Coulomb

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: 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: Under the 2019 redefinition of the SI base units, which took effect on 20 May 2019, the elementary charge (the charge of the proton) is exactly 1.602176634×10−19 coulombs. Thus the coulomb is exactly the charge of 1/(1.602176634×10−19) protons, which is approximately 6.2415090744×1018 protons (1.036×10−5 mol). The same number of electrons has the same magnitude but opposite sign of charge, that is, a charge of −1 C. 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 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 (the numerical values of those quantities are the multiplicative inverses of each other). The ampere is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634×10−19 coulomb. Thus one coulomb is the charge of 6241509074460762607.776 protons, where the number is the reciprocal of 1.602176634×10−19 C,. 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 electric current, and the coulomb as the unit of electric charge. At that time, the volt was defined as the potential 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. See also Metric prefix.