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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,[2] 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.

# Name and notation

The coulomb is an named after Charles-Augustin de Coulomb. As with every SI unit named for a person, its symbol starts with an upper case letter (C), but when written out it follows no special casing, following whatever would contextually befit a common noun; i.e., "coulomb" becomes capitalised at the beginning of a sentence and in titles.[3]

# Definition

The SI system defines the coulomb in terms of the ampere and second: 1 C = 1 A × 1 s.[4] 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.[5]

Thus one coulomb is the charge of 6241509074460762607.776 protons, where the number is the reciprocal of 1.602176634×10−19 C,[6].

By 1873, the British Association for the Advancement of Science had defined the volt, ohm, and farad, but not the coulomb.[7] 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.[8] 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.[9]

# Conversions

• The magnitude of the electrical charge of one mole of elementary charges (approximately 6.022×1023, the Avogadro number) is known as a faraday unit of charge (closely related to the Faraday constant). One faraday equals 96485.33212... coulombs.[10] In terms of the Avogadro constant (NA), one coulomb is equal to approximately 1.036×10−5 mol × NA elementary charges.
• One ampere hour equals 3600 C, hence 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.

# In everyday terms

• The charges in static electricity from rubbing materials together are typically a few microcoulombs.[11]
• The amount of charge that travels through a lightning bolt is typically around 15 C, although for large bolts this can be up to 350 C.[12]
• 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.[13]