The coulomb (symbol: C) is the unit of electric charge in the

International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. E ...

(SI). It is equal to the electric charge delivered by a 1

ampere The ampere (, ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to elect ...

current in 1

second The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds ...

and is defined in terms of the

elementary charge The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a fundame ...

''e'', at about .

Name and history

By 1878, the

British Association for the Advancement of Science The British Science Association (BSA) is a charity and learned society founded in 1831 to aid in the promotion and development of science. Until 2009 it was known as the British Association for the Advancement of Science (BA). The current Chie ...

had defined the volt, ohm, and farad, but not the coulomb. In 1881, the

International Electrical Congress The International Electrical Congress was a series of international meetings, from 1881 - 1904, in the then new field of applied electricity. The first meeting was initiated by the French government, including official national representatives, lead ...

, now the

International Electrotechnical Commission The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and r ...

(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 .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 The emu () (''Dromaius novaehollandiae'') is the second-tallest living bird after its ratite relative the ostrich. It is endemic to Australia where it is the largest native bird and the only extant member of the genus ''Dromaius''. 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.

Definition

The SI defines the coulomb by taking the value of the elementary charge ''e'' to be , but was previously defined in terms of the force between two wires. The coulomb was originally defined, using the latter definition of the ampere, as . The 2019 redefinition of the ampere and other SI base units fixed the numerical value of the

when expressed in coulombs and therefore fixed the value of the coulomb when expressed as a multiple of the fundamental charge. One coulomb is approximately (and is thus not an integer multiple of the elementary charge), where the number is the reciprocal of The coulomb is exactly

1 ~ \mathrm = \frac ~ e .

SI prefixes

Like other SI units, the coulomb can be modified by adding a

prefix A prefix is an affix which is placed before the Word stem, stem of a word. Adding it to the beginning of one word changes it into another word. For example, when the prefix ''un-'' is added to the word ''happy'', it creates the word ''unhappy'' ...

that multiplies it by a power of 10.

Conversions

* The magnitude of the electrical charge of one mole of

s (approximately , the Avogadro number) is known as a faraday unit of charge (closely related to the Faraday constant). One faraday equals In terms of the Avogadro constant (''N''_A), one coulomb is equal to approximately × ''N''_A elementary charges. * Every

farad The farad (symbol: F) is the unit of electrical capacitance, the ability of a body to store an electrical charge, in the International System of Units (SI). It is named after the English physicist Michael Faraday (1791–1867). In SI base unit ...

of capacitance can hold one coulomb per volt across the capacitor. * One

ampere hour An ampere hour or amp hour (symbol: A⋅h or A h; often simplified as Ah) is a unit of electric charge, having dimensions of electric current multiplied by time, equal to the charge transferred by a steady current of one ampere flowing for on ...

equals , hence = . * One statcoulomb (statC), the obsolete CGS electrostatic unit of charge (esu), is approximately or about one-third of a nanocoulomb.

In everyday terms

* The charges in

static electricity Static electricity is an imbalance of electric charges within or on the surface of a material or between materials. The charge remains until it is able to move away by means of an electric current or electrical discharge. Static electricity is na ...

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 for large bolts this can be up to 350 C. * The amount of charge that travels through a typical

alkaline In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a base (chemistry), basic, ionic compound, ionic salt (chemistry), salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as ...

AA battery from being fully charged to discharged is about = ≈ ., "The capacity range of an AA battery is typically from 1100–2200 mAh." * A typical smartphone battery can hold ≈ .

Notes and references