The force exerted by one charge q on another charge Q a distance r away is given by:

Coulomb's Law :   F

=

k q Q r2

The use of the unit vector tells us that the force is directed along the line connecting the two charges.

The constant k = 8.99 x 10⁹ N m² / C². We will often round that off to 9.0 x 10⁹ N m² / C², particularly when we're doing no-calculator problems.

Force is a vector. When more than one charge exerts a force on another charge, the net force is the vector sum of the individual forces.

The force between charges is similar to the gravitational force between interacting masses. The equations are very similar, and in both cases the force goes as 1 / r².