. the unit of electrical charge, defined as the quantity of electrical charge transferred by 1 ampere in 1 second. Symbol C.
2. the SI unit of exposure to radiation. 1 coulomb per kilogram = 3876 roentgen; abbreviated C/kg.

To convert from coulombs to:

statcoulombs, multiply by 2.998E+09.
faradays, multiply by 1.036E-05.

Unit of electric charge. A negative coulomb charge consists of 6.24 × 10¹⁸ electrons.

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