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Homework Help: Flux for an Infinite charge line through a cylinder

  1. Feb 1, 2012 #1
    1. The problem statement, all variables and given/known data
    An infinite line of charge with charge density λ1 = 2.9 μC/cm is aligned with the y-axis as shown. a = 7.9 cm. h = 8.8 cm. λ2 = -8.7 μC/cm. Another infinite line of charge with charge density λ2 = -8.7 μC/cm parallel to the y-axis is now added at x = 3.95 cm as shown

    What is the total flux Φ that now passes through the cylindrical surface? Enter a positive number if the net flux leaves the cylinder and a negative number if the net flux enters the cylnder. (This question is what got me confused).

    2. Relevant equations


    3. The attempt at a solution

    I was able to correctly find the net E along x for point P and I was also able to find the flux for the centered line for the cylinder but once the second line was added I wasn't sure where to go from there because E is no longer constant around the cyclinder. I am sure I am missing something very fundamental but I can't wrap my mind around what that is.
  2. jcsd
  3. Feb 2, 2012 #2


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    Staff Emeritus
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    Homework Helper

    The left-hand-side is the basic definition of flux, and provides one way of calculating it (where it is the component of E perpendicular to A that is used.

    However, it turns out the flux is also equivalent to the right-hand-side of the above equation, so that provides an alternative way to calculate the same quantity.

    As you said, it's difficult to calculate the integral since E is not constant everywhere around the cylinder -- also E is not everywhere perpendicular to A. Therefore, I suggest using qencl/ε to calculate the flux instead.
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