A French Scientist, Coulomb (In 1875): First experimentally measured the force of attraction (or repulsion) between two charges. On the basis of his measurements, Coulomb enunciated a law, called Coulomb's Law.
"Two charges attract (or repel) each other with the force, which is directly proportional to the product of the magnitude of the charge and inversely proportional to the square of the distance between them."
The force is along the straight line joining them, as shown in figure below. This law is based on the inverse square law.
Let $Q_1$ and $Q_2$ be the two charges, separated by a distance $(r)$ then,
Mathematically,
$F = k.\frac{Q_1 Q_2}{r^2}$ .......... (i)
Where, $k$ is the constant of proportionality. It's value depends upon the medium between the charges. If the charges are situated in vacuum (or air) in SI Unit,
$F = \frac{1}{4 \pi \epsilon_0} \frac{Q_1 Q_2}{r^2}$ .......... (ii)
Where, $\epsilon_0$ = permittivity of free space (or vacuum) = $8.854 * 10^{-12} \frac{C^2}{N\;m^2}$.
If the two charges have the same sign, the electrostatic force between them is repulsive. If they have different signs, the electrostatic force between them is attractive.
