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Electric Field at a Point

  1. Feb 24, 2013 #1
    1. The problem statement, all variables and given/known data
    A charged rod of length L = 1.00 m lies centered on the x axis as shown. The rod has a linear charge density which varies according to λ = ax where a = −90.0 μC/m.
    What is the x component of the electric field at a point on the x axis a distance of D = 2.00 m from the end of the rod?

    2. Relevant equations
    charge density = Q/A

    3. The attempt at a solution
    I really do not understand...Where do I get the r from? Since its not uniformly distributed how can there be any field outside?
    Would my r = (D-(half of L))?
    I'm assuming lambda is just regular charge density at a location of x?

    Attached Files:

  2. jcsd
  3. Feb 24, 2013 #2


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    Hi Broem! :smile:
    Yes. :smile:

    (and of course it's a linear charge density, ie coulombs per metre, not per metre3 :wink:)
    Not following you. :confused:
  4. Feb 24, 2013 #3
    Thanks for the quick response!!
    Ok so here's where I am:

    I now know that lambda = Q/L so Q = L * (lambda)

    So I'm left with

    This however is not the correct answer...Where can I go from here?
  5. Feb 24, 2013 #4


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    oh, no, you'll have to integrate

    use coulomb's law to find the electric field from a tiny section [x,x+dx] (whose charge will be λdx, and which you can assume is concentrated entirely at x), and integrate from -L/2 to L/2 :wink:
  6. Feb 25, 2013 #5
    I got it!!!
    Thank you so much for your help!!!
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