《E导体与电介质》PPT课件.ppt

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1、1 Conductors in the Electrostatic Field2 Dielectrics and Polarization in the Electric Field 3 Capacitance and Capacitor4 The Energy of Electric field Chapter 6 Conductors and Dielectrics in the Electrostatic Field5 Application 1 Electrostatic EquilibriumIn electric conditions all the free electron a

2、re at rest2 Conditions of Electrostatic EquilibriumThe electric field is zero inside a conductorProving 1 No tangential component can existab+Proving 21 Conductors in the Electrostatic Field Corollary 1Theconducingsurfaceisanequipotentialsurface,andallpointsintheinteriorareatthesamepotentialasthesur

3、faceCorollary 2Thefieldatthesurfaceoftheconductor(andjustoutside)isperpendiculartothesurface3 Distribution of Chargea)All of the charges is on the surface of the conductor Prove：b)The electric field on the surface of a conductor sNote:The field at the surface of the conductor(and just outside)c)Dist

4、ribution of Charge of Spherical ConductorRestricting factors：the electric field is zero inside a conductor.E=0q4Conductor shell（腔）腔）5(1)No charge in a conductor shell a The electric field is zero inside inside conductor and shell 导体内、空腔内场强皆为零；导体内、空腔内场强皆为零；b The conductor and shell are equipotential导

5、体、空腔为等势体；导体、空腔为等势体；c The charge must lie on the surface 内表面处处没有电荷，电荷只分布在外表面内表面处处没有电荷，电荷只分布在外表面.(2)A charge q in a conductor shell Ontheinnersurfaceoftheshellthechargeis-q.On the outer surface of the shell the charge is q.It is determined by Gausss law.5 Electrostatic Screening 静电屏蔽静电屏蔽Electrostatic

6、Screening:Insulating the influence of electric field using conductor shell No matter the change of charge A the force on B charge always is zero.A电荷变化电荷变化,B电荷受力总是为零。电荷受力总是为零。The charge inside shell can be shielded 壳可以屏蔽内部壳可以屏蔽内部If the charge B is changed,the force on the charge A is changed too.B电荷变

7、化，电荷变化，A电荷受力会变化。电荷受力会变化。The charge A outside shell can not be shielded 壳不能屏蔽外边电荷壳不能屏蔽外边电荷A。A。BIf the shell is the ground,the influence between A and B is not exist.接地导体壳，可以使壳内外的电荷彼此不影响。接地导体壳，可以使壳内外的电荷彼此不影响。例：球形壳C，电荷A带q,B带Q，r 远大于R。求:A，B，C彼此之作用力。解：解：(1)A、B之间作用力：之间作用力：(2)A、C对对B来说是带电量来说是带电量q的点电荷，的点电荷，B受

8、力：受力：(3)静电屏蔽，静电屏蔽，A受力受力 F=0+QBA+qCrR(4)A、C之间作用力：之间作用力：(5)B、C之间作用力之间作用力F0(6)C接地，接地，A、B受力为零受力为零问题：导体壳是否真的能够屏蔽掉外电场使其不能进入腔内？答：接地导体壳答：接地导体壳return电介质中不存在自由电荷，在电场中会发生极化现象。电介质中不存在自由电荷，在电场中会发生极化现象。极性介质：极性介质：HCl、SO2、H2O 非极性介质：非极性介质：O O2 2 H H2 2 CO CO2 21 Dielectric polarization无外电场时，电偶极矩取向杂乱或为零，整个介质呈电中性。当有外电

9、场时，非极性分子产生极化，极性分子趋向外场排列，介质表面产生束缚电荷。E-2 Polarization 极化的强弱程度用电极化强度矢量描述。由于每个分子为偶极子，偶极矩P越大，单位体积内偶极子越多，极化越强。线性各向同性介质线性各向同性介质2 Dielectrics and Dielectric Polarization 3 The relationship between electric polarization and bound charge4 Electric displacement Gausss law in mediumElectric displacement 电位移矢量电位

