Problem 71-4    Source:  Proof 1 - PUCRJ - RJ - 2010.

What happens to two electrical charges, +Q y -q placed at a distance d between each other, if we change the charge + Q by +4 Q, the charge -q by +3 q and the distance d by 2 d ?

Solution of the Problem 71-4   

In the first case, we have the following force of attraction between the charges:

In the second case, we replace the Q charge with 4 Q and the - q charge with +3 q, in addition to doubling the distance between them. In the first case we had a force of attraction, because the charges had charge of opposite signs. In the second case, the two charges have the same signal, soon we will obtain a repulsion force.

In summary we can write that:

Therefore, the repulsion force in the second case is three times greater than the attraction force in the first case.