The statcoulomb (statC) or electrostatic unit (esu) is the physical unit for electrical charge used in the cgs system of units. The SI system of units uses the coulomb (C) instead. The conversion is
- 1 statC = 3.3356 × 10-10 C
In the cgs system, electrical charge is a fundamental quantity defined via the electrostatic force (as will be described below); in the SI system, electrical current is fundamental and defined via the magnetric force while electrical charge is a derived quantity.
The statcoulomb is defined as follows: if two objects each carry a charge of 1 statC and are 1cm apart, they will repel each other with a force of 1 dyne. As a result, if all quantities are measured in the cgs system, then Coulomb's law describing the force F between two charges q1 and q2 a distance r apart takes the simple form
- F = q1 q2 / r2
Note that in order for the above formula to work, the dimension of statcoulombs must be [mass]1/2 [length]3/2 [time]-1. This is different from the dimension of coulombs which accounts for the fact that the factor k mentioned above is not dimensionless.
The coulomb is an extremely large charge rarely encountered in electrostatics, while the statcoulomb is closer to everyday charges.