Hello, maybe this is the wrong place to post this, but I'll give it a try. It's not a homework problem, but rather how to interpret my textbook. I'm using Tipler & Mosca and I find the section about the electric field kind of slippery. It defines the electric field as in many other sources, e.g. wikipedia; if you put a small positive test charge q at some point near e.g. three point charges, there will be a net force exerted on q by the three other charges. That's okay. The part I don't understand is where it says that "because each of these forces is proportional to q, the net force will be proportional to q." Can that be shown using Coulomb's Law? So, the greater the charge is, the greater the attraction between the charges will be? Then it says that, in addition, q will exert a force on each of the other point charges. And because these forces on the other charges might cause some of the other charges to move, the charge q must be so small that the forces it exerts on the other charges are negligible. But if anything, the forces from q on the other charges will be equal to the forces from the other charges on q? If not, that keeps the whole idea from being what we talked about. To me, the way in which this is shown just doesn't make any sense.