電磁場與電磁波第10講電流密度電動勢電壓電流定理

1、Field and Wave Electromagnetic電磁場與電磁波電磁場與電磁波第第10講講2作業(yè)情況作業(yè)情況1班：人班：人合計：人合計：人情況情況:3yxyxdo 1 2211120,AyByydyd200021211121211()ln()/lndddyyQQQdVE dlaa dydySSSydSQCVd 0001020();();()yryyVVVEaDaDaddd 121222001200221122()1rVVVVrrVVD EdvD EdvdvdvddLxwdLx wdx 41. Poissons and Laplaces Equations 泊松方程和拉普拉斯方程泊松方

2、程和拉普拉斯方程2V 20V222222222aaaaaaxyzxyzVVVVxyVVVVVVxyzxyzz In Cartesian coordinates:Review 4. Solution of Electrostatic Problems53. Method of Images 鏡像法鏡像法4. Boundary-Value Problems in Cartesian Coordinates 笛卡爾坐標中的邊值問題笛卡爾坐標中的邊值問題2. Uniqueness of Electrostatic Solutions 靜電問題的解法靜電問題的解法uniqueness theorem:

3、means that a solution of Poissons equation (of which Laplaces equation is a special case) that satisfies the given boundary conditions is a unique solution. Essence: The effect of the boundary is replaced by one or several equivalent charges, and the original inhomogeneous region with a boundary bec

4、omes an infinite homogeneous space.( , , )( ) ( ) ( )V xyzX x Y y Z z0dd222XkxXx 0dd222YkyYy0dd222ZkzZz6Main topic Steady Electric Currents3. Equation of Continuity and Kirchhoffs Current Law 連續(xù)性方程和基爾霍夫電流定律連續(xù)性方程和基爾霍夫電流定律1. Current Density and Ohms Law 電流密度和歐姆定律電流密度和歐姆定律2. Electromotive Force and Kir

5、chhoffs Voltage Law 電動勢和基爾霍夫電壓定律電動勢和基爾霍夫電壓定律71. Current Density and Ohms LawElectrolytic current(電解電流電解電流): are the result of migration of positive and negative ions; （正離子和負離子徙動的結(jié)果正離子和負離子徙動的結(jié)果）Convection current(對流電流對流電流): result from motion of electrons and/or ions in a vacuum or rarefied gas;（電子和（

6、或）離子在電子和（或）離子在真空中的運動真空中的運動）Conduction current(傳導電流傳導電流): in conductors and semiconductors are caused by drift motion of conduction electrons and/or holes;（導體和半導體中的電子和（或）空穴的移動引起的導體和半導體中的電子和（或）空穴的移動引起的）Current is the time rate of change of charge, so QIt 8unasThe amount of charge Q passing through the

7、 element of surface S with the time t, is (C)nQNqu as tNqus t Current is the time rate of change of charge, so QINqusJst Consider the steady motion of one kind of charge carriers, each of charge q (which is negative for electrons), across an element of surface s with a velocity u. If N is the number

8、 of charge carriers per unit volume, then in time t each charge carrier moves a distance u t 9Define vector current density(電流密度矢量電流密度矢量):;IJs SIJ dsA2 ( /)JuA mIt is convenient to define a vector point function, volume current density, or simply current density J, in amperes per square meter,2 (A/m

9、 )JNquThe total current I flowing through an arbitrary surface S is then the flux of the J vector through S:JNquNqWe may rewrite:which is the relation between the convection current density and the velocity of the charge carrier.10電流場：電流場：大塊導體中各點的電流密度矢量大塊導體中各點的電流密度矢量 J J，在不同的點，在不同的點有不同的大小和方向，給定空間每一點

10、處的電流密度矢量有不同的大小和方向，給定空間每一點處的電流密度矢量J J的大小和方向，構(gòu)成一個矢量場，的大小和方向，構(gòu)成一個矢量場，即：電流場。即：電流場。電流線：電流線：電流場由電流場由電流線電流線來描述，電流線上每點的切來描述，電流線上每點的切線方向都和該點的電流密度矢量方向一致。線方向都和該點的電流密度矢量方向一致。電流管：電流管：由一束電流線圍成的管狀區(qū)域叫由一束電流線圍成的管狀區(qū)域叫電流管，電流管，通通過同一個電流管的各個截面的電流強度都相等過同一個電流管的各個截面的電流強度都相等 電流線：電流場J1S2S11In the case of conduction currents th

11、ere may be more than one kind one charge carriers drifting with different velocities 2 (A/m )iiiiJN quIt can be justified analytically that for most conducting materials the average drift velocity is directly proportional to the electric field intensity. For metallic conductors we write ( / )euEm sW

