- #1
razidan
- 75
- 1
Homework Statement
A conducting sphere, radius R, charged with Q is inside a conducting shell (2R<r<3R) with charge 2Q. Find the electric potential and the energy.
Homework Equations
[itex] \Phi =-\int_{r_1}^{r_2} \vec{E}\cdot\vec{dl}[/itex]
[itex]U=\int_{V}E^2dV[/itex]
The Attempt at a Solution
I think i got it right, and I'm mostly looking for confirmation:
I started with calculating the field everywhere:
[itex]
\vec{E} (r) =
\begin{cases}
0 & \quad \text{if } \text{ r<R}\\
\frac{kQ}{r^2} \hat{r} & \quad \text{if } \text{ R<r<2R}\\
0 & \quad \text{if } \text{ 2R<r<3R}\\
\frac{3kQ}{r^2} \hat{r} & \quad \text{if } \text{ r>3R}\\
\end{cases}
[/itex]
this leads to:
[itex]
\Phi (r) =
\begin{cases}
\frac{kQ}{2r} & \quad \text{if } \text{ r<R}\\
\frac{3kQ}{3R}+\frac{kQ}{2R}-\frac{kQ}{r} \hat{r} & \quad \text{if } \text{ R<r<2R}\\
\frac{3kQ}{3R} & \quad \text{if } \text{ 2R<r<3R}\\
\frac{3kQ}{r} \hat{r} & \quad \text{if } \text{ r>3R}\\
\end{cases}
[/itex]
[itex] \text{and the energy is} \quad 14\pi\frac{k^2Q^2}{R} [/itex]
Thanks,
R