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Homework Help: Finding the electric field of a charged rod

  1. Jan 10, 2014 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    See above

    3. The attempt at a solution

    I messed with this for hours. I also searched all over the web and can't find anything that is similar. None of the problems I could find gave a value, they just gave lambda. Anyway, I have no idea what I'm doing wrong here. Any help would be greatly appreciated.
  2. jcsd
  3. Jan 10, 2014 #2
    Your method is correct but the units are not written. Note that the units of charge density given are pC not C.

  4. Jan 10, 2014 #3


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    Homework Helper

    Your method is correct, but:

    The linear charge density is λ=-3.5 pC/m. What do you think pC means?

    Plug in the value of the constant k, to get the final result. What is k?

    You need to the include the units in the final result.

  5. Jan 10, 2014 #4
    Wow, thank you. I had no idea, nor did our professor ever bother to bring that up when he assigned this homework. I actually though it may have been a ρ symbolizing the density. It threw me off because I couldn't find any similar problem that defined λ.

    I really appreciate the feedback. I made the adjustment and my answer checks out. I know I had my sign wrong (went from the point to dx instead of dx to the point for r), but I wasn't so much worried about that as I was figuring out what I was doing wrong.
  6. Jan 10, 2014 #5


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    According to the vectorial form of the Coulomb force , the electric field at a point x on the x axis, produced by a charge Q at x=X0 is

    [tex]F=k\frac{Q}{|x-X_0|^2} \frac{x-X_0}{|x-X_0|}[/tex]
    So you have to measure the distance from the charge to the point, you did it well, but you have too multiply kdq/(x-0.1)2 with the sign of x-0.1. It is negative: you missed a - sign.

    YOu need to be familiar with the prefixes of SI units : http://www.physics.nist.gov/cuu/Units/prefixes.html

  7. Jan 11, 2014 #6
    Got it and thanks for the link to the prefixes too.
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