Statcoulomb

In physics, a statcoulomb (symbol statC), formerly known as esu of charge, is the unit of electric charge in the  cgs-esu (centimeter-gram-second electrostatic system) of units.

By definition, a point charge has magnitude one statcoulomb if it repels a point charge of equal magnitude at a distance of 1 centimeter with a force of one dyne.
 * 1 statC = 1 esu of charge = C/(10&sdot;c) &asymp; 3.335&thinsp;640&thinsp;95&sdot;10&minus;10 C

where C (coulomb) is the SI unit of charge and c is the SI speed of light (c &asymp; 3&sdot;108 m/s).

Conversion to coulomb
The conversion of statC to C can be done as follows. Write
 * 1 C = k statC,

and compute k. Consider two charges of 1 C each, 1 m apart, then the force between them is according to Coulomb's law in SI units,

F = \frac{1}{4\pi \varepsilon_0} = 10^{-7} c^2\quad \textrm{[N]} $$ (see electric constant for the SI value of &epsilon;0). The same system of two charges has in cgs units a force in dyne (1 dyn = 10&minus;5 N):

F' = \frac{k^2}{100^2}\; \Longrightarrow\; F' = 10^{-4} k^2\quad\textrm{[dyn]}\;\Longrightarrow\; F' = 10^{-9}k^2\quad \textrm{[N]}, $$ where we applied Coulomb's law in electrostatic units. Hence

10^{-9}k^2 = 10^{-7} c^2 \;\Longrightarrow\;k = 10 c. $$