Problem 72-9    Source:  Problem elaborated by the author of the site.

Repeat problem 72-08 considering that we must shift a charge of + 12 µC from point B to point A.

Solution of the Problem 72-9   

All the values we need have already been calculated in problem 72-08. In this way, for the calculation of the work we continue to use the eq. 72-07, changing the sign in the load value. With that, we found

Carrying out the calculation, we find

Note that the work is now negative as the charges repel each other. So to approximate the charges, we must have a force contrary to the force of repulsion. Or, we can say that the displacement is contrary to the force of repulsion.