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Homework Help: Potential energy question

  1. Mar 12, 2006 #1

    wml

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    An infinite sheet of charge that has a surface charge density of 21 nC/m2 lies in the yz plane, passes through the origin and is at a potential of 1.3 kV.
    A long wire having a linear charge density of 50 nC/m lies parallel to the y axis and intersects the x axis at 3.2m.
    The permittivity of a vacuum is 8.85419*10^-12 C2/N.m2

    What is the potential energy of a 2.4 nC charge placed at x = 1.5 m in units of J.


    can anybody help me..
    I dont know how to use the 1.3 kV potential
    I used V=(入/ 2Eoл)× Ln(r1/r2) , U=V×q
    but I entered the answer, it was wrong
    :cry:
     
    Last edited: Mar 12, 2006
  2. jcsd
  3. Mar 12, 2006 #2

    Meir Achuz

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    The problem is not stated correctly.
    If the infinite sheet has a uniform charge density, and there are other charges present, it can’t be an equipotential.
     
  4. Mar 12, 2006 #3

    wml

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    never mind~ I got the right answer~!!!! ^^: finally

    Vb-Va=-21n×1.5/2Eo -> Vb=1300-1779.661= -479.661

    VB-VA(0)=50n/ (2лEo) × Ln(3.2/1.7) = 568.7519 3.2-1.5=1.7

    U=q(Vb+VB) = 2.4n × (568.7519-479.661)=2.13818×10^-7
     
  5. Mar 13, 2006 #4

    Meir Achuz

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    But that answer is wrong because of my first post.
    If there is a charged wire and a point charge present, the plane cannot be at a constant potential of 1.3 kV.
    Is it stated that that is the potential at the origin or on the whole plane?
    Also, watch your sig. figs.
     
  6. Mar 13, 2006 #5

    wml

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    well, actually I'm still confused but I entered the answer and it was right >,< I didn't skip any words... what I put on the first post is exactly I have on my paper
     
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