- #1
KEØM
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Homework Statement
Let's define the radial vector [tex]\vec{v}(r) = \hat{r}/r^{2}[/tex] where [tex]\vec{r} = \vec{OP}[/tex] (O being the origin of our coordinate system and P being our observation point at point (x, y, z)). Using spherical coordinates, demonstrate that [tex]\vec{\nabla
} \cdot\vec{v}(r) = 0[/tex] everywhere except at r = 0. At r = 0, demonstrate that [tex]\vec{\nabla}\cdot\vec{v}(r)[/tex] is going to infinity.
I can show the first part but I can't show how it goes to infinity when r = 0.
Homework Equations
[tex]\vec{\nabla}\cdot\vec{v} = \frac{1}{r^{2}}\frac{\partial(r^{2} v_{r})}{\partial r} + \frac{1}{r sin\theta}\frac{\partial}{\partial\theta}(sin\theta v_{\theta}) + \frac{1}{r sin\theta}\frac{\partial v_{\phi}}{\partial\phi} [/tex]
The Attempt at a Solution
For the first part, because [tex]\vec{v}[/tex] is only a function of r the two last terms in the divergence formula will equal zero. The first term becomes
[tex]\vec{\nabla}\cdot\vec{v}(r) = \frac{1}{r^{2}}\frac{\partial}{\partial r}(r^{2}\cdot\frac{1}{r^{2}}) = 0[/tex]
zero as well, satisfying part a.
For part b, if we let r = 0 then [tex]\vec{v}(r)[/tex] becomes infinitely large and I am not sure how to take the divergence of that.
Any help will be appreciated.
Thanks in advance,
KEØM