电子信息类专业英语(第三版)李白萍章课件8.pptx

1、Unit8 Electromagnetic Field and WaveUnit8 Electromagnetic Field and WavePassageA The Foundation of Electromagnetic FieldPassageB Maxwell Equations PassageC EBG and MetamaterialsUnit8 Electromagnetic Field and WavePassageA The Foundation of Electromagnetic FieldWhat is the field?Is it a scalar field

2、or a vector field?What is the nature of afield?Is it a continuous or a rotational field?How is the magnetic field produced by a current-carrying coil?How does a capacitor store energy?How does a piece of wire(antenna)radiate or receive signals?How do electromagnetic fields propagate in space?What re

3、ally happenswhen electromagnetic energy travels from one end of hollow pipe(waveguide)to the other?Unit8 Electromagnetic Field and Wave Prior to undertaking the study of electromagnetic fields,we must define the concept ofafield.When we define the behavior of a quantity in a given region in terms of

4、 a set of values,one for each point in that region,we refer to this behavior of the quantity as field.1 The value at each point of afield can either be measured experimentally or predicted by carrying our certain mathematical operations on some other quantities.Unit8 Electromagnetic Field and Wave A

5、ccording to the theory of electromagnetism,the vectors E and B are the fundamental field vectors that define the force acting on a charge moving in an electromagnetic field.Two associated field vectors D and H,known as the electric flux density(or the displacement flux density)and the magnetic field

6、 intensity,respectively,take into account the dielectric and magnetic properties,respectively,of material media.Materials contain charged particles that under the application of external fields respond giving rise to three basic phenomena known as conduction,polarization,and magnetization.Unit8 Elec

7、tromagnetic Field and Wave Although a material may exhibit all three properties,it is classified as a conductor,a dielectric,or a magnetic material depending upon whether conduction,polarization,or magnetization is the predominant phenomenon.While these phenomena occur on the atomic or“microscopic”s

8、cale,it is sufficient for our purpose to characterize the material based on“macroscopic”scale observations,that is,observations averaged over volumes large compared with atomic dimensions.2 Unit8 Electromagnetic Field and Wave In the case of conductors,the effect of conduction is to produce a curren

9、t in the material known as the conduction current.Conduction is the phenomenon whereby the free electrons inside the material move under the influence of the externally applied electric field with an average velocity proportional in magnitude to the applied electric field,instead of accelerating,due

10、 to the frictional mechanism provided by collisions with the tomic lattice.Unit8 Electromagnetic Field and WaveFor linear isotropic conductors,the conduction currentdensity,having the units of amperes per square meter(A/m2),is related to the electric field intensity in the manner where is the conduc

11、tivity of the material,having the units siemens permeter(S/m).In semiconductors,the conductivity is governed by not only electrons but also holes.Unit8 Electromagnetic Field and Wave While the effect of conduction is taken into account explicitly in the electromagnetic field equations through Eq.(8.

12、1),the effect of polarization is taken into account implicitlythrough the relationship between D and E,which is given by for linear isotropic dielectrics,where is the permittivity of the material having the units coulomb squared per newton-squared meter,commonly known as farads permeter(F/m),where a

13、 farad is a coulomb square per newton-meter.Unit8 Electromagnetic Field and WavePolarization is the phenomenon of creation and net alignment of electric dipoles,formed by the displacements of the centroids of the electron clouds of the nuclei of the atoms within the material,along the direction of a

14、n appliedel ectric field.4 The effect of polarization is to produce a secondary field that acts in superposition with the applied field to cause the polarization.Thus the situation is as depictedin Figure8.1.Toimplicitly take this into account,leading to Eq.(8.2),we begin withUnit8 Electromagnetic F

15、ield and WaveFigure8.1 illustrates the effect of polarization in dielectric materialUnit8 Electromagnetic Field and Wave Where,0 is the permittivity of free space,having the numerical value8.8541012,or approximately 10-9/36,and P is the polarization vector,or the dipole moment per unitvolume,having

16、the units coulombs persquare meter.Note that this gives the units of coulombs per square meter for D.The term accounts for the relationship between D and Eif the medium were frees pace,and the quantity P represents the effect of polarization.For linear isotropic dielectrics,P is proportionalt o EUni

17、t8 Electromagnetic Field and Wave Where,a dimensionless quantity is the electric susceptibility,a parameter that signifies the ability of the material to getpolarized.Combining Eqs.(8.3)and(8.4),we haveWhere r(=1+e)is the relative permittivity of the material.Unit8 Electromagnetic Field and Wave In

