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Homework Help: When does work equal negative electric potential energy

  1. Dec 9, 2014 #1
    1. The problem statement, all variables and given/known data
    Let EPE be change in electric potential energy

    so if a positive charge is moving towards the negative terminal with the electric field it would mean that it is losing PE therefore work = - EPE, and if a negative charge is moving against the electric field towards the positive terminal it would gain potential energy? therefore W = + Delta EPE

    can someone confirm if this concept is right and i should use it for life.

    2. Relevant equations
    W = EPE = q*DELTA V

    3. The attempt at a solution
  2. jcsd
  3. Dec 9, 2014 #2
  4. Dec 9, 2014 #3


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    Homework Helper

    Yes, you are right.

    Let we have capacitor plates A and B d distance apart. Let be the potential UA at A (on the left) and UB at B and UA>UB. The electric field is E=(UA-UB)/d, and points to the right, away from the positive plate. The force acting on charge q is F= qE, and the work done by the field when q moves from A to B is W=q(UA-UB). We define change of potential by Ufinal -Uinitial, and the change of potential energy is ΔEPE=q(Ufinal -Uinitial). Uinitial=UA, Ufinal=UB , so ΔEPE=q(UB-UA) which is opposite to the work done by the electric field. W = -ΔEPE.

    Last edited: Dec 9, 2014
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