如图 1所示,由于长直共轴共焦抛物导体柱板在空间产生的电场与轴无关,是垂直于轴的横截面上的二维场问题.文献[1-3]分别采用抛物柱坐标、高斯定理和复势函数法对此问题进行了研究;文献[4]采用不同坐标系的度规系数简化求解拉普拉斯方程.本文引入导体曲面函数,将解偏微分方程问题转化为一般的积分运算,简便直观地导出了共轴共焦抛物导体柱板间的电场分布.
设f(x, y, z)为具有连续一、二阶偏导数的函数,c为参数,则
$ f\left( {x, y, z} \right) = c $ | (1) |
表示一空间曲面族[5].若任取2个满足f(x, y, z)的薄导体曲面并带电,则曲面间的电势u满足拉普拉斯方程,而该电势在薄导体曲面上的取值为常量,说明电势u是f(x, y, z)的函数.
因为
$ \nabla u = \frac{{{\text{d}}u}}{{{\text{d}}f}}\frac{{\partial f}}{{\partial x}}{{\bf{e}}_{\bf{x}}} + \frac{{{\text{d}}u}}{{{\text{d}}f}}\frac{{\partial f}}{{\partial y}}{{\bf{e}}_{\bf{y}}} + \frac{{{\text{d}}u}}{{{\text{d}}f}}\frac{{\partial f}}{{\partial z}}{{\bf{e}}_{\bf{z}}}, $ |
所以
$ \begin{gathered} {\nabla ^2}u = \frac{\partial }{{\partial x}}\left( {\frac{{{\text{d}}u}}{{{\text{d}}f}}\frac{{\partial f}}{{\partial x}}} \right) + \frac{\partial }{{\partial y}}\left( {\frac{{{\text{d}}u}}{{{\text{d}}f}}\frac{{\partial f}}{{\partial y}}} \right) + \hfill \\ \frac{\partial }{{\partial z}}\left( {\frac{{{\text{d}}u}}{{{\text{d}}f}}\frac{{\partial f}}{{\partial z}}} \right) = \frac{{{{\text{d}}^2}u}}{{{\text{d}}{f^2}}}{\left| {\nabla f} \right|^2} + \frac{{{\text{d}}u}}{{{\text{d}}f}}{\nabla ^2}f. \hfill \\ \end{gathered} $ |
令
$ \frac{{{\text{d}}u'}}{{u'}} = \frac{{{\nabla ^2}f}}{{{{\left| {\nabla f} \right|}^2}}}{\text{d}}f. $ |
积分2次得曲面间的电势
$ u = A\int {{{\text{e}}^{-\frac{{{\nabla ^2}f}}{{{{\left| {\nabla f} \right|}^2}}}}}{\text{d}}f + B, } $ | (2) |
其中,A、B为积分常数.
2 抛物板间的电势分布图 1为具有共同焦点(0, 0)的两抛物板l1和l2,对应的方程为
$ f\left( {x, y} \right) = c =-y + \sqrt {{x^2} + {y^2}}, $ | (3) |
其中抛物板l1对应c=c1,电势为u1;抛物板l2对应c=c2,电势为u2,由于板间电势u满足拉普拉斯方程,故将相应参量代入式(2),即可计算抛物板间的电势.
由
$ u = A\int {\frac{{{\text{d}}f}}{{\sqrt f }}} + B = 2A\sqrt f + B. $ | (4) |
下面由边界条件确定积分常数A和B.将抛物柱板对应的u1, c1,u2, c2分别代入式(4)有
$ A = \frac{{{u_2}-{u_1}}}{{2\left( {\sqrt {{c_2}}-\sqrt {{c_1}} } \right)}}, B = \frac{{{u_1}\sqrt {{c_2}}-{u_2}\sqrt {{c_1}} }}{{\sqrt {{c_2}} - \sqrt {{c_1}} }}, $ |
所以
$ \begin{gathered} u\left( {x, y} \right) = \frac{{{u_2}-{u_1}}}{{\sqrt {{c_2}}-\sqrt {{c_1}} }}\sqrt {-y + \sqrt {{x^2} + {y^2}} } + \hfill \\ \;\;\;\;\;\;\;\;\;\;\;\;\;\frac{{{u_1}\sqrt {{c_2}} - {u_2}\sqrt {{c_1}} }}{{\sqrt {{c_2}} - \sqrt {{c_1}} }}, \hfill \\ \end{gathered} $ | (5) |
式(5)给出的两抛物导体柱板间的电势分布与文献[1]结果相同.
