Coulomb's Law

Coulomb's law describes the force of interaction between two point charges:

$F=k\frac{|q_1\,q_2|}{r^2}$

where $k=8.99\cdot10^9\ \tfrac{\text{N·m}^2}{\text{C}^2}$, $q_1,q_2$ are the charges, and $r$ is the distance between them. The force decreases with the square of the distance — doubling the separation weakens it fourfold.

The sign of the product of the charges decides its character: like charges (the same sign) repel, unlike charges attract. You can enter the charges in scientific notation, e.g. 1e-6 (one microcoulomb).