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Homework Help: Cylindrical shell- electric potential problem

  1. Feb 28, 2012 #1
    A very long insulating cylindrical shell of radius 6.40 cm carries charge of linear density 8.90μC/m spread uniformly over its outer surface. What would a voltmeter read if it were connected between the surface of the cylinder and a point 4.00 cmabove the surface?

    λ=dq/dr
    V=k∫dq/r


    V=λk*ln([r+x]/r)
    V=(8.9e-6)*9e9*ln(.104/.064)
    V=3.89e4


    The actual answer is 7.78e4 (twice my answer). I can't figure out whats wrong with my integral to make the answer half of what it should be. Thanks!
     
  2. jcsd
  3. Feb 29, 2012 #2

    ehild

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    Homework Helper

    λ is the linear charge density, λ=dq/dL.
    Check also the formulae for V. What is the electric field around a very long cylindrical shell according to Gauss' Law?

    ehild
     
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