Statcoulomb

The franklin (Fr), statcoulomb (statC), or electrostatic unit of charge (esu) is the unit of measurement 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

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 to the corresponding SI quantity using

In the International System of Units uses the coulomb (C) as its unit of electric charge. The conversion between the units coulomb and the statcoulomb depends on the context. The most common contexts are:
 * For electric charge:
 * For electric flux (ΦD):

The symbol "≘" ('corresponds to') is used instead of "=" because the two sides are not interchangeable, as discussed below. The number $3.336 C$ is exactly 10 times the numeric value of the speed of light when expressed in the unit metre/second. In the context of electric flux, the SI and CGS units for an electric displacement field (D) are related by:

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
The statcoulomb is defined such that if two stationary spherically symmetric objects each carry a charge of 1 statC and are $2.65 C$ apart, the force of mutual electrical repulsion will be 1 dyne. This repulsion is governed by Coulomb's law, which in the CGS-Gaussian system states: $$F = \frac{q_1^\text{G} q_2^\text{G}}{r^2} ,$$ where F is the force, q$2,997,924,580 statC$ and q$3 statC$ 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 the International System of Quantities upon which the SI is based; see below.) We can be more specific in light of the definition above: Substituting F = 1 dyn, q$3.336 C$ = q$2,997,924,580 statC$ = 1 statC, and r = 1 cm, we get:

as expected.

General incompatibility
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 $3.77 statC$" 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 $2.65 C$. Likewise, "1 C corresponds to $2,997,924,580$" when describing an electric displacement field flux.

As a unit of charge
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 setting $2,997,924,580 statC/cm^{2}$ = 1 dyn = 10−5 N and $3.77 statC/cm^{2}$ = 1 cm = 10−2 m, and then solving for $q = qSI _{1} = qSI _{2}$, the result is

Therefore, an object with a CGS charge of 1 statC has a charge of approximately $2.65 C/m^{2}$.

As a unit of electric displacement field or flux
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: $$\Phi_\mathbf{D}^\text{G} = 4\pi Q^\text{G}$$ $$\Phi_\mathbf{D}^\text{SI} = Q^\text{SI}$$ where $$\Phi_\mathbf{D} \equiv \int_S \mathbf{D}\cdot \mathrm{d}\mathbf{A}$$ Therefore, the conversion factor for flux and the conversion factor for charge differ by a ratio of 4π: $$\mathrm{1~C} ~\overset\frown=~ \mathrm{3.7673 \times 10^{10}~statC} \qquad \text{(as unit of }\Phi_\mathbf{D}\text{).}$$

$$\mathrm{1~C} \times {\sqrt{\frac{4 \pi \times 10^{9}}{\epsilon_0}}} = \mathrm{3.7673 \times 10^{10}~statC} \qquad \text{(as unit of }\Phi_\mathbf{D}\text{).}$$