工程电磁场(国外)浅析.ppt

Engineering ElectromagneticsW.H. Hayt Jr. and J. A. Buck Chapter 5: Conductors and Dielectrics Current and Current Density Current Density as a Vector Field Relation of Current to Charge Velocity Relation of Current Density to Charge Velocity Continuity of Current Energy Band Structure in Three Material Types Electron Flow in Conductors Resistance General Expression for Resistance Electrostatic Properties of Conductors Tangential Electric Field Boundary Condition Boundary Condition for the Normal Component of D Summary Method of Images Forms of Image Charges Example of the Image Method Example (continued) Example (concluded) Electric Dipole and Dipole Moment Model of a Dielectric Polarization Field Polarization Field (with Electric Field Applied) Migration of Bound Charge Bound Charge Motion as a Polarization Flux Polarization Flux Through a Closed Surface Bound and Free Charge Gauss Law for Free Charge Charge Densities Electric Susceptibility and the Dielectric Constant Isotropic vs. Anisotropic Media Boundary Condition for Tangential Electric Field Boundary Condition for Normal Electric Flux Density Example of the Use of Dielectric Boundary Conditions Region 1 Region 2 n ?2 ?1 We use the fact that E is conservative: So therefore: Leading to: More formally: * Current is a flux quantity and is defined as: Current density, J, measured in Amps/m2 , yields current in Amps when it is integrated over a cross-sectional area. The assumption would be that the direction of J is normal to the surface, and so we would write: n In reality, the direction of current flow may not be normal to the surface in question, so we treat current density as a vector, and write the incremental flux through the small surface in the usual way: where ?S = n da Then, the current through a large surface is found through the flux integral: Consider a charge ?Q, occupying volume ?v, moving in the positive x direction at velocity vx In terms of the volume charge density, we may write: S