Chapter 11: Problem 5
In the circular cylindrical coordinates \(r, \theta, z\) defined by $$ x=r \cos \theta, \quad y=r \sin \theta, \quad z=z $$ Laplace's equation is $$ u_{r r}+(1 / r) u_{r}+\left(1 / r^{2}\right) u_{\theta \theta}+u_{z z}=0 $$ (a) Show that if \(u(r, \theta, z)=R(r) \Theta(\theta) Z(z),\) then \(R, \Theta,\) and \(Z\) satisfy the ordinary differential equations $$ \begin{aligned} r^{2} R^{\prime \prime}+r R^{\prime}+\left(\lambda^{2} r^{2}-n^{2}\right) R &=0 \\ \Theta^{\prime \prime}+n^{2} \Theta &=0 \\\ Z^{\prime \prime}-\lambda^{2} Z &=0 \end{aligned} $$ (b) Show that if \(u(r, \theta, z)\) is independent of \(\theta,\) then the first equation in part (a) becomes $$ r^{2} R^{\prime \prime}+r R^{\prime}+\lambda^{2} r^{2} R=0 $$ the second is omitted altogether, and the third is unchanged.
Short Answer
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Key Concepts
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