Physics of Electric Materials[ESE 242]

Sheet 2

1. Show that the local field within a dielectric material is given by

where E is the applied electric field on the dielectric material.2. Show that the electronic polarization (e) within a dielectric material has the following relationship with its dielectric constant (r)

3. Amorphous selenium (a-Se) is a high resistivity semiconductor that has a density of approximately 4.3 g cm-3 and an atomic number and mass of 34 and 78.96 g/mol respectively. Its relative permittivity at 1 kHz has been measured to be 6.7. Calculate the relative magnitude of the local field in a-Se. Calculate the polarizability per Se atom in the structure. What type of polarization is this? How will r depend on the frequency? 4. Consider a material has a FCC lattice structure, an electric field is then applied on the material, its atomic number is 29 and the lattice constant is 3.6 A, the average displacement of electron relative to the nucleus is 1.810-8 A. Calculate the polarization is this material.

5. KCl has the NaCl crystal structure with a lattice parameter of 0.629 nm. Calculate the relative permittivity of a KCl crystal at optical frequencies given that the electronic polarizability of K+ is 1.264 10-40 F m2 and that of Cl- is 3.408 10-40 F m2. 6. Consider a 1 nF polyester capacitor that has a polymer (PET) film thickness of 1 m at 50 C. Calculate the equivalent circuit of this capacitor at 50 C and at 120 C for operation at 1 kHz. The real part of the relative permittivity at 50 C, r = 2.6 and at 120 C r = 2.8, and the loss tangent or loss factor tan = 0.001 at 50 C, and at 120 C tan = 0.01. What is your conclusion?Dr. Waleed El Nahal Page 1

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