3 抛物板间的电场强度由
$ \begin{gathered} \mathit{\boldsymbol{E}} =- \frac{{\partial u}}{{\partial x}}{\mathit{\boldsymbol{e}}_\mathit{\boldsymbol{x}}}- \frac{{\partial u}}{{\partial y}}{\mathit{\boldsymbol{e}}_\mathit{\boldsymbol{y}}} =- \frac{1}{2}\frac{{{u_2} - {u_1}}}{{\sqrt {{c_2}} - \sqrt {{c_1}} }} \times \hfill \\ \left[{\frac{{x{\mathit{\boldsymbol{e}}_x} + \left( {y-\sqrt {{x^2} + {y^2}} } \right){\mathit{\boldsymbol{e}}_y}}}{{\sqrt {{x^2} + {y^2}} \times \sqrt {-y + \sqrt {{x^2} + {y^2}} } }}} \right], \hfill \\ \end{gathered} $ | (6) |
由式(6)可得电场线满足微分方程:
$ \frac{{{\text{d}}y}}{{{\text{d}}x}} = \frac{{{E_y}}}{{{E_x}}} = \frac{y}{x}-\sqrt {1 + {{\left( {\frac{y}{x}} \right)}^2}}, $ | (7) |
令
$ \frac{{{\text{d}}t}}{{\sqrt {1 + {t^2}} }} =-\frac{{{\text{d}}x}}{x}, $ |
两边积分得
即
$ y + \sqrt {{x^2} + {y^2}} = A\left( {A为任意常数} \right), $ | (8) |
式(8)即为抛物柱板间的电场线方程.
由式(6)可知,柱板间电场强度大小为
$ E = = \frac{{\sqrt 2 }}{2} \times \frac{{{u_2}-{u_1}}}{{\sqrt {{c_2}}-\sqrt {{c_1}} }}{\left( {\frac{1}{{{x^2} + {y^2}}}} \right)^{\frac{1}{4}}}, $ | (9) |
故抛物板上电荷面密度为
$ \sigma = {\varepsilon _0}E = = \frac{{\sqrt 2 {\varepsilon _0}}}{2} \times \frac{{{u_2}-{u_1}}}{{\sqrt {{c_2}}-\sqrt {{c_1}} }}{\left( {\frac{1}{{{x^2} + {y^2}}}} \right)^{\frac{1}{4}}}, $ | (10) |
说明抛物导体板上的电荷分布是不均匀的,越靠近抛物柱弧顶,电荷密度越大,而在较远处,电荷密度几乎为0.
通过导体曲面函数,利用积分导出了共轴共焦的抛物导体柱板间的电场分布,并给出了导体板上的电荷分布情况.该方法为计算等势面规范的带电导体产生的电场提供了新思路,有利于对静电问题的研究.
[1] |
朱国斌, 陈钢, 陈梦姣. 用抛物柱坐标求解两共焦抛物板间的电势和电场[J].
大学物理, 2011, 30(11): 56–57.
ZHU G B, CHEN G, CHEN M J. Distribution of electric potential between two charged confocal parabolic conductor plates by parabolic cylindrical coordinates[J]. College Physics, 2011, 30(11): 56–57. DOI:10.3969/j.issn.1000-0712.2011.11.016 |
[2] |
郑民伟. 用高斯定理计算两共焦抛物柱板间的电场和电容[J].
大学物理, 2015, 34(10): 20–22.
ZHENG M W. Calculation of two confocal parabolic cylinder plate between the electric field and capacitance using Gauss theorem[J]. College Physics, 2015, 34(10): 20–22. |
[3] |
郑民伟. 用复势函数法计算两共焦抛物柱板间的电场和电容[J].
广州航海学院学报, 2016, 24(2): 18–20.
ZHENG M W. Using the complex potential function method to calculate the electric field between the two confocal parabolic column plate and capacitance[J]. Journal of Guangzhou Maritime Institute, 2016, 24(2): 18–20. |
[4] |
陈钢, 林焰清. 等值坐标带电板的电势分布[J].
大学物理, 2010, 29(8): 31–32.
CHEN G, LIN Y Q. The electric potential of equivalent coordinate charged cylindrical shells or planes[J]. College Physics, 2010, 29(8): 31–32. |
[5] |
张之翔. 等势面族的条件及例子[J].
大学物理, 2008, 27(6): 3–5.
ZHANG Z X. Conditions and examples of equipotential[J]. College Physics, 2008, 27(6): 3–5. |