18、asimilar manner,the effect of magnetization is taken into account implicitly hrough the relationship between H and B,which is given byfor linear isotropic magnetic materials,where is the permeability of the material,having the units newtons per amperes quared,commonly known as henrys permeter(H/m),w

19、here a henry is a newton-meter per amperes quared.Unit8 Electromagnetic Field and WaveNotes 1 When we define the behavior of a quantity in a givenre gion in terms of a set of values,one for each point in that region,we refer to this behavior of the quantity as field.当定义一个量在一个给定区域中针对一整套数值所呈现出的行为特征,且在

20、这一给定的区域中,每一个点对应一个该量时,定义这一量的这些行为特征为场。Unit8 Electromagnetic Field and Wave 2 While these phenomena occur on the atomic or“microscopic”scale,it is sufficient for our purpose to characterize the material based on“macroscopic”scale observations,that is,observations averaged over volumes large compared wi

21、th atomic dimensions.虽然这些现象出现在原子或者微观层面,但也足以达到了目的,即在宏观层面上定义材料的特性,将问题平均到与原子维度相比较大的体积上。Unit8 Electromagnetic Field and Wave 3 Conduction is the phenomenon whereby the free electrons inside the material move under the influence of the externally applied electric field with an average velocity proportion

22、al in magnitude to the applied electric field,instead of accelerating,due to the frictional mechanism provided by collisions with the atomic lattice.传导是一种现象,自由电子在外加电场的作用下在介质中移动,平均的移动速度与所施加的电场的幅度成正比,由于与原子晶格的碰撞所形成的摩擦机理,所以这不是一种加速过程。.frictional mechanism 的意思是摩擦机理。atomic lattice 的意思是原子晶格。Unit8 Electromag

23、netic Field and Wave 4Polarization is the phenomenon of creation and net alignment of electric dipoles,formed by the displacements of the centroids of the electron clouds of the nuclei of the atoms within the material,along the direction of an applied electric field.极化是电偶极子沿着一个外加电场的方向创建和基于网格基准进行取位的一

24、种现象,其中,电偶极子是由材料中原子核的电子云的重心的移位所形成的。electric dipoles 的意思是电偶极子。电偶极子是指两个等量异号点电荷组成的系统。Unit8 Electromagnetic Field and WaveExercises 1.Fillintheblanks.(1)What really when electromagnetic energy travels from one end of hollow pipe(waveguide)to the other?(2)When we define the behavior of a quantity in a reg

25、ion in terms of a set of values,one for each point in that region,we this behavior of the quantity asfield.Unit8 Electromagnetic Field and Wave(3)Twoassociatedfieldvectors D and H,knownas the electric flux density(or the displacement flux density)and the magnetic field intensity,take into account th

26、edielectric and magnetic properties,of material media.(4)Conduction is the phenomenon the free electrons inside the material move under the influence of the externally applied electric field with an average velocity proportional in magnitude to the applied electric field,accelerating,thefrictional m

27、echanism provided by collisions with the atomic lattice.Unit8 Electromagnetic Field and Wave (5)While the effect of conduction is taken into account in the electromagnetic field equations through Eq.(8.1),the effect of polarization is taken into account through the relationship between D and E.(6)Th

28、e effect of polarization is to produce a secondary field that acts the applied field to cause the polarization.Unit8 Electromagnetic Field and Wave 2.Fillintheblankswiththebestchoice.(1)undertaking the study of electromagnetic fields we must define the concept of afield.a.With respect to b.Owing to

29、c.Regarding to d.Prior to (2)Although a material may exhibit all three properties,it is classified as a conductor,a dielectric,or a magnetic material depending upon whether conduction,polarization,or magnetization is the phenomenon.a.main b.predominant c.principal d.Foremost (3)In semiconductors,the

30、 conductivity is by not only electrons but also holes.a.ruled b.defined c.governed d.managedUnit8 Electromagnetic Field and Wave (4)Polarization is the phenomenon of creation and net of electric dipoles,formed by the displacements of the centroids of the electron clouds of the nuclei of the atoms wi

31、th in the material,along the direction of an applied electric field.a.alignment b.benchmark c.criteria d.specification (5)A dimensionless quantity P is the electric susceptibility,a parameter that the ability of the material to get polarized.a.signifies b.symbolizes c.denotes d.specifiesUnit8 Electr

32、omagnetic Field and Wave 3.TranslatethefollowingparagraphintoChinese.Electric and magnetic phenomena at the macroscopic level are described by Maxwells equations,as published by Maxwell in 1873.This work summarized the state of electromagnetic science at that time and hypothesized from theoretical c

33、onsiderations the existence of the electrical displacement current,which led to the experimental discovery by Hertz of electromagnetic wave propagation.Maxwell s work was based on a large body of empirical and theoretical knowledge developed by Gauss,Ampere,Faraday,and others.Unit8 Electromagnetic F

