A capacitor stores electric potential energy in terms of its charges, which reside in the cathode and anode with the same magnitude but opposite signs.

7.1 By considering the electric field function in a parallel plate capacitor of area A and distance d apart, determine the capacitance of a parallel-plate capacitor, and likewise for cylindrical and spherical capacitor. 

Note that 7.1 done right should show that the capacitance is solely dependent on the dimensions of the capacitor, although in the case of a parallel-plate capacitor, it requires an assumption of small d.

7.2 The distance between the plates of a plane capacitor is d, and the area of each plate is A. As shown in the figure, both plates of the capacitor are earthed and a small body carrying charge Q is held in position between them, at a distance x from one plate. What charge will accumulate on each plate?