i would like to ask a question regarding electric charge. I see that charge has 3 units

  1. Elementary charge
  2. Coulomb
  3. Esu

For example A proton has a charge of $+1e$ or of $+1.602 * 10^{-19}$ Coulombs..

In terms of the elementary charge unit, I sometimes I see people write A proton has a charge of $+1$ instead of $+1e$. Is there any name for the kind of expression or it's just about habit's stuff ?

I apologize if i have violated a rule. I also ask this question in chemistry section because I don't know if this explanation is clear in either of the two subjects..


1 Answer 1


The esu (electrostatic unit) is an old unit of charge which preceded the SI system of units.
The system had the centimetre, the gramme and the second as the base units hence the name cigs units.
In this system of units Coulomb's law was $F = \frac{q_1q_2}{r^2}$ with the charge measured in esu or statcoulomb or franklin.
The whole topic of conversion between esu and SI units is a minefield because there was another set of units for magnetism called emu.
There is information about the conversions factors here.

What you have called elementary charge is really relative charge and can be compared with quoting the masses of atoms in atomic mass units.
Masses of atoms are compared to the mass of a twelfth of a Carbon-12 atom.
Charges are compared to the charge on an electron.

So a charge of $+2$ means that the particle is positively charged and has a charge whose magnitude is twice that of the charge on the electron.


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