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Unformatted text preview: 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 Ohms Law- One of the most important electrical characteristics of a solid material is the ease with which it transmits electric current. Ohms Law: V=I*R- The value of R is influenced by specimen configuration, and for many materials is independent of current.- Resistivity ( ) is independent of specimen geometry but related to R through the expression o = R*A / l (lowercase L) o = V*A/ I*l (Current * length) Electrical Conductivity- Sometimes electrical conductivity ( ) is used to specify the electrical character of a material o = 1/ - 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- 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 will have the atomic energy levels and electron configuration as if isolated...
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This note was uploaded on 04/11/2008 for the course MSE 209 taught by Professor Kelly during the Spring '08 term at UVA.
- Spring '08