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在静电场中,我们研究了穿过闭合曲面的电场通量,导出了静电场的高斯定理。类比于电场通量,我们用同样的方法定义磁通量。
请看这幅图,考虑一个任意形状的曲面上某一点处的面元矢量dA. 面元矢量dA垂直于该点处的面元,大小等于面元的面积。
穿过面元dA的磁通量定义为B和dA的点乘. 其中,B为面元处的磁感强度。
因此,穿过整个曲面的通量就等于磁感强度沿整个曲面的面积分。磁通量的国际单位为特斯拉平方米,我们定义为韦伯。
我们考虑一种特殊情形。平面A放置于匀强磁场B中,B与面元dA法向之间的夹角为θ。则磁通量等于BAcosθ,
如图所示,如果磁场平行于平面,或者说,磁场垂直于平面法向,则θ等于90度,磁通量为零。
另一方面,如果磁场垂直于平面,或者说,磁场平行于平面法向,则θ等于零,磁通量最大,为B乘以A。
现在我们来看一个例子。请看这幅图,一根长直载流导线,通恒定电流I。
一个宽为a长为b的矩形线圈位于导线一旁,长直导线与线圈的近边相距c.
导线平行于矩形框的长边。我们计算通过矩形框的磁通量。
我们知道长直载流导线的磁场线是圆,它们穿过矩形线圈,与到导线垂直距离r成反比。
因此,穿过线圈的磁场是非均匀的,我们必须先确定穿过面元dA 的磁通量dΦb,再通过积分得到总磁通量。
如图,我们选择长为b宽为dr的面元dA. 磁场量由此式计算。
注意到线圈平面各点的B与面元矢量dA平行,所以B点乘dA等于BdA. 代换B和dA,然后进行积分,积分下限为c, 上限为a+c. 由此得到穿过整个矩形线圈的磁通量。
根据高斯定理,我们知道穿过闭合面的电场通量与闭合面所围的净电荷成正比。
换句话说,穿出闭合面的电场线数目仅仅取决于面内的净电荷。这是因为电场线总是从正电荷出发,终止于负电荷。
磁场线的情况就完全不同了,磁场线是闭合曲线。如图所示的是条形磁铁的磁场线。显然磁场线无头无尾。
如图,对于任意一个闭合面,如虚线所示,穿进闭合面的磁场线数目等于穿出闭合面的磁场线数目。
因此,穿过闭合面的净通量必为零。这与电场线的情形完全不同,如图,对于任意闭合面,只要面内的净电荷不为零,电通量就不为零。
由此我们得到磁场的高斯定理,磁场的高斯定理表明通过任意闭合曲面的磁通量必等于零,如此方程所示。
磁场的高斯定理也说明到目前为止,自然界还未发现单磁极。
至此我们讨论的是真空中的磁场。假如空间中有磁介质,情况会怎样?我们下一讲探讨这个问题。
-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