Two different examples are given below. Notice that the charges on capacitors A, C, E and F are given. The goal of the two examples is to determine the charges on capacitors B and G.
A capacitor consists of two metal plates separated by a narrow insulating gap. No charge can pass through the gap. The circuit at right is identical to the one above except a portion of the circuit has been highlighted. The capacitor plate that is colored green has a charge that is known. The top plate of capacitor B is colored red to remind you that you are trying to determine B's charge. Because of the gaps between the plates of capacitors A and B, the highlighted portion of the circuit is not touching anything. If it is not touching anything, then no charge can be transferred to it. Since the highlighted portion was neutral before the battery was connected to the circuit, the highlighted portion will still be neutral after the battery is connected.
Since the highlighted portion of the circuit consists of two capacitor plates only, if one plate is negatively charged, the other must bear an equal and opposite positive charge. The charge on capacitor A is 4A. Hence the charge on capacitor B must be the same.
Notice that capacitors A and B are connected together in series. The charges on capacitors that are connected in series are always equal.
The circuit at right is identical to the circuit in the previous example, except that a different part of the circuit is highlighted. As before, the bottom plate of capacitor F which has a known charge is colored green. The lower plate of capacitor C is colored red to remind you that its charge is unknown.
Note that capacitors F and G are connected in series. The charges on capacitors that are connected together in series are always equal.