Symbol, esu. In this system an electric unit was provided by describing the size of a charge in terms of a force. If you have ever attracted or repelled with a comb rubbed against your clothing, you have observed a force acting between charged bodies.
Imagine two bodies with electric charges, Q1 and Q2 respectively. Coulomb's Inverse Square Law for electrostatic charges describes the force between them:
where F is the force, r is the distance (which has to be considerably larger than the bodies themselves), and ε (epsilon) is what is called the permittivity of the medium between the charges.
In the cgs system force is in dynes. The dyne has the base units
so let r be in centimeters. To define the unit of charge in the cgs electrostatic system of units, ε is disposed of by defining its value as equal to 1 for a vacuum. Inserting the other dimensions into Coulomb's law, the base units of the unit of charge are seen to be:
This unit is called the esu unit of charge, the electrostatic unit of charge, or the statcoulomb. All the other esu units are defined in terms of the esu unit of charge and the centimeter, gram and second. To distinguish cgs electrostatic units from units in the international system, they were sometimes given the prefix “stat-”.
|statvolt||electric potential difference, electromotive force|
|stattesla||magnetic flux density|
|statmho||conductance, admittance, susceptance|
If you found this page interesting, you might also enjoy learning about another system of units:
Are you interested in properties of systems of units?
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Last revised: 8 March 2008.