The franklin (Fr), statcoulomb (statC), or electrostatic unit of charge (esu) is the physical unit for electrical charge used in the centimetre–gram–second electrostatic units variant (CGS-ESU) and Gaussian systems of units. It is a derived unit given by

Unit systemGaussian, CGS-ESU
Unit ofelectrical charge
SymbolFr, statC, esu
1 Fr in ...... is equal to ...
   CGS base units   1 cm3/2⋅g1/2⋅s−1
   SI (charge)   ≘ ~3.33564×10−10 C
   SI (flux)   ≘ ~2.65×10−11 C
1 statC = 1 dyn1/2⋅cm = 1 cm3/2⋅g1/2⋅s−1.

That is, it is defined so that the CGS-ESU quantity that corresponds to the Coulomb constant is a dimensionless quantity equal to 1.

It can be converted using

1 newton = 105 dyne
1 cm = 10−2 m

The SI system of units uses the coulomb (C) instead. The conversion between C and statC is different in different contexts. The most common contexts are[a]:

  • For electric charge:
    1 C ≘ ~2997924580 statC3.00×109 statC
    ⇒ 1 statC ≘ ~3.33564×10−10 C.
  • For electric fluxD):
    1 C ≘ ~4π × 2997924580 statC3.77×1010 statC
    ⇒ 1 statC ≘ ~2.65×10−11 C.

The symbol "≘" ('corresponds to') is used instead of "=" because the two sides are not interchangeable, as discussed below. The number 2997924580 is 10 times the numeric value of the speed of light expressed in metres/second, and the conversions are exact except where indicated. The second context implies that the SI and CGS units for an electric displacement field (D) are related by:

1 C/m2 ≘ 4π × 2997924580×10−4 statC/cm23.77×106 statC/cm2
⇒ 1 statC/cm2 ≘ ~2.65×10−7 C/m2

due to the relation between the metre and the centimetre. The coulomb is an extremely large charge rarely encountered in electrostatics, while the statcoulomb is closer to everyday charges.

Definition and relation to CGS base units edit

The statcoulomb is such that if two stationary objects each carry a charge of 1 statC and are 1 cm apart, they will electrically repel each other with a force of 1 dyne. This repulsion is governed by Coulomb's law, which in the CGS-Gaussian system states:

where F is the force, qG
and qG
are the two charges, and r is the distance between the charges. Performing dimensional analysis on Coulomb's law, the dimension of electrical charge in CGS must be [mass]1/2 [length]3/2 [time]−1. (This statement is not true in SI units; see below.) We can be more specific in light of the definition above: Substituting F = 1 dyn, qG
= qG
= 1 statC, and r = 1 cm, we get:
1 statC = g1/2⋅cm3/2⋅s−1

as expected.

Dimensional relation between statcoulomb and coulomb edit

General incompatibility edit

Coulomb's law in the Gaussian unit system and the SI are respectively:


Since ε0, the vacuum permittivity, is not dimensionless, the coulomb is not dimensionally equivalent to [mass]1/2 [length]3/2 [time]−1, unlike the statcoulomb. In fact, it is impossible to express the coulomb in terms of mass, length, and time alone.

Consequently, a conversion equation like "1 C = n statC" is misleading: the units on the two sides are not consistent. One cannot freely switch between coulombs and statcoulombs within a formula or equation, as one would freely switch between centimetres and metres. One can, however, find a correspondence between coulombs and statcoulombs in different contexts. As described below, "1 C corresponds to 3.00×109 statC" when describing the charge of objects. In other words, if a physical object has a charge of 1 C, it also has a charge of 3.00×109 statC. Likewise, "1 C corresponds to 3.77×1010 statC" when describing an electric displacement field flux.

As a unit of charge edit

The statcoulomb is defined as follows: If two stationary objects each carry a charge of 1 statC and are 1 cm apart in vacuum, they will electrically repel each other with a force of 1 dyne. From this definition, it is straightforward to find an equivalent charge in coulombs. Using the SI equation


and plugging in F = 1 dyn = 10−5 N, and r = 1 cm = 10−2 m, and then solving for q = qSI
= qSI
, the result is q = (1/2997924580) C ≈ 3.34×10−10 C. Therefore, an object with a charge of 1 statC has a charge of 3.34×10−10 C.

This can also be expressed by the following conversion, which is fully dimensionally consistent, and often useful for switching between SI and CGS formulae:[b]


As a unit of electric displacement field or flux edit

An electric flux (specifically, a flux of the electric displacement field D) has units of charge: statC in CGS and coulombs in SI. The conversion factor can be derived from Gauss's law:

Therefore, the conversion factor for flux is 4π different from the conversion factor for charge:
The dimensionally consistent version is:[b]

Notes edit

  1. ^ As of the 2019 redefinition of the SI base units, there is no exact equality possible here.
  2. ^ a b As of the 2019 redefinition of the SI base units, this equality is not exact.