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Question about work on an electric charge

  1. Jan 19, 2017 #1
    1. The problem statement, all variables and given/known data
    If the electric field does positive work on a negative charge as the charge undergoes a displacement from point A to point B within an electric field, then the electric potential energy is

    a) negative
    b) positive
    c) increases
    d) decreases
    e) electric fields can not do work.

    The "Right" answer is D, but I do not understand why that is right. I picked C to be the answer.
    2. Relevant equations
    Conceptual question. No equations needed.

    3. The attempt at a solution

    Say I have two points

    A B

    I move that negative charge from A to B. If there is positive work done on it during its displacement the electric force will be directed from point A to B. Since the force is done on a negative charge, the electric field is going from point B to point A. Point B is at a higher potential than A. Thus, if the negative charge moves closer to point B which is at a higher potential, then its electric potential energy increases. Right?

    Thanks in advance for the help in helping me figure out what is the right answer. C or D.
     
  2. jcsd
  3. Jan 19, 2017 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    the negative charge moves closer to point B which is at a higher potential, then its electric potential energy increases. Wrong.
    How is electric potential defined? It is the potential energy of an unit positive charge. The potential energy is charge times potential.
     
  4. Jan 20, 2017 #3
    U = q*deltaV

    Yes, because the charge q is negative and the change in potential is positive, so the change in electric potential energy is negative.
    I see how this works with the formula.
    Thanks for the help. I get it now!
     
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