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Chapter 18 Notes

Chapter 18 Notes - Chapter 18 Electrical Properties...

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Chapter 18: Electrical Properties Introduction - The objective of this chapter is to explore the electrical properties of materials in  response to an applied electric field Ohm’s Law - One of the most important electrical characteristics of a solid material is the ease with  which it transmits electric current.  Ohm’s Law: V=I*R - The value of R is influenced by specimen configuration, and for many materials is  independent of current. - Resistivity ( ρ ) is independent of specimen geometry but related to R through the  expression  o ρ  = R*A / l (lowercase L) o ρ  = V*A/ I*l (Current * length) Electrical Conductivity - Sometimes electrical conductivity ( σ ) is used to specify the electrical character of a  material  o σ  = 1/ ρ - The units for conductivity are the reciprocal of ohm-meters - Solid materials exhibit an amazing range of electrical conductivities (ranging in over 27  orders of magnitude) - One way of classifying solid materials is according to the ease with which they conduct  an electric current; within this classification scheme, there are three groupings:  conductors, semiconductors, and insulators. - Metals   Semi-Conductors   Insulators (in decreasing conductivities) Electronic and Ionic Conduction
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- An electric current results from the motion of electrically charged particles in response to  forces that act on them from an externally applied electric field. Positively charged  particles are accelerated in the field direction, negatively charged particles in the  direction opposite.  - Within most solids, a current arises from the flow of electronics (electronic conduction) - A net motion of charged ions is possible that produces a current (ionic conduction) Energy Band Structures in Solids - In all conductors, semiconductors, and many insulating materials, only electronic  conduction occurs, and the magnitude of the electrical conductivity is strongly dependent  on the number of electrons available to participate in the conduction process - Not all electrons in every atom will accelerate in the presence of an electric field - The # of electrons available for electronic conduction is related to the arrangement of  electron states/levels with respect to energy, and then the manner in which these states  are occupied by electrons - A solid maybe be thought of as consisting of a large number of atoms initially separated  from one another, which are subsequently brought together and bonded to form the  ordered atomic arrangement found in the crystalline material - At relatively large separation distances, each atom is independent of all the others and 
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