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According to Coulomb's law, the magnitude of attractive or repulsive electric force between two charged particles $q_1$ and $q_2$ is proportional to the product of magnitude of charges and inversely proportional to the distance squared $r^2$ from each other and is founded by \[F=k\frac{\left|q_1q_2\right|}{r^2}\] 
Or in vector form as 

Where $\hat{r}$ is the unit vector along the line joining the particles to one another.

In SI units, $F\to \mathrm{N}$ , $r\to \mathrm{m}$, $q\to \mathrm{C}$ and $k=9\times {10}^9\ \mathrm{N.}{\mathrm{m}}^{\mathrm{2}}\mathrm{/}{\mathrm{C}}^{\mathrm{2}}$

Note 1: the electric forces which two charged particle exert on each other have the same magnitude and are in the same direction and in opposite direction (according to Newton's third law)

Note 2: $k$, which called the Coulomb's constant, is related to the vacuum permittivity by
\[k=\frac{1}{4\pi {\epsilon }_0}\]