The parallel-plate capacitor is the simplest theoretical and practical arrangement and its capacitance value is, for ideal conditions, easy to calculate. For a pair of parallel plates of equal area A, separation d, the capacitance is given by:
The quantity εε0 is a universal constant called the permittivity of free space, and it has the fixed value of 8.84×10−12 farads per metre. Air has approximately this same value of permittivity also, but other insulating materials have values of permittivity that are higher by the factor εr, a pure number with no units, which is different for each material. Values of this quantity, now called relative permittivity (formerly called dielectric constant), are shown in Table 2.1 for some common materials. The formula can be recast, using units of cm2 for area, mm of spacing, and result in pF, as:
Table Typical Values Of Relative Permittivity At 20◦C
These units are more practical for small plate sizes but some allowance must be made for edge effects (the capacitance is slightly less than the predicted value) and for stray capacitance between any conductor and the metal that surrounds it. Even a completely isolated piece of metal will have some capacitance and in some circumstances this may be significant.