My physics textbook defines the electric potential energy of a system to be equal to the external work that must be done to assemble the system, bringing each charge in from an infinite distance. And then it goes on the explain an example: "Figure 24-15 shows two point charges q1 and q2, separated by a distance r. To find the electric potential energy of this two-charge system, we must mentally build the system, starting with both charges infinitely far away and at rest.When we bring q1 in from infinity and put it in place, we do no work because no electrostatic force acts on q1. However, when we next bring q2 in from infinity and put it in place, we must do work because q1 exerts an electrostatic force on q2 during the move." Now, my question is: Why do we not do work when we bring q1 in from infinity? We are exerting a force over a distance, are we not? Why is it dependent on the presence of an electrostatic force / field?