Seite - 18 - in Adaptive and Intelligent Temperature Control of Microwave Heating Systems with Multiple Sources

2. IntroductionofHEPHAISTOS tion altering of the external electric field is so high that the dipole re-orientation fails to follow. As a consequence the friction becomes smallerandthegeneratedheat isalso less. External electric field E + - - + - + - Dielectric material + - + - + - + Dipole Figure2.2. Dipolepolarizationofdielectric. The above interpretation was firstly formulated by the physicist Peter Debye [Aru10], using the well-known complex permittivity equation [GC99], suchas ε(ω) =ε′(ω)− jε′′(ω) =ε∞+ εs−ε∞ 1+jωτ , (2.2) where ε(ω) is the complex permittivity of a dielectric as a function of angular frequencyω = 2pif, ε′(ω) and ε′′(ω) are the real and imagi- nary part of ε(ω) respectively. The parameter εs is the permittivity of the material at low frequencies, ε∞ is the permittivity of the material at very high frequencies. The relaxation time τ represents the time of form and decay of the polarization within the dielectric when the externalelectricfield isappliedandvanishedrespectively. Fromtheaboveequation, it canbederivedthat ε′=ε∞+ εs−ε∞ 1+(ωτ) 2 , ε′′= ωτ (εs−ε∞) 1+(ωτ) 2 . (2.3) 18