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Homework Help: Electric Field magnitude problem help

  1. Feb 7, 2009 #1
    1. The problem statement, all variables and given/known data

    A very long uniform line of charge has charge per unit length 4.88E-6 C/m and lies along the x-axis. A second long uniform line of charge has charge per unit length -2.44E-6 C/m and is parallel to the x-axis at y1 = 0.408 m.

    What is the magnitude of the net electric field at point y2 = 0.198 m on the y-axis?

    2. Relevant equations

    E = kq/r^2

    3. The attempt at a solution

    E1 = (9E9)(4.88E-6)/(.198)^2
    E2 = (9E9)(-2.44E-6)/(.21)^2

    net E = E1 + E2 (both point in the positive y direction), but I'm not getting the right answer. Apparently, my answer is close, but not correct. Can somebody help me to spot my mistake?

    Also, I know the problem gave linear charge density, but it didn't say how long the wire was. Maybe that's where my error lies. If so, how do I correct it?
  2. jcsd
  3. Feb 8, 2009 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    One cannot simply take the expression for the electric field of a point charge and apply it to an infinite line charge. Conceptually, to find the electric field do to a line charge one must sum up the contributions from each individual point charge. Since the charge distribution is continuous one does this via an integral rather than a sum.

    See this page: http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/elelin.html for more information.
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