We know that the force between two charges is given by Coulomb's law: $F = k \frac{q_1 q_2}{r^2}$, where $F$ is the force, $k$ is the electrostatic constant ($8.99 \times 10^9 N m^2 C^{-2}$), $q_1$ and $q_2$ are the charges, and $r$ is the distance between them.
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