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E Field of a line charge

  1. Feb 12, 2009 #1
    1. The problem statement, all variables and given/known data
    Infinite unknown line charges of 5nC/m lie along the (positive and negative) x and y axes in free space. Find E at P(0,0,4)


    2. Relevant equations
    E = (pl/2(pi)(eo)p)ap
    pi = 3.14...
    eo - permitivity of free space 8.854 x 10^-12
    pl = line charge density
    ap = unit vector

    3. The attempt at a solution
    really just need to know how to solve for the unit vector pl then I can do the rest.
    I was thinking along the lines of ap = (z-4) az/ sqrt((z-4)^2) then plug in z... not really sure. Any help would be appreciated.

    The answer is 45az but how?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Feb 12, 2009 #2

    LowlyPion

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    Homework Helper

    Why the unit vector pl?

    Through symmetry (You're on the z-axis) isn't the set up equivalent to just a single line charge with density 10 taken from the radius of 4?
     
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