Everything about the Statcoulomb totally explained
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 (cgs) electrostatic system of units. The
SI system of units uses the
coulomb (C) instead. The conversion is: 1 statC = 0.1 A·m/
c ≈ 3.33564 C
The conversion factor (≈ 3.33564) is equal to 10 divided by the numerical value of the
speed of light,
c, expressed in cm/s.
In the electrostatic cgs system, electrical charge is a fundamental quantity defined via the electrostatic force (see below); in the SI system, electrical current is fundamental and defined via the electromagnetic force while electrical charge is a derived quantity. The electrostatic system derives the electric charge from
Coulomb's law and takes the
permittivity as a dimensionless quantity whose value in a vacuum is 1/(4π). Also the use of the
permeability of vacuum,
, is avoided, having the consequence that the speed of light appears explicitly in some of the equations interrelating quantities in this system.
The statcoulomb is defined as follows: if two objects each carry a charge of 1 statC and are 1 cm apart, that'll repel each other with a force of 1
dyne. As a result, in the electrostatic cgs system, Coulomb's law describing the force
F between two charges
q1 and
q2 a distance
r apart takes the simple form:
(where is the permittivity of vacuum) has to be used. Several other laws of electromagnetism also become easier when all quantities are expressed in electrostatic cgs units; this is the main reason that the cgs system of units is still in use in physics and electrical engineering. The main drawback of this approach is that two other sets of cgs units and equations are defined, the electromagnetic and symmetrical systems (the latter system mixes the first two). The equations in all three systems are usually written in non-rationalized form, so-called because the factors 2π or 4π appear often in unexpected places (in situations not involving circular or spherical symmetry, respectively). It is possible, albeit less often done, to write each set of equations in rationalized form.
The coulomb is an extremely large charge rarely encountered in electrostatics, while the statcoulomb is closer to everyday charges.
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