1. Why delta PE=-W? In gravitation’s case, If we look at it numerically, PE= mg (b-a), W=F (b-a), then they are the same, where does the negative sign come from? Maybe that has to do with against gravitation or not? Vba=-Wba/q has the same concept. But here is one question: What minimum work must be done by an external force to bring a charge from a great distance away to a point from another charge. This is the general information, I didn’t put the numbers. Solution is W=q(Vb-Va), so why here there is no negative sign? because of the external force? 2. If the electric field is between two point charges, then are these two charges the source charges of the field? Or only one of them? Thanks.