- #1
ghostfolk
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Homework Statement
A spherical conductor of radius ##a## carries a charge ##q## and also there is a jelly of constant charge density ##\rho## per unit volume extending from radius a out to radius ##b##. Find the electrostatic energy stored in the configuration.
Homework Equations
##\oint \vec{E} \cdot d\vec{a}=\frac{Q_{enc}}{\epsilon_0}##
##U=\frac{\epsilon_0}{2}\int E^2 d^3r##
The Attempt at a Solution
[/B]We first find the electric field.
##\oint \vec{E} \cdot d\vec{a}=4\pi r^2##
##Q_{enc}=\int_a^r 4\pi r'^2 \rho dr'+q=\frac{4\pi}{3}(r^3-a^3)\rho+q##
So then,
##E=\begin{cases}
0, r<a& \\\
\rho \frac{(r^3-a^3)}{3r^2 \epsilon_0}+ \frac{q}{4\pi r^2\epsilon_0} \hat{r}, a<r<b\\
\rho \frac{(b^3-a^3)}{3r^2 \epsilon_0} +\frac{q}{4\pi r^2\epsilon_0} \hat{r}, b\le r
\end{cases}##
Then
##E^2=\begin{cases}
0, r<a& \\\
\rho^2 \frac{(r^3-a^3)^2}{9r^4 \epsilon_0^2}+ 2q \rho \frac{(r^3-a^3)}{12 \pi r^4\epsilon_0^2}+\frac{q}{16 \pi^2 r^4 \epsilon_0^2}, a<r<b\\
\rho^2 \frac{(b^3-a^3)^2}{9r^4 \epsilon_0^2}+ 2q \rho \frac{(b^3-a^3)}{12 \pi r^4\epsilon_0^2}+\frac{q}{16 \pi^2 r^4 \epsilon_0^2}, b\le r
\end{cases}##
Now,
##U=\frac{\epsilon_0}{2} \int_a^b (\rho^2 \frac{(r^3-a^3)^2}{9r^4 \epsilon_0^2}+ 2q \rho \frac{(r^3-a^3)}{12 \pi r^4\epsilon_0^2}+\frac{q}{16 \pi^2 r^4 \epsilon_0^2})4 \pi r^2dr+\frac{\epsilon_0}{2} \int_b^\infty (\rho^2 \frac{(b^3-a^3)^2}{9r^4 \epsilon_0^2}+ 2q \rho \frac{(b^3-a^3)}{12 \pi r^4\epsilon_0^2}+\frac{q}{16 \pi^2 r^4 \epsilon_0^2}) 4 \pi r^2dr##
So far, have I done everything correctly?