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Homework Help: Calculating change in energy of a particle

  1. Jun 19, 2013 #1
    1. The problem statement, all variables and given/known data

    An electron moves from an initial location between parallel plates where the electric potential is Vi = 30 V and Vf = 150 V

    a) Determine the change in the electrons potential energy

    2. Equations
    J/C = V = N/C * m

    3. The attempt at a solution
    a) The electron is naturally moving to a location of higher energy, therefore its potential energy decreases as its kinetic energy increases.

    Potential energy change = Ep2 - Ep1 = Vfq - Viq = q(Vf-Vi) = q(120 V) = -1.92 * 10^-17 J.

    The sign makes sense to me, since the electron has more potential energy at Vi than Vf.

    However, the book says the answer is +1.92*10^-17 J. Can someone help me out with why I am is wrong? Thanks
  2. jcsd
  3. Jun 19, 2013 #2
    Well your book looks like its wrong. Your qualitative reasoning seems correct to me.
  4. Jun 19, 2013 #3

    rude man

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    Homework Helper
    Gold Member

    Are you sure they didn't ask for the change in kinetic energy? Because otherwise I have to agree with your answer as being negative.
  5. Jun 19, 2013 #4
    Yes I'm sure they asked for potential energy. I guess the book is wrong. Thanks for confirming this.
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