Coulomb's Law

Two charges a distance r apart push or pull each other with a force:

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

$k = 9\times10^{9}\ \mathrm{N\cdot m^2/C^2}$. Use the sizes of the charges: the signs only give the direction (like repel, opposite attract).

Both charges feel forces of the same size, in opposite directions, along the line between them.

The distance is squared: twice as far apart, the force is four times smaller; twice the charge on one of them, twice the force.