Abstract: It is investigated in this paper how the capacitance of parallel circular-plate and rectangular-plate

capacitors varies with the size，respectively. For each kind of capacitor，first it is assumed that charge is distributed

uniformly on the plate，then approximate values of capacitance can be obtained conveniently，afterwards more accurate

values are obtained via finite element simulation using Ansoft. By comparison，it is discovered that results obtained

using the approximation of uniform charge distribution can reflect correctly the tendancy how the capacitance

varies with the size，which is better than the capacitance formula of parallel-plate capacitor in textbooks; however，

those values are smaller，since charge is not strictly distributed uniformly on the plate，but less in the middle and

more on the edge.

Key words: parallel-plate capacitor, size effect, finite element simulation