- #1
ELB27
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- 15
Homework Statement
[problem 3.49 from Griffiths' Introduction to Electrodynamics 4th edition. The relevant equations from the book are reproduced in "relevant equations" below] In Ex. 3.9, we obtained the potential of a spherical shell with surface charge ##\sigma(\theta) = k\cos\theta##. In Prob. 3.30, you found that the field is pure dipole outside; it's uniform inside. Show that the limit ##R## → 0 reproduces the delta function term in Eq. 3.106.
Homework Equations
Dipole moment of this charge configuration:
[tex] \vec{p} = \frac{4}{3}\pi kR^3\hat{z}[/tex]
Potential inside shell:
[tex] V_{in} = \frac{k}{3\epsilon_0}r\cos\theta[/tex]
Potential outside shell:
[tex] V_{out} = \frac{k}{3\epsilon_0}\frac{R^3}{r^2}\cos\theta[/tex]
Eq. 3.106 from the book:
[tex] \vec{E}_{dip}(\vec{r}) = \frac{1}{4\pi\epsilon_0}\frac{1}{r^3}[3(\vec{p}⋅\hat{r})\hat{r} - \vec{p}] - \frac{1}{3\epsilon_0}\vec{p}\delta^3(\vec{r})[/tex]
The Attempt at a Solution
First of all I'm not sure how am I supposed to get a delta function out of a limit.
From the above potentials, I calculated the electric field inside and outside the shell (from the definition of the potential: ##\vec{E} = -\nabla V##).
Electric field outside shell:
[tex] \vec{E}_{out} = \frac{k}{3\epsilon_0}\frac{R^3}{r^3}(2\cos\theta\hat{r} + \sin\theta\hat{\theta})[/tex]
It should be mentioned that the above expression is identical to the first term (the one without the delta function) in Eq. 3.106 cited above, the only difference being that in Eq. 3.106 it is written in coordinate-free form.
Electric field inside shell:
[tex] \vec{E}_{in} = -\frac{k}{3\epsilon_0}(\cos\theta\hat{r} - \sin\theta\hat{\theta})[/tex]
I reasoned that in the limit ##R## → 0, the electric field everywhere but the origin will be zero (by taking the limit of ##\vec{E}_{out}## above) or at least an infinitesimal quantity depending on the infinitesimal ##R##. At the origin (or at an infinitesimal sphere of radius ##R##), the field will be uniform (in the sense that it doesn't depend on ##R##).
Now I'm stuck - I can't figure out how to derive the delta function.
Any help will be greatly appreciated - this problem really interests me now :p