12、here e is the electron mobility 電子遷移率電子遷移率in (m/Vs). We have2 (A/m )eeJNquEE Where e=-Ne is the charge density of the drifting electrons and is a negative quantity. =- e e, is a macroscopic constitutive parameter of the medium called conductivity.1212lIVIRIsGllRss-resistance電阻電阻1GR-conductance電導電導1-

13、resistivity電阻電阻率率-conductivity 電導率電導率parallel： series：12sRRR1212111 ppRRRGGG13The conductivities of several mediaunit in S/mMediaConductivitiesMediaConductivitiesSilverSea waterCopperPure waterGoldDry soilAluminumTransformer oilBrassGlassIronRubber71017. 671080. 531071010. 451071054. 3111071057. 112

14、10710151041419111911年荷蘭物理學家發(fā)現(xiàn)電阻完全消失的現(xiàn)象僅發(fā)生在物質(zhì)處于極端年荷蘭物理學家發(fā)現(xiàn)電阻完全消失的現(xiàn)象僅發(fā)生在物質(zhì)處于極端物理狀態(tài)的溫度物理狀態(tài)的溫度- -臨界溫度。臨界溫度。19331933年，德國科學家發(fā)現(xiàn)超導體的抗磁效應(yīng)。年，德國科學家發(fā)現(xiàn)超導體的抗磁效應(yīng)。19621962年，英國劍橋大學博士研究生證明兩個以薄的絕緣層相隔的超導年，英國劍橋大學博士研究生證明兩個以薄的絕緣層相隔的超導體之間會產(chǎn)生一種特殊現(xiàn)象體之間會產(chǎn)生一種特殊現(xiàn)象- -隧道效應(yīng)。隧道效應(yīng)。19621962年，美國年，美國WestinghoseWestinghose公司用鈦化鈮合金拉制成超導電

15、線。（公司用鈦化鈮合金拉制成超導電線。（15-25K)15-25K)19861986年，在蘇黎世附近一家年，在蘇黎世附近一家IBMIBM實驗室工作的兩位瑞士物理學家宣布，實驗室工作的兩位瑞士物理學家宣布，他們研制成含鑭和鋇的銅酸鹽（氧化銅）化合物，其臨界溫度為他們研制成含鑭和鋇的銅酸鹽（氧化銅）化合物，其臨界溫度為30K30K。更令人驚奇的是此物質(zhì)為一種陶瓷材料，在常溫下具有絕緣體的所用更令人驚奇的是此物質(zhì)為一種陶瓷材料，在常溫下具有絕緣體的所用特征。特征。在隨之而來的高溫超導熱中又發(fā)現(xiàn)了一批具有較高臨界溫度的化合物，在隨之而來的高溫超導熱中又發(fā)現(xiàn)了一批具有較高臨界溫度的化合物，目前臨界溫度的

16、最高紀錄約是目前臨界溫度的最高紀錄約是150K150K（-123-1230 0C C）。）。15超導材料的基本物理特征超導材料的基本物理特征：v零電阻現(xiàn)象零電阻現(xiàn)象v完全抗磁性完全抗磁性（邁斯納效應(yīng)）（邁斯納效應(yīng)）v超導態(tài)并非僅取決于溫度超導態(tài)并非僅取決于溫度( (臨界電流和臨界磁場臨界電流和臨界磁場) )普通普通導體導體超導體超導體16高溫超導材料制備所面臨的問題：高溫超導材料制備所面臨的問題： 材料制造成本高材料制造成本高, 價格昂貴價格昂貴。 在長距離超導線材的制造上面仍然在長距離超導線材的制造上面仍然有很大的難度。（氧化物高溫超導陶有很大的難度。（氧化物高溫超導陶瓷材料各向異性和短的電

17、子相干長度瓷材料各向異性和短的電子相干長度以及大量晶界的存在嚴重影響線材的以及大量晶界的存在嚴重影響線材的超導電性。）超導電性。） 高溫超導材料臨界電流和臨界磁場高溫超導材料臨界電流和臨界磁場的提高仍是科學家研究的難題。的提高仍是科學家研究的難題。17節(jié)省大量資金節(jié)省大量資金緩解環(huán)境污染緩解環(huán)境污染超導電纜、超導發(fā)電超導電纜、超導發(fā)電機、超導電纜機、超導電纜預(yù)預(yù) 測測低電力低電力低能耗低能耗靈敏度度高靈敏度度高釔鋇銅超導薄膜釔鋇銅超導薄膜-應(yīng)用于諧振器、濾波應(yīng)用于諧振器、濾波器、天線等有源器件器、天線等有源器件商品化商品化低電力低電力低能耗低能耗釔鋇銅超導超導塊材釔鋇銅超導超導塊材-用于磁懸浮

