statcoulomb

Statcoulomb, symbol statC, is the unit of electric charge in the CGS electrostatic (Gaussian) system, also known as the electrostatic unit (esu) of charge. It is part of the CGS-ESU family of electromagnetic units that were widely used in physics before the broader adoption of SI units.

In Gaussian CGS units, Coulomb's law is written as F = q1 q2 / r^2, with F in dynes, r in centimeters, and charges q in statcoulombs. This leads to the practical definition: two point charges of 1 statC separated by 1 centimeter exert a force of 1 dyne on each other. In these units, the electric constant 1/(4π ε0) does not appear explicitly; ε0 is effectively unity in the Gaussian system.

Conversions to SI are commonly cited: 1 statC is approximately 3.33564 × 10^−10 coulombs, so 1 coulomb is about 2.99792458 × 10^9 statC. The statcoulomb is primarily of historical importance, as SI units have become standard in most disciplines. It continues to appear in discussions of Gaussian CGS electrodynamics and in older scientific literature, where the distinction between CGS-ESU and SI units is relevant.

See also: Coulomb's law, Gaussian (CGS) units, electrostatic unit, esu.