34、ield and WaveA first course in electromagnetics usually follows this historical(ordeductive)approach,and it is assumed that the reader has had such a course as a prerequisite to the present material.Several references are available here that provide a good treatment of electromagnetic theory at the

35、undergraduate or graduate level.Unit8 Electromagnetic Field and WavePassageB Maxwell Equations We have introduced the different field vectors and associated constitutive relations for material media.The electric and magnetic fields are governed by a set of four laws,known as Maxwells equations,resul

36、ting from several experimental findings and a purely mathematical contribution.Together with the constitutive relations,Maxwells equations form the basis for the entire electromagnetic field theory.Unit8 Electromagnetic Field and WaveNow,we shall consider the time variations of the fields to be arbi

37、trary and introduce these equations and an auxiliary equation in the time domain form.In view of the irexperimental origin,the fundamental form of Maxwells equations is the integral form.In the following,we shall first present all four Maxwells equations in integral form and the auxiliary equation,t

38、he law of conservation of charge,and then discuss several points of interest pertinent to them.It is understood that all field quantities are real functions of position and time.Unit8 Electromagnetic Field and Wave Among the Maxwell Equation Groups,the Faraday s Law isThe Amperes Circuital Law isThe

39、 GaussLaw for the Electric Field isUnit8 Electromagnetic Field and WaveThe GaussLaw for the Magnetic Field isAnd the Law of Conservation of Charge isThere are certain procedures and observations of interest pertinent to Eqs.(8.7)(8.11),as follows.Unit8 Electromagnetic Field and Wave 1.That the magne

40、tic flux and the displacement flux,respectively,are to be evaluated in accordance with the right-hand screw rule(RHS rule),that is,in the sense of advance of aright-hand screw as it is turned around C in the sense of C.The RHS rule is a convention that is applied consistently for all electromagnetic

41、 field laws involving integration over surfaces bounded by closed paths.Unit8 Electromagnetic Field and Wave 2.In evaluating the surface integrals in E q s.(8.7)and(8.8),any surface S bounded by C can be employed.In addition in Eq.(8.7),the same surfaces must be employed for both surface integrals.T

42、his implies that the time derivative of the magnetic flux through all possible surfaces bounded by C is the same in order for the emf around C to be unique.Likewise,the sum of the current due to flow of charges and the displacement current through all possible surfaces bounded C is the same in order

43、 for the mmf around C to be unique.Unit8 Electromagnetic Field and Wave 3.The minus sign on the right side of Eq.(8.7)tells us that when the magnetic flux enclosed by C is increasing with time,the induced voltage is in the sense opposite to that of C.If the path C is imagined to be occupied by a wir

44、e,then a current would flow in the wire that produces a magnetic field so as to oppose the increasing flux.Similar considerations apply for the case of the magnetic flux enclosed by C decreasing with time.These are in accordance with Lenzs law,which states that the sense of the induced emf is such t

45、hat any current it produces tends to oppose the change in the magnetic flux producing it.Unit8 Electromagnetic Field and Wave 4.If loop C contains more than one turn,such as in an N-turn coil,then the surface tightly wound coil,this is equivalent to the situation in which N separate,identical,single

46、-turn loops are stacked so that the emf in duced in the N-turn coil is N times the emf induced in one turn.Thus,for an N-turn coil.Where is the magnetic flux computed as though the coil is a one-turn coil.Unit8 Electromagnetic Field and Wave 5.Since magnetic force acts perpendicular to the motion of

47、 a charge,the magnetomotive(mmf)force,that isdoes not have a physical meaning similar to that of the electromotive force.The terminology arises purely from analogy with electromotive forceforUnit8 Electromagnetic Field and Wave 6.The charge density in Eq.(8.9)and the currentdensity J in Eq.(8.8)pert

48、ain to true charges and currents,respectively,due to motion of true charges.They do not pertain to charges and currents resulting from the polarization and magnetizationphenomena,since these are implicitly taken into account by the formulation of these two equations in terms of D and H,instead of in

49、 terms of E and B.Unit8 Electromagnetic Field and Wave 7.The displacement current is not a true current,that is,it is not a current due to actual flow of charges,such as in the case of the conduction current in wires or a convection current due to motion of a charged cloud in space.Mathematically,it

50、 has theunits of d(C/m2)m2/d t or amperes,the same as the units for a true current,as it should be.Physically,it leads to the same phenomenon as a true current does,even in free space for which P is zero,and D is simply equal to“Without it,the uniqueness of the mmf around a given closed path C is no