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Homework Help: Magnitude of electric field due to line of charge

  1. Mar 16, 2009 #1
    1. The problem statement, all variables and given/known data

    A charge (uniform linear density = 9.4 nC/m) lies on a string that is stretched along an x axis from x = 0 to x = 3.0 m. Determine the magnitude of the electric field at x = 5.5 m on the x axis.

    2. Relevant equations

    [tex]\stackrel{\rightarrow}{dE}=\frac{\lambda dx}{4 \pi \epsilon_{0} r^2}\hat{r}[/tex]

    3. The attempt at a solution

    I believe the unit vector can eliminated since the point is on the same axis as the line of charge (cos(0)=1). My concern is with my limits of integration and with my "r" argument. My final expression was:

    [tex]E = \frac{\lambda}{4 \pi \epsilon_{0} 5.5^2} \int^{3}_{0} dx[/tex] which got me an 8.38244 N/C, a wrong answer.
  2. jcsd
  3. Mar 16, 2009 #2


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    Homework Helper

    Looks like your r is a function of x.

    For any dq isn't r = (5.5 - x) ?
  4. Mar 16, 2009 #3
    Yes! I knew the influence of dq would vary with distance from it. I just wasn't sure how to incorporate that. I guess I'm having trouble visualizing r as a function of x that way. Is the x in (5.5 - x) the position of dq?
  5. Mar 16, 2009 #4


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    As I see it, yes. That should be the r for a particular dq located at x, and relative to the point 5.5.
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