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Homework Help: Work and Electrostatics - a bit of Clarification, please

  1. Jan 4, 2008 #1
    Work and Electrostatics - Help!!!!

    1. The problem statement, all variables and given/known data

    I need to place equal and oppositve charges on a piano and at the top of a 5.0 meter ramp in order to move a 1000 kg box up a 30 degree frictionless ramp. The box has a charge of +Q and the top has a -Q.

    A) What magnitude charges do I need to keep the box at the bottom and not moving?

    B) What is the initial energy of the box?

    *Why is it negative?

    *Or is it zero?

    C) How much work does it take to separate the charges from 5 m to infinity?

    D) How much work does it take to push the box by hand?
    Work = Fd
    How do I find the force that's necessary?

    2. Relevant equations

    3. The attempt at a solution

    A) Solved

    I solved B, but I just wanted to check that B and C, the answers should be equal? I got an answer of -24.5 kJ for B and is C 24.5 kJ (positive?)

    * Or is the answer to B actually zero? I'm told to "make sure" I define my zeros for potential energy. And I found an explanation on handysolver.com (i don't think it's a trustworthy site) that says that B is zero because the zeros were all there, but doesn't the +Q charge still exert a force?

    Thanks a lot in advance, I realize my work is a little scatterbrained...
    Last edited: Jan 5, 2008
  2. jcsd
  3. Jan 4, 2008 #2
    He set the Fg=Fe and solved for Q
  4. Jan 4, 2008 #3


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    L outside the radical is the same as L^2 inside the radical...
  5. Jan 5, 2008 #4
    And is B zero? I guess someone else had the exact same problem, because I found a solution:

    http://handysolver.com/problemsandso...problem_id=418 [Broken]

    that says that B is zero, but I really don't think so.
    Last edited by a moderator: May 3, 2017
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