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The statcoulomb (statC) or franklin (Fr) or electrostatic unit of charge (esu) is the physical unit for electrical charge used in the centimetre-gram-second system of units (cgs) and Gaussian units. It is a derived unit given by
- 1 statC = 1 g1/2 cm3/2 s−1 = 1 erg1/2 cm1/2.
- For electric charge:
- 1 C ↔ 2997924580 statC ≈ 3.00×109 statC
- ⇒ 1 statC ↔ ~3.33564×10−10 C.
- For electric flux (ΦD):
- 1 C ↔ 4π×2997924580 statC ≈3.77×1010 statC
- ⇒ 1 statC ↔ ~2.65×10−11 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/cm2 ≈ 3.77×106 statC/cm2
- ⇒ 1 statC/cm2 ↔ ~2.65×10−7 C/m2
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:
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
Dimensional relation between Statcoulomb and Coulomb
||This section possibly contains original research. (February 2013)|
||This section may stray from the topic of the article into the topic of another article, Gaussian units #Major differences between Gaussian and SI units. (February 2013)|
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
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 ≈ 3.34×10−10 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:
As a unit of electric displacement field or flux
Therefore the conversion factor for flux is 4π different from the conversion factor for charge:
- (as unit of ΦD).
The dimensionally consistent version is:
- (as unit of ΦD).