Coulomb constant
The Coulomb constant, the electric force constant, or the electrostatic constant (denoted , or ) is a proportionality constant in electrostatics equations. In SI units it is equal to . It was named after the French physicist Charles-Augustin de Coulomb (1736–1806) who introduced Coulomb's law. Value of the constant. The Coulomb constant is the constant of proportionality in Coulomb's law, where is a unit vector in the -direction. In SI: where formula_3 is the vacuum permittivity. This formula can be derived from Gauss' law, Taking this integral for a sphere, radius , centered on a point charge, the electric  field points radially outwards and is normal to a differential surface element on the sphere with constant magnitude for all points on the sphere. Noting that for some test charge , Coulomb's law is an inverse-square law, and thereby similar to many other scientific laws ranging from gravitational pull to light attenuation. This law states that a specified physical quantity is inversely proportional to the square of the distance.formula_5In some modern systems of units, the Coulomb constant has an exact numeric value; in Gaussian units , in Lorentz–Heaviside units (also called "rationalized") . This was previously true in SI when  the vacuum permeability was defined as . Together with the speed of light in vacuum , defined as , the vacuum permittivity can be written as , which gave an exact value of Since the redefinition of SI base units, the Coulomb constant is no longer exactly defined and is subject to the measurement error in the fine structure constant, as calculated from CODATA 2018 recommended values being Use. The Coulomb constant is used in many electric equations, although it is sometimes expressed as the following product of the vacuum permittivity constant: The Coulomb constant appears in many expressions including the following: Coulomb constant