- #1
Karl86
- 40
- 3
- Homework Statement
- Two infinitely long rods parallel to the ##z##-axis have uniform charge density ##\lambda##. I need to calculate the energy density ##U## per unit length along the ##z##-axis.
- Relevant Equations
- ##\frac{\epsilon_0}{2} \int E^2 d\tau##?
Let ##(x_1,y_1)## and ##(x_2,y_2)## be the point where the rods intersect the ##x,y## plane. I know that on any given point there will be the superpositions of ##E_1=\frac{2\lambda}{4\pi \epsilon_0}\frac{1}{(x-x_1)^2+(y-y_1)^2}\hat{r}_1## and ##E_2=\frac{2\lambda}{4\pi \epsilon_0}\frac{1}{(x-x_2)^2+(y-y_2)^2}\hat{r}_2##, which would mean ##E^2=\frac{4\lambda^2}{(4\pi\epsilon_0)^2}\frac{1}{(x-x_1)^2+(y-y_1)^2}\frac{1}{(x-x_2)^2+(y-y_2)^2}##. So my thinking was to sum them as vectors and then integrate the square of this sum, which will be a scalar, over the ##x,y## plane, and I though this would give me the energy density per unit length along the $z$-axis. Can you criticize my reasoning? The result should involve a logarithm of the distance between the two rods and that really seems a far cry from what my calculation points to.
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