#### fluidistic

Gold Member

- 3,615

- 94

**1. Homework Statement**

I tried to derive the electric field of an infinite (non conductor and conductor, I believe it is the same) charged rod.

**2. Homework Equations**

[tex]\oint \vec E d \vec A = \frac{Q_{\text {enclosed}}}{\varepsilon _0}[/tex].

**3. The Attempt at a Solution**

I could do all, except at the end... when he wrote that [tex]\oint \vec E d \vec A=E 2 \pi rL[/tex]. I understand that [tex]\oint d\vec A = 2\pi rL[/tex], but I don't understand how he could pass the [tex]E[/tex] outside the line integral, as if [tex]E[/tex] was constant. Because it isn't constant, [tex]E[/tex] depends on [tex]r[/tex].

I would have understood this step if we were to derive the electric field due to an infinite charged plane, where [tex]E[/tex] does not depend on the distance between a charged particle and the plane.

Can you explain me why does [tex]\oint \vec E d \vec A=E 2 \pi rL[/tex]?

Thanks in advance!

Here's the link : http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/ElectricForce/LineChargeDer.html [Broken].

(look at the very bottom of the page)

Last edited by a moderator: