## ${\mathbf {F}}=k_{e}{\frac {q_{1}q_{2}}{r^{2}}}$

Where $q_{1}$ and $q_{2}$ are two point charges, $r$ is the distance between them, and $k_{e}$ is Coulomb's constant
(ke = 8.99×10^{9} N m^{2} C^{-2}).

Coulomb's law describes the force between two charged particles, which is attractive if the charges have opposite signs, and repulsive if they have the same sign. It can be used to derive Gauss's Law.

### Pendulum point charges

Demo using both the Coulomb force and the force of gravity ($F = m g$). Gravity is acting on the two charged sphere in one direction: down. The Coulomb force is acting in a direct line between the two charged spheres, and the strength depends on the square of the distance between the two. Size of sphere indicates mass, $m$, length of strings is $L$, the charge on each is $q$, and the distance between them is $d$.