Coulomb

The coulomb (symbol C) is the SI unit of electric charge. It is defined as the amount of charge transported by a constant current of one ampere in one second, equivalently one coulomb equals one ampere-second. Because the elementary charge e is about 1.602176634×10^-19 coulombs, a single electron carries a charge of −e, and roughly 6.2415×10^18 elementary charges constitute one coulomb.

The unit is named after Charles-Augustin de Coulomb, a French physicist who in the 18th century formulated Coulomb's law describing the electrostatic interaction between point charges. The law states that the magnitude of the electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation: F = k |q1 q2| / r^2. In vacuum, the constant k equals 1/(4π ε0) and is approximately 8.9875×10^9 N·m^2/C^2, where ε0 is the vacuum permittivity (~8.854187817×10^-12 F/m). This inverse-square form underpins classical electrostatics and the concept of an electric field.

In practical terms, the coulomb is a derived SI unit, with electric charge often measured in coulombs and current defined as charge flow per unit time (1 A = 1 C/s). The coulomb is central to calculations involving charge storage and transfer, such as Q = C·V for capacitors and in coulometry, the measurement of charge in electrochemical reactions.