# Statcoulomb

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

1 statC = 1 g1/2 cm3/2 s−1 = 1 erg1/2 cm1/2.

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:

• For electric charge:
1 C ↔ 2997924580 statC ≈ 3.00×109 [[statC
⇒ 1 statC ↔ ~3.33564×1010 C.
• For electric fluxD):
1 C ↔ 4π×2997924580 statC ≈3.77×1010 [[statC
⇒ 1 statC ↔ ~2.65×1011 C.

The symbol "↔" is used instead of "=" because the two sides are not necessarily interchangeable, as discussed below. The number 2997924580 is 10 times the value of the speed of light expressed in meters/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 ↔ ~2.65×107 [[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

The statcoulomb is defined as follows: 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 Gaussian-cgs system states:

$F=\frac{q_1q_2}{r^2}$

where F is the force, q1 and q2 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: Plugging in F = 1 dyne, q1 = q2 = 1 statC, and r = 1 cm, we get:

1 statC = g1/2 cm3/2 s−1

as expected.

## Dimensional relation between Statcoulomb and Coulomb

### General incompatibility

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

$F=\frac{q_1q_2}{r^2}$ (cgs-Gaussian)
$F=\frac{q_1q_2}{4\pi\epsilon_0 r^2}$ (SI)

Since ε0, the vacuum permittivity, is not dimensionless, the coulomb (the SI unit of charge) 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" can be 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 centimeters and meters. 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

The statcoulomb is defined as follows: 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. From this definition, it is straightforward to find an equivalent charge in SI coulombs. Using the SI equation

$F=\frac{q_1q_2}{4\pi\epsilon_0 r^2}$ (SI),

and plugging in F = 1 dyne = 10−5 N, and r = 1 cm = 10−2 m, and then solving for q = q1 = q2, the result is q = (1/2997924580)C ≈ 3.34×1010 C. Therefore an object with a charge of 1 statC has a charge of 3.34×1010 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:

$1 \; \mathrm{C}/\sqrt{4 \pi \epsilon_0} = 2997924580 \; \mathrm{statC}$

### 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:

$\Phi_D = 4\pi Q$ (cgs)
$\Phi_D = Q$ (SI)

where

$\Phi_D \equiv \int_S \mathbf{D}\cdot \mathrm{d}\mathbf{A}$

Therefore the conversion factor for flux is 4π different from the conversion factor for charge:

$1 \; \mathrm{C} \text{ corresponds to } 3.7673 \times 10^{10} \; \mathrm{statC}$ (as unit of ΦD).

The dimensionally consistent version is:

$1 \; \mathrm{C}\sqrt{4 \pi /\epsilon_0} = 3.7673 \times 10^{10} \; \mathrm{statC}$ (as unit of ΦD).