= Coulomb force
{c}
{title2=$\mathbf F=k_e q_1q_2\widehat{\mathbf r}/r^2$}
The <Coulomb force> is the electrostatic <force> between <electric charges>. For point charges in a uniform isotropic medium of <permittivity> $\epsilon$, the <force> on charge $q_2$ from $q_1$ is $\mathbf F_{2\leftarrow1}=q_1q_2\hat{\mathbf r}_{12}/(4\pi\epsilon r^2)$, where $\hat{\mathbf r}_{12}$ points from the first charge to the second.
Equal charge signs give repulsion and opposite signs give attraction. The inverse-square dependence allows <superposition> to construct fields of extended <charge distributions>. Nearby conductors or spatially varying dielectrics can redistribute charge and alter the field, so the simple uniform-medium pair formula does not automatically solve those boundary problems.
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