notes7 - Version 1, 4/14/99 Chapt. 4, Chang, Direct...

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1 Version 1, 4/14/99 Chapt. 4, Chang, Direct Currents In these notes, we consider the motion of charges constituting direct (DC) currents. In contrast to time varying or alternating (AC) currents, DC currents do not vary with time Point Form of Ohm’s Law There are two types of electric currents: convection and conduction currents. Convection currents arise from the transport of charged particles such as ions and electrons in the ionosphere or electrons in a CRT beam. Conduction currents arise from the drift motion of charges through a medium. In linear media, the conduction current density is proportional to the applied electric field: JE E == σ ρ r where = 1/ r [S/m] is the conductivity and r [ Ω -m] is the resistivity of the material . This equation is clearly related to the Ohm’s law of circuit theory; in field theory, it is called the point form of Ohm’s law . A table of resistivities and conductivities of common materials follows. Table of Material Resistivities and Conductivities Material r [ Ω -m] [S/m=mho/m] Classification Aluminum 2.83 10 -8 × 353 10 7 . × Good conductor Copper 11 0 -8 .69 × 58 10 7 . × Good conductor Gold 21 0 -8 .44 × 410 10 7 . × Good conductor Nickel 7.24 10 -8 × 138 10 7 . × Good conductor Silver 0 -8 .62 × 617 10 7 . × Good conductor
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2 Germanium 045 . 2 2 0 Semiconductor Silicon 640 156 10 3 . × Semiconductor Seawater 0.25 4.00 Semiconductor Amber 500 10 14 . × 200 10 15 . × Good Insulator Glass 10 10 10 14 10 10 10 14 −− Good Insulator Petroleum Oil 10 14 10 14 Good Insulator Quartz 750 10 17 . × 130 10 18 . × Good Insulator In conductors, the conductivity can be factored further into material properties called the electron mobility , μ e , and the free-electron charge density , ρ e : σ ρμ = − ee , the negative sign arising because of the negative charge density of electrons. In semiconductors, there are often two species of free charges that may exist: electrons and holes. Holes are atoms which have lost a valence electron, leaving a net positively charged atom; if a hole is “filled” by an electron borrowed from a neighboring atom, the neighbor becomes the new hole and we effectively have a movement of positive charge.
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This note was uploaded on 10/25/2011 for the course ECE 2317 taught by Professor Staff during the Fall '08 term at University of Houston.

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notes7 - Version 1, 4/14/99 Chapt. 4, Chang, Direct...

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