Really dumb question but, suppose I have three charges in a line, an -8.0 microcoulomb charge, a 3.0 microcoulomb charge, and a -4.0 microcoulomb charge. The first is .3 m from the second, and the second is .2 m from the third. The book says that the net force on the third is about 1.5 N to the left, and I got this myself. I'm not satisfied though, because I know that the negative charge on the left would be attracted toward the middle positive charge and other such interactions. How would this system evolve over time? Is there an equation for this?