Two parallel plate capacitors differ only in the spacing between their (very thin) plates; one, AB, has a spacing of 5 mm and a capacitance of 20 pF, the other, CD, has a spacing of 2 mm. Plates A and C carry charges of +1 nC, whilst B and D each carry −1 nC. What are the potential differences {{V}_{AB}} and {{V}_{CD}} after the capacitor CD is slid centrally between and parallel to the plates of AB without touching them? Would it make any difference if CD were not centrally placed between A and B?