18、、儲能飛用于磁懸浮、儲能飛輪等方面輪等方面即即 將將實業(yè)化實業(yè)化預(yù)計在預(yù)計在20202020年年左右會形成左右會形成1500-20001500-2000億美億美元的超導市場，元的超導市場，其中高溫超導其中高溫超導占一半占一半18summary1. Current Density and Ohms Law SIJ dsA2 ( /)JuA m2 (A/m )JE12lIVIRIsG192 (A/m )JEThis equation is a constitutive relation of the conducting medium. Isotropic materials for which

19、the linear relation holds are called ohmic media. It is generally referred to as the point form of Ohms law. It holds at all points in space. The unit for is ampere per volt-meter (A/Vm), or siemens per meter (S/m). The reciprocal of conductivity is called resistivity, in ohm-meter ( m). 203. Equati

20、on of Continuity and Kirchhoffs Current LawThe principle (law) of conservation of charge: Electric charges may not be created nor destroyed, but merely transported; all charges either at rest or in motion must be conserved for at all time；any change of charge in a region must be accompanied by a flo

21、w of charge across the surface bounding the region.SnaVJISVVVVdQdIJ dsdVdVdtdttJdVdVt 21Since the equation must hold regardless of the choice of V, the integrands must be equal. Thus we have3 (A/m )Jt This point relationship derived from the principle of conservation of charge is called the equation

22、 of continuity.For the steady currents, / t=0, the equation of continuity is :0J0SJ ds0jjI Kirchhoffs current law :the algebraic sum of all the currents out of a junction in an electric circuit is zero.a1I2I3I22We are now in a position to prove this statement and to calculate the time it takes to re

23、ach an equilibrium.tEJEJt E0twhere 0 is the initial charge density at t = 0.The time constant is called the relaxation time(馳豫時間）馳豫時間）.銅，銅， 1.521.5210-19SAn initial charge density 0 will decay to 1/e or 36.8% of its value in a time equal to0ttCee232. Electromotive Force and Kirchhoffs Voltage LawThi

24、s equation tells us that a steady current cannot be maintained in the same direction in a closed circuit by an electrostatic field. A steady current in a circuit is the result of the motion of charge carriers, which, in their paths, collide with atoms and dissipate energy in the circuit. This energy

25、 must come from a non-conservative field, These electrical energy sources, when connected in an electric circuit, provide a driving force for the charge carriers. This force manifests itself as an equivalent impressed electric field intensity Ei.100CCE dJ dll24iEE Chemical action creates accumulatio

26、n of positive and negative charges at electrodes 1 and 2, respectively. These charges give rise to an electrostatic field intensity E both outside and inside the battery. Inside the battery, E must be equal in magnitude and opposite in direction to the non-conservative Ei produced by chemical action

27、, since no current flows in the open-circuited battery and the net force acting on the charge carriers must vanish.EConducting mediumP 1N 2EImpressed sourceEi25The electromotive force is a measure of the strength of the non-conservative source, denoted by , we have1122dd (Inside the source)iEl =El 2

28、11200CE dE dE dlllOutside the sourceInside the source1121212221dd =d-VV ViEl =El El Outside the sourceInside the sourceWe have expressed the emf of the source as a line integral of the conservative E and interpreted it as a voltage rise. In spite of the non-conservative nature of Ei, the emf can be

29、expressed as a potential difference between the positive and negative terminals. EConducting mediumP 1N 2EImpressed sourceEi26EConducting mediumP 1N 2EImpressed sourceEi()1()iiCCCJEEIEEdJ ddRISlll =If there are more than one source of electromotive force and more than one resistor (including the int

30、ernal resistances of the sources) in the closed path, we generalize (V)jkkjkR IEquation is an expression of Kirchhoffs voltage law. It states that around a closed path in an electric circuit the algebraic sum of the emfs (voltage rises) is equal to the algebraic sum of the voltage drops across the r

31、esistances.27summary1. Current Density and Ohms Law SIJ dsA2 ( /)JuA m2 (A/m )JE12lIVIRIsG282. Electromotive Force and Kirchhoffs Voltage Law1121212221dd =d-VV ViEl =El El Outside the sourceInside the source (V)jkkjkR I29homeworkThank you! Bye-bye!Thank you! Bye-bye!答疑安排答疑安排時間：周一時間：周一 下午下午 14：0016：00地點：地點：1401， 1403P.5-8, 5-1330summary1. Current Density and Ohms Law SIJ dsA2 ( /)JuA m2 (A/m )JE12lIVIRIsG313. Equation of Continuity and Kirchhoffs Current Law2. Electromotive Force and Ki