tailieunhanh - Electromagnetic Field Theory: A Problem Solving Approach Part 17

Electromagnetic Field Theory: A Problem Solving Approach Part 17. Electromagnetic field theory is often the least popular course in the electrical engineering curriculum. Heavy reliance on vector and integral calculus can obscure physical phenomena so that the student becomes bogged down in the mathematics and loses sight of the applications. This book instills problem solving confidence by teaching through the use of a large number of worked problems. To keep the subject exciting, many of these problems are based on physical processes, devices, and models. This text is an introductory treatment on the junior level for a two-semester electrical engineering. | chapter 3 polarization and conduction 136 Polarization and Conduction The presence of matter modifies the electric field because even though the material is usually charge neutral the field within the material can cause charge motion called conduction or small charge displacements called polarization. Because of the large number of atoms present X 1023 per gram molecular weight Avogadro s number slight imbalances in the distribution have large effects on the fields inside and outside the materials. We must then self-consistently solve for the electric field with its effect on charge motion and redistribution in materials with the charges resultant effect back as another source of electric field. 3-1 POLARIZATION In many electrically insulating materials called dielectrics electrons are tightly bound to the nucleus. They are not mobile but if an electric field is applied the negative cloud of electrons can be slightly displaced from the positive nucleus as illustrated in Figure 3-la. The material is then said to have an electronic polarization. Orientational polarizability as in Figure 3-1A occurs in polar molecules that do not share their No field Electronic polarization f -------- F E Torque dx E pxE F -9E p 9d Orientation and ionic polarization M b Figure 3-1 An electric dipole consists of two charges of equal magnitude but opposite sign separated by a small vector distance d. a Electronic polarization arises when the average motion of the electron cloud about its nucleus is slightly displaced b Orientation polarization arises when an asymmetric polar molecule tends to line up with an applied electric field. If the spacing d also changes the molecule has ionic polarization. Polarization 137 electrons symmetrically so that the net positive and negative charges are separated. An applied electric field then exerts a torque on the molecule that tends to align it with the field. The ions in a molecule can also undergo slight relative displacements that gives rise