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Homework Help: Nonconducting Rod electric pot.

  1. Dec 15, 2009 #1
    1. The problem statement, all variables and given/known data

    There is a nonconducting rod of negligible thickness located along the x axis; its ends have coordinates x = 0 and x = L. It has a positive, nonuniform, linear charge density (lambda) = (alpha)x; alpha is constant. An infinite distance away, th eelectric potential is zero. Show that th electric potential at the location x=L+d is given by:

    V= ( alpha/4pi(epsilon0) ) ( (L+d) ln(1+L/d) -L )

    2. Relevant equations

    V= q/4pi epsilon r

    3. The attempt at a solution

    V = integral of dv
    dv= dq/4pi(epsilon)r
    dq= alpha x dx

    dV = ( (alpha) x dx) / (4 pi epsilon (d-x) )

    V=constants <integral> xdx/ d-x <===== integration table

    <integral> udu/a+bu = 1/b2 (a + bu - a*ln(a + bu) <evaluate from 0 to L>

    when I evaluate i get:

    (constants) * d-d-L d*ln( d / d - L )

    and thats not what im supposed to get =/

  2. jcsd
  3. Dec 16, 2009 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Your expression d - x for the distance between your charge element and the location of interest is incorrect. Read the problem. The point of interest is at distance d from the end of the rod at x = L, so any point on the rod must be at distance greater than d from the point of interest. Draw a picture and see for yourself what that distance ought to be.
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