NOTES7 - 11/16/03 Direct Currents In these notes, we...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
11/16/03 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 through a vacuum or gas. Examples include moving ions 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: 2 [A/m ] r σ ρ == E JE where = 1/ r [S/m (Siemens/meter)] is the conductivity and r [ ] 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. m (Ohm meters) Ω⋅ Table of Material Resistivities and Conductivities Material r [ -m] [S/m=mho/m] Classification Aluminum 2.83 10 -8 × 353 10 7 . × Good conductor Copper 11 0 -8 .69 × 58 7 . × Good conductor Gold 21 0 -8 .44 × 410 7 . × Good conductor Nickel 7.24 10 -8 × 138 7 . × Good conductor Silver 0 -8 .62 × 617 7 . × Good conductor 1
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Germanium 045 . 2 2 0 Semiconductor Silicon 640 156 10 3 . × Semiconductor Seawater 0.25 4.00 Semiconductor Amber 500 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 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 (conductivity is a positive quantitiy). 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. The atom may “fill” the hole by borrowing an electron from a neighboring atom; when the borrowing process proceeds from atom-to- atom in the material, we effectively have a movement of positive charge. For this positive charge movement, a hole mobility h and
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 08/05/2011 for the course ECE 2317 taught by Professor Staff during the Spring '08 term at University of Houston.

Page1 / 8

NOTES7 - 11/16/03 Direct Currents In these notes, we...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online