Electric Fields and Capacitors 85permittivity of each dielectric. Thus relative permittivity is deﬁ ned as the ratio of the capacitance with that dielectric to the capacitance with a vacuum dielectric i.e. rεCC21(3.9) where C1 is with a vacuum and C2 is with the other dielectric. Note: Dry air has the same effect as a vacuum so the relative permittivity for air dielectrics 1. 3.10 Absolute Permittivity (ε ) For a given system the ratio of the electric ﬂ ux density to the electric ﬁ eld strength is a constant, known as the absolute permittivity of the dielectric being used. so farad/metreεDE(3.10) but we have just seen that a dielectric (other than air) is more effective than a vacuum by a factor of εr times, so the absolute permittivity is given by:εε εor farad/metre(3.11) 3.11 Calculating Capacitor Values From the equation (3.10): εεDDQ AV dQdVAEEbut / and /soalso, /capacitance therefore QVCCdAε and transposing this for C we haveCAdAdεεεorfarad(3.12)