* The charges are as close together as possible. The force of electrostatic attraction or repulsion is inversely proportional to the square of the distance between the charges. So, as the distance decreases, the force increases dramatically.
* The magnitudes of the charges are as large as possible. The force is directly proportional to the product of the charges. Therefore, larger charges will exert a stronger force.
In summary, the force between two similar charges will be maximum when the charges are close together and have large magnitudes.
