For calculating the capacitance of a parallel plate capacitor which has only dielectric between two of its plate, do I have to use only relative permitivity or do I have to consider both permitivities (i.e. permitivity of free space and relative permitivity)?
Oops, should have read the reference more carefully. Here is the quote: The relative static permittivity (or static relative permittivity) of a material under given conditions is a measure of the extent to which it concentrates electrostatic lines of flux. It is the ratio of the amount of stored electrical energy when a voltage is applied, relative to the permittivity of a vacuum. , so ε0 enters into the computation, too.