chapter 05 - Chapter 5 ELECTRIC FIELDS IN MATERIAL SPACE...

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Chapter 5 ELECTRIC FIELDS IN MATERIAL SPACE The 12 Principles of character: (1) Honesty, (2) Understanding, (3) Compassion, (4) Appreciation, (5) Patience, (6) Discipline, (7) Fortitude, (8) Perseverance, (9) Humor, (10) Humility, (11) Generosity, (12) Respect. —KATHRYN B. JOHNSON J.1 INTRODUCTION In the last chapter, we considered electrostatic fields in free space or a space that has no materials in it. Thus what we have developed so far under electrostatics may be regarded as the "vacuum" field theory. By the same token, what we shall develop in this chapter may be regarded as the theory of electric phenomena in material space. As will soon be evident, most of the formulas derived in Chapter 4 are still applicable, though some may require modification. Just as electric fields can exist in free space, they can exist in material media. Materi- als are broadly classified in terms of their electrical properties as conductors and noncon- ductors. Nonconducting materials are usually referred to as insulators or dielectrics. A brief discussion of the electrical properties of materials in general will be given to provide a basis for understanding the concepts of conduction, electric current, and polarization. Further discussion will be on some properties of dielectric materials such as susceptibility, permittivity, linearity, isotropy, homogeneity, dielectric strength, and relaxation time. The concept of boundary conditions for electric fields existing in two different media will be introduced. 2 PROPERTIES OF MATERIALS In a text of this kind, a discussion on electrical properties of materials may seem out of place. But questions such as why an electron does not leave a conductor surface, why a current-carrying wire remains uncharged, why materials behave differently in an electric field, and why waves travel with less speed in conductors than in dielectrics are easily an- swered by considering the electrical properties of materials. A thorough discussion on this subject is usually found in texts on physical electronics or electrical engineering. Here, a 161
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162 • Electric Fields in Material Space brief discussion will suffice to help us understand the mechanism by which materials influ- ence an electric field. In a broad sense, materials may be classified in terms of their conductivity a, in mhos per meter (U/m) or Siemens per meter (S/m), as conductors and nonconductors, or techni- cally as metals and insulators (or dielectrics). The conductivity of a material usually depends on temperature and frequency. A material with high conductivity (a » 1) is re- ferred to as a metal whereas one with low conductivity (a <sC 1) is referred to as an insu- lator. A material whose conductivity lies somewhere between those of metals and insula- tors is called a semiconductor. The values of conductivity of some common materials as shown in Table B. 1 in Appendix B. From this table, it is clear that materials such as copper and aluminum are metals, silicon and germanium are semiconductors, and glass and rubber are insulators.
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This note was uploaded on 12/20/2010 for the course E E 330_315 taught by Professor Dinavahiandiyer during the Fall '10 term at University of Alberta.

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chapter 05 - Chapter 5 ELECTRIC FIELDS IN MATERIAL SPACE...

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