电磁场与电磁波第8讲边界条件电容能量要点解析.ppt

作业情况 1班：人 2班：人 合计：人 情况: summary Since V’ can be any volume that includes all the charges, we may choose it to be a very large sphere with radius R (R→?). For a linear medium, we have D=?E, * Field and Wave Electromagnetic 电磁场与电磁波 2009. 3. 26 1. Electric Potential Review 2. Conductors in Static Electric Field 3. Dielectrics in Static Electric Field Inside a conductor (under static conditions) Boundary Conditions (at a Conductor/Free Space Interface) 4. Electric Flux Density and Dielectric Constant Main topic 1. Boundary Conditions for Electrostatic Fields 2. Capacitance and Capacitors 3. Electrostatic Energy 1. Boundary Conditions for Electrostatic Fields Electromagnetic problems often involve media with different physical properties and require the knowledge of the relations of the field quantities at an interface between two media. For instance, we may wish to determine how the E and D vectors change in crossing an interface.we now consider an interface between two general media (1) the tangential component of E Let us construct a small path abcda with sides ab and cd in media 1 and 2 respectively, both being parallel to the interface and equal to ?w. The integral form is assumed to be valid for regions containing discontinuous media, is applied to this path. If we let sides bc=da=?h approach zero, their contributions to the line integral of E around the path can be neglected. Thus we have E1 E2 ? 2 ? 1 at ?w ?h a c d b an2 The tangential components of the electric field intensities in both sides of the interface between two dielectrics are equal. Or say, the tangential components of the electric field intensities are continuous. For linear isotropic dielectrics, we have Where the reference unit normal an2 is ourward from medium ② to ①. (2) the normal component of D In order to find a relation between the normal components of the fields at a boundary, we construct a small pillbox with its top face in medium 1 and bottom face in medi