Factors affecting the capacitance of a parallel-plate capacitor

The three factors which affect the capacitance of a parallel-plate capacitor are the area of the plates, the distance apart of the plates and the nature of the insulating material or dielectric between them. These factors may be investigated by the following experiments.

(1) Area of plates
The effect of altering the area of the plates may be demonstrated by setting up a charged insulated plate connected to an electroscope, alongside an earthed tinfoil roller blind, as illustrated in Fig. 33.10. The tinfoil is slowly unrolled and the leaf divergence noted. As the blind is unrolled, the leaf divergence progressively decreases. An increase in the effective area of the plates is therefore seen to bring about a decrease in potential difference between the plates, and hence an increase in capacitance.

(2) Distance apart of the plates
A parallel-plate capacitor is set up as shown in Fig. 33.11 (a), one plate being earthed and the other connected to an electroscope and charged. The distance apart of the plates is now varied. and the effect of this on the leaf divergence noted. It is found that the closer the plates are together, the smaller is the divergence, and hence the lower the potential. It follows that the capacitance increases as the plates are moved closer together.

(3) Dielectric between the plates
The plates of the charged capacitor are placed a suitable fixed distance apart and slabs of various materials of equal thickness, e.g., polythene, glass, paraffin wax, etc., are placed in turn, between the plates (Fig. 33.11 (b)). In every case a decrease in the leaf divergence is noticed. As before, this indicates a decrease in potential, and hence an increase in capacitance.