When deriving capacity of a parallel plate capacitor, we consider the electric field (sigma/epsilon), i.e. the field due to one of the plates while both the plates are charged with Q and -Q respectively...so should not be the field due to both the plates, i.e. (2sigma/epsilon) since the direction of both fields are same?? And when doing for two parallel wire of equal and opposite charges, my book says the field between these wires is the addition of fields due to each wire.....Why so?? Please someone clarify this. Thanks.