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Statcoulomb

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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⋅c) ≈ 3.335 640 95⋅10−10 C

where C (coulomb) is the SI unit of charge and c is the SI speed of light (c ≈ 3⋅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,

(see electric constant for the SI value of ε0). The same system of two charges has in cgs units a force in dyne (1 dyn = 10−5 N):

where we applied Coulomb's law in electrostatic units. Hence