Notes+8+Fall+2004 - Notes 8 Fall 2004.doc Wave propagation...

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Notes 8 Fall 2004.doc Wave propagation in dielectrics When classifying different media it is useful to compare relative conductivities. The range of value for conductivity is extreme as we go from the best insulators to semiconducting materials to the best conductors. In siemens per meter the conductivity, σ , ranges from for fused quartz, for poor plastic insulators, roughly unity for semiconductors to almost for metallic conductors at room temperature. These values cover a range of 25 orders of magnitude! HUGE! 17 10 7 10 8 10 Within this spectrum of values exist materials whose molecules exhibit a polar nature, i.e., the positive and negative centers of charge are slightly separated in space. When subjected to an external electric field the atoms of the material will line up with the field to form dipoles thereby storing electric energy like a spring. When the field is removed the atoms relax to a random neutral position due to internal molecular forces and give up the stored energy. Nonpolar molecules do not have this dipole arrangement until after the field is applied. These are not free charges and do not contribute to the conduction current. They are referred to as bound charges. However, depending on the quality of the dielectric material, conduction currents will be present. In a perfect dielectric no conduction current exists. An electromagnetic wave traveling through a dielectric material will experience a loss of energy due to three major mechanisms: a. bound electron oscillations b. dipole relaxation c. conduction currents The extent to which each of these contributes to the energy loss is determined by the properties of the medium and the frequency of the wave. In many instances of interest the conduction current is the dominant loss mechanism and will be our focus. Solving the wave equation in free space we previously determined that the free space wave number
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This note was uploaded on 01/20/2012 for the course EE 4460 taught by Professor Czarnecki during the Fall '10 term at LSU.

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Notes+8+Fall+2004 - Notes 8 Fall 2004.doc Wave propagation...

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