Statcoulomb

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

The conversion factor 3.3356 × 10^-10 is equal to 10 divided by the numerical value of the speed of light, expressed in cm/s.

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

whereas in SI units a proportionality constant, the electrostatic constant k = 1/(4πε₀) (where ε₀ is the permittivity of vacuum) has to be used. Several other laws of electromagnetism also become easier when all quantities are expressed in cgs units; this is the main reason that the cgs system of units is still in use in physics and electrical engineering.

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.