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Homework Help: Electric potential of a line of charge

  1. Sep 11, 2011 #1
    1. The problem statement, all variables and given/known data

    Find the electric potential at a point d perpendicular to one end of a line of charge with a positive uniform linear charge density lambda, length L, and negligible thickness.

    2. Relevant equations

    A = 2πdL
    V_f - V_i = -integral(Eds)

    3. The attempt at a solution

    for E i get λ/(2πDε_0) by drawing a gaussian cylinder around the line of charge and then i plug that into the integral for V which turns into V = Ed*ln|(L+(d^2+L^2 )^(1/2))/d|

    the answer i get is V= λ/(2πε_0 ) ln|(L+(d^2+L^2 )^(1/2))/d| which is correct except that i should have a 4πε_0 instead of a 2πε_0 but i dont understand why since the area of my gaussian cylinder is 2πdL
  2. jcsd
  3. Sep 11, 2011 #2
    Are you *sure* the area is 2*pi*d*L?
  4. Sep 11, 2011 #3
    .........yes? lol i mean the way i drew my diagram charge would only be coming out of the middle of the cylinder. i mean i acknowledge that im making a mistake somewhere in there but i cant for the life of me see why.
  5. Sep 11, 2011 #4


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    Staff Emeritus
    Science Advisor
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    Gauss's Law isn't applicable here.

    There's not sufficient symmetry.

    What is the electric potential at a distance R from a point charge with a charge of Q ?
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