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上一讲我们讨论了法拉第电磁感应定律。现在我们讨论一个典型的应用—发电机。请看这幅图，这是交流发电机的简易模型图。
由两个磁铁产生磁场。可以转动的线圈处于两个磁铁之间的区域。线圈的两端与两个滑环相接，通过滑环上的两个静止电刷与外部电路相连，作为发电机的输出端口。
我们来看发电机的工作原理。当线圈在磁场中转动，磁感强度B和线圈平面之间的夹角随时间变化，使得穿过线圈平面的磁通量随时间变化，从而在线圈中产生了感应电动势。
我们可以计算出任意时刻t穿过线圈所围面积的磁通量: Φb 等于B点乘S，等于B乘以S乘以cosθ。
为方便起见，假设初始时刻，磁感强度B与线圈平面法向同向，则θ等于ωt.
如果有N匝线圈，磁场线穿过线圈N次，则磁通量应该等于N乘以Φb。
根据法拉第电磁感应定律，线圈中的感应电动势应该是：ε等于N乘以dΦb 比 dt 等于 NBAωsinωt。
这是感应电动势emf 随时间的正弦变化函数。
显然，当磁场垂直于线圈平面法向，emf 为最大值NBAω。当磁场平行于线圈平面法向，emf等于零。
现在如果我们需要一个直流发电机，应该怎样设计？我们知道对于直流发电机，在线圈转动过程中，感应电动势的极性必须保持不变，如图所示。
请看直流发电机的简易模型示意图。类似地，由两个磁铁产生磁场，可以转动的线圈处于两个磁铁之间的区域。
但我们可以发现连接线圈两端的不是交流发电机的两个滑环，而是一个分裂环，我们称为换向器。
可以理解，每半个周期，感应电动势的极性会反向。由于这个结构，在相同时间间隔内，连接分裂环的电刷也会反向，因此输出电压可以保持不变，从而我们得到直流电。
好，下一讲，我们将讨论电感。
-Lecture 1 Electric Charges
-Lecture 1 Electric Charges--作业
-Lecture 2 Coulomb’s Law
-Lecture 2 Coulomb’s Law--作业
-Lecture 3 Electric Fields
-Lecture 3 Electric Fields--作业
-Lecture 4 Evaluation of Electric Fields（I）
--Lecture 4 Evaluation of Electric Fields（I）
-Lecture 4 Evaluation of Electric Fields（I）--作业
-Lecture 5 Evaluation of Electric Field (II)
--Lecture 5 Evaluation of Electric Field (II)
-Lecture 5 Evaluation of Electric Field (II)--作业
-Lecture 6 Electric Field Lines
--Lecture 6 Electric Field Lines
-Lecture 6 Electric Field Lines--作业
-Lecture 7 Electric Flux
-Lecture 7 Electric Flux--作业
-Lecture 8 Gauss’s Law
-Lecture 8 Gauss’s Law--作业
-Lecture 9 Application of Gauss’s Law
--Lecture 9 Application of Gauss’s Law
-Lecture 9 Application of Gauss’s Law--作业
-Lecture 10 Electric Potential
--Lecture 10 Electric Potential
-Lecture 10 Electric Potential--作业
-Lecture 11 Evaluation of Electric Potential
--Lecture 11 Evaluation of Electric Potential
-Lecture 11 Evaluation of Electric Potential--作业
-Lecture 12 Finding E from V
-Lecture 12 Finding E from V--作业
-Lecture 13 Properties of a conductor in electrostatic equilibrium （I）
--Lecture 13 Properties of a conductor in electrostatic equilibrium
-Lecture 13 Properties of a conductor in electrostatic equilibrium （I）--作业
-Lecture 14 Properties of a conductor in electrostatic equilibrium (II)
--Lecture 14 Properties of a conductor in electrostatic equilibrium (II)
-Lecture 14 Properties of a conductor in electrostatic equilibrium (II)--作业
-Lecture 15 Dielectrics
-Lecture 15 Dielectrics--作业
-Lecture 16 Capacitor & Capacitance
--Lecture 16 Capacitor & Capacitance
-Lecture 16 Capacitor & Capacitance--作业
-Lecture 17 Capacitors with dielectrics
--Lecture 17 Capacitors with dielectrics
-Lecture 17 Capacitors with dielectrics--作业
-Lecture 18 Energy stored in a capacitor
--Lecture 18 Energy stored in a capacitor
-Lecture 18 Energy stored in a capacitor--作业
-Lecture 19 Magnetism
-Lecture 20 Current & Current Density
--Lecture 20 Current & Current Density
-Lecture 20 Current & Current Density--作业
-Lecture 21 Steady current & Electromotive force
--Lecture 21 Steady current & Electromotive force
-Lecture 22 Magnetic Fields
-Lecture 22 Magnetic Fields--作业
-Lecture 23 Charged Particle in a Magnetic Field
--Lecture 23 Charged Particle in a Magnetic Field
-Lecture 23 Charged Particle in a Magnetic Field--作业
-Lecture 24 Magnetic Force on a Current—Carrying Conductor
--Lecture 24 Magnetic Force on a Current—Carrying Conductor
-Lecture 24 Magnetic Force on a Current—Carrying Conductor--作业
-Lecture 25 Sources of a Magnetic Field
--Lecture 25 Sources of a Magnetic Field
-Lecture 25 Sources of a Magnetic Field--作业
-Lecture 26 Calculation of a magnetic field by applying the Biot-Savart Law
--Lecture 26 Calculation of a magnetic field by applying the Biot-Savart Law
-Lecture 26 Calculation of a magnetic field by applying the Biot-Savart Law--作业
-Lecture 27 Ampere’s Law
-Lecture 27 Ampere’s Law--作业
-Lecture 28 Application of Ampere’s Law
--Lecture 28 Application of Ampere’s Law
-Lecture 28 Application of Ampere’s Law--作业
-Lecture 29 Gauss’s Law in Magnetism
--Lecture 29 Gauss’s Law in Magnetism
-Lecture 29 Gauss’s Law in Magnetism--作业
-Lecture 30 Origin of the magnetic effect of materials
--Lecture 30 Origin of the magnetic effect of materials
-Lecture 30 Origin of the magnetic effect of materials--作业
-Lecture 31 Magnetization
-Lecture 31 Magnetization--作业
-Lecture 32 Magnetic Materials
--Lecture 32 Magnetic Materials
-Lecture 32 Magnetic Materials--作业
-Lecture 33 Faraday’s Law
-Lecture 33 Faraday’s Law--作业
-Lecture 34 Motional EMF
-Lecture 34 Motional EMF--作业
-Lecture 35 Induced Electric Field
--Lecture 35 Induced Electric Field
-Lecture 35 Induced Electric Field--作业
-Lecture 36 Generators
-Lecture 36 Generators--作业
-Lecture 37 Inductance
-Lecture 37 Inductance--作业
-Lecture 38 Energy in a Magnetic Field
--Lecture 38 Energy in a Magnetic Field
-Lecture 38 Energy in a Magnetic Field--作业
-Lecture 39 Maxwell’s Equations
--Lecture 39 Maxwell’s Equations
-Lecture 39 Maxwell’s Equations--作业
