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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

\[\vec{F}=k\frac{\left|q_1q_2\right|}{r^2}\hat{r}\]

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

\[k=\frac{1}{4\pi {\epsilon }_0}\]