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The statcoulomb (statC), franklin (Fr), 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. In terms of the Gaussian base units, it is
In the CGS-ESU system, charge q is therefore has the dimension to M 1/2 L 3/2 T −1. Other units in the CGS-ESU system include the statampere (1 statC/s) and statvolt (1 erg/statC). In CGS-ESU, all electric and magnetic quantities are dimensionally expressible in terms of length, mass, and time, and none has an independent dimension.
The esu-cgs (or "electrostatic cgs") units are one of several systems of electromagnetic units within the centimetre–gram–second system of units; others include CGS-EMU (or "electromagnetic cgs units"), Gaussian units, and Heaviside–Lorentz units. In the cgs-emu system, the unit of electric current is the abampere. The unit of current in ...
The statvolt is a unit of voltage and electrical potential used in the CGS-ESU and gaussian systems of units. In terms of its relation to the SI units, one statvolt corresponds to c cgs 10 −8 volt, [a] i.e. to 299.792458 volts. [2] [b] The statvolt is also defined in the CGS system as 1 erg per statcoulomb. [2]
One difference between the Gaussian and SI systems is in the factor 4π in various formulas that relate the quantities that they define. With SI electromagnetic units, called rationalized, [3] [4] Maxwell's equations have no explicit factors of 4π in the formulae, whereas the inverse-square force laws – Coulomb's law and the Biot–Savart law – do have a factor of 4π attached to the r 2.
In the ESU system, the permittivity of free space is set to unity by definition. This results in unit conversions to the SI system containing a power of the speed of light, c, as well as a power of 10. In the case of the statmho, the conversion from siemens (S) is given by: [2] 1 S ≘ c 2 × 10 −5 s 2 /m 2 × 1 statmho.
In the centimetre–gram–second system of units (CGS), the corresponding quantity is 4.803 2047... × 10 −10 statcoulombs. [b] Robert A. Millikan and Harvey Fletcher's oil drop experiment first directly measured the magnitude of the elementary charge in 1909, differing from the modern accepted value by just 0.6%.
The emu-cgs (or "electromagnetic cgs") units are one of several systems of electromagnetic units within the centimetre–gram–second system of units; others include esu-cgs, Gaussian units, and Heaviside–Lorentz units. In these other systems, the abampere is not one of the units; the "statcoulomb per second" or statampere is used instead.