10、移矢量Relative permittivity 相对电容率相对电容率The field in medium 介质中的场：介质中的场：(Absolute)permittivity 电容率电容率解：解：QR例1一点电荷Q被半径为R的介质1包围，球外是2的介质，求空间电场强度。例2平行极板间极化强度矢量，求介质表面束缚电荷密度解：解：选取圆柱形高斯面选取圆柱形高斯面return110-310-610-910-12FmFF微nF纳pF皮1 The capacitance of an Isolated ConductorUnit：Farads法拉TheradiusoftheearthR6370kmC=

11、7.0810-4F3 Capacitance Capacitor Capacitance is a measure of amount of charge a conductor holds at a given potential difference 2 Capacitance of a CapacitorCapacitor：Itisconsistsoftwoconductorplatesbeingveryclosed3 Combinations of Capacitors 电容器的联接电容器的联接(1)Connected in Series 串联(2)Connected in Paral

12、lel 并联并联Forcapacitorsconnectedinseriesthechargeoneachcapacitoristhesame.C1C2C1C2CapacitorsconnectedinParallelhavethesamepotentialdifferenceacrosstheirplateComparing 弹簧的串并联，电阻的串并联，物理中由几个相同量纲量组成另一个同量纲量时，只有上面的组合。4 Calculating the Capacitance of a Capacitor(1)A Parallel-plate CapacitorThecapacitanceofac

13、apacitordependsontheseparationdistanced,theareaSandthepermittivityRARBq-qSD=0-QdABQ(2)A Spherical Capacitor(3)A Cylindrical Capacitor-取高为取高为h 圆柱形高斯面圆柱形高斯面RARBr hNote：1 Thecapacitanceofacapacitordependsonthegeometryofthecapacitorandthepermittivity,independentof thepotentialdifferenceandthecharge2 The

14、sphericalcapacitorisastandardcapacitorEx1 A parallel-plate capacitor consists of two square plates,each of sides 4.0 cm separated by 2.0 mm.(a)Calculate the capacitance.(b)How many electrons can be stored on a plate if the capacitor is charged to 50 V?Solution(a)(b)Q=CU=7.110-1250=3.510-10CThemagnit

15、udeofthechargeofoneelectronis1.610-19C,sothenumberNofelectronsthatcanbestoredis=7.110-12F=7.1pF.Ex2Asphericalcapacitorconsistsoftwothinconcentricsphericalshellsofradiiaandb,withab.Acharge Qisplacedontheinnershell,andQ ontheoutershell,whichisgrounded.Calculatethecapacitance.InordertouseC=,weneedfirst

16、tocalculatethepotentialdifferencebetweentheshells,whichinturndependsonthefield.Aconcentricsphereofradiusr,wherearEB DA=DBreturn5 Application例例1无限长均匀带电圆柱面内外的场强分布：0，R面内面内面外面外方向垂直柱面向外解：电荷分布具有柱对称性，解：电荷分布具有柱对称性，电场分布具有柱对称性；电场分布具有柱对称性；利用高斯定理求场强分布，利用高斯定理求场强分布，选取柱形高斯面选取柱形高斯面rR例例2无限长均匀带电圆柱体内外的场强分布：0，R柱体内柱体内柱体外柱体外方向垂直圆柱体轴线向外方向垂直圆柱体轴线向外rR例例3无限长均匀带电圆柱面，半径为R，单位长带电量为+，试求其电势分布。解：解：电场强度分布：rR：rQB：（1）证明两板内侧电荷密度符号相反，数值相等；外侧的电荷密度符号相同，数值相等。（2）计算板上的电荷面密度和两板间的电势差。解：解：(1)证明（2）电荷守恒定律：ABQAQBdP1P2P1：静电平衡导体内部场强为零A、B间总场：例6把一块原来不带电的大金属板B移近一块带电Q的A板，B接地和不接地，两极板的电势差是否一样？B接地B不接地ABreturn