Walker, Halliday & Resnick — Coulomb's Law: Problem 16

16 In Fig. 21-15, three identical conducting spheres initially have the following charges: sphere A, $4Q$; sphere B, $-12Q$; and sphere C, $0$. Spheres A and B are fixed in place, with a center-to-center separation that is much larger than the spheres. Two experiments are conducted. In experiment 1, sphere C is touched to sphere A, then (separately) to sphere B, and then (separately) to sphere A again, and then it is removed. In experiment 2, starting with the same initial states, the procedure is changed: Sphere C is touched to sphere B and then (separately) to sphere A, and then it is removed. What is the ratio of the electrostatic force between A and B at the end of experiment 2 to that at the end of experiment 1?