Hi. My question is straightforward: how do I know when the charge inside a surface is zero? I've read my textbook chapter twice, but it doesn't offer any explanation on this. In one example problem with a hollow shell, it simply makes the automatic assumption that "there's no charge inside the hollow shell and therefore none inside the guassian surface." I'm baffled as to how they arrived at that conclusion. Am I to assume that anytime a surface has a hollow/empty space in it, the enclosed charge equals zero? Worse yet, my textbook appears to contradict itself: it states that "the electric field exists at every point in space." But if in some surfaces there's no charge (like the hollow shell), how can the electric field exist!? Any clarification would be greatly appreciated, especially if it's dumbed down. I feel like this is an elementary concept, but I'm struggling to understand it.