What is the difference between emu and esu? My textbook contains the following two statements:


*

*In the CGS system the unit of charge is electrostatic unit of charge (E.S.U). It is also called Stat Coulomb (StatC).


*In the CGS system, the unit of charge is electromagnetic unit (E.M.U).

How can e.m.u and e.s.u both be the units of charge in the same system?
 A: Quoting from the Wikipedia page on the CGS system:

*

*The e.s.u of charge, also called the franklin or statcoulomb, is the charge such that two equal $q=1\:\mathrm{statC}$ charges at a distance of $1\:\mathrm{cm}$ from each other exert an electrostatic force of $1\:\mathrm{dyn}$ on each other.


*The e.m.u. of current, also called the biot or abampere, is the current such that two infinitely-long straight, parallel conductors carrying $1\:\mathrm{abA}$ of current and separated by $1\:\mathrm{cm}$ exert a magnetostatic force of $2\:\mathrm{dyn}$ on each other.


*The relations between these units are such that
$$\frac{1\:\mathrm{statC}}{1\:\mathrm{abA\times 1\:s}}=\frac{1\:\mathrm{statC}}{1\:\mathrm{abC}}=\frac{1}{c}=\frac{1\:\mathrm{statA}}{1\:\mathrm{abA}}=\frac{1\:\mathrm{statC/s}}{1\:\mathrm{abA}},$$
where $c$ is the speed of light.
The ESU and EMU systems of electromagnetic units are different systems and they should generally be considered as separate and independent (if relatively similar), and they do not coincide with the gaussian set of electromagnetic units. For example, the electric displacement vector $\mathbf D$ is defined as $\mathbf E+4\pi\mathbf P$ in the ESU system and $\frac{1}{c^2}\mathbf E+4\pi\mathbf P$ in the EMU system, so you cannot interchangeably use formulas for one system in another without the use of a formula dictionary like the one at the end of Jackson's Classical Electrodynamics:



A: http://www.britannica.com/science/electromagnetic-unit-of-charge

electric charge
  Electric charge
  ...× 10−19 coulomb. In the centimetre–gram–second system there are two units of electric charge: the electrostatic unit of charge, esu, or statcoulomb; and the electromagnetic unit of charge, emu, or abcoulomb. One coulomb of electric charge equals about 3,000,000,000 esu, or one-tenth emu.

