| statcoulomb | |
| Standard: | Gaussian, CGS-ESU |
| Quantity: | electrical charge |
| Symbol: | Fr |
| Symbol2: | statC, esu |
| Extralabel: | Derivation |
| Extradata: | dyn1/2⋅cm |
| Units1: | CGS base units |
| Inunits1: | 1 cm3/2⋅g1/2⋅s−1 |
| Units2: | SI (charge) |
| Inunits2: | ≘ ~ |
| Units3: | SI (flux) |
| Inunits3: | ≘ ~ |
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 the proportionality constant in Coulomb's law 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:
The symbol "≘" ('corresponds to') is used instead of "=" because the two sides are not interchangeable, as discussed below. The number 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.
The statcoulomb is defined such that if two stationary spherically symmetric objects each carry a charge of 1 statC and are 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:where F is the force, q and q 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 = q = 1 statC, and r = 1 cm, we get:as expected.
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 " 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 . Likewise, "1 C corresponds to " when describing an electric displacement field flux.
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 equationand setting = 1 dyn = 10−5 N and = 1 cm = 10−2 m, and then solving for, the result isTherefore, an object with a CGS charge of 1 statC has a charge of approximately .
An electric flux (specifically, a flux of the electric displacement field) has units of charge: statC in CGS and coulombs in SI. The conversion factor can be derived from Gauss's law:whereTherefore, the conversion factor for flux and the conversion factor for charge differ by a ratio of 4π: