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Electric Potential and Electric Potential E changes

  1. Apr 18, 2015 #1
    I just want to make sure my understanding in this topic is correct. Positive charges will spontaneously move in the direction that lowers their electric potential (V), where as negative charges will spontaneously move in the direction that increases their electric potential. This in turn makes positive charges going with a uniform E field lines, and negative charges going in the direction opposing the direction of the E field lines. However, both charges will move in the direction that decreases their potential energy, and thus stabilize the system more.

    Here is a question then that I am in conflict with:

    "A charged particle goes from point i to point f in the presence of and electric field E. The change in the potential energy as the particle moves from i to f depends on the charge" True or False

    I stated false because the change in potential energy as the particle moves from i to f will decrease independent of the type of charge. If this were talking about the electric potential then I would state true. The answer key however disagreed with my choice and stated the answer as True. I'm thinking it must be a typo, but require confirmation.
  2. jcsd
  3. Apr 18, 2015 #2
    It's asking about electrical potential energy. I'll use this analogy: does the change in gravitational potential energy of an object depend on its mass?
  4. Apr 18, 2015 #3
    PEg = Gm1m2/r2 so yes it does. But unlike charge mass cannot be positive or negative and will only vary by magnitude. Ultimately for both positive and negative test charges there will be a negative change in electric potential energy correct, in other words the change in potential energy doesn't depend on the type of charge because it will have to decrease in order for which ever type of charge to move spontaneously in its respective direction when placed in a E field. Does this logic make sense? Thanks.
  5. Apr 18, 2015 #4
    It didn't specify "positive or negative" charge. It just says "depends on charge", which means "will it be different with a small charge than it is with a big charge?"

    On top of that, it doesn't specify the path, but consider an electric field caused by a positive charge Q. Now imagine moving a positive charge towards Q. This requires that we put energy into the system. Does this charge gain of lose potential energy? Now imagine taking a negative charge and taking the same path. The force caused by the field is no longer pushing against this charge's movement. Does this charge gain or lose potential energy?

    Perhaps the issue is with the wording of the question.

    You're not supposed to consider which path the charge would naturally take if you left it in the field. The question is "if the charge takes this specific path, will the change in potential energy be the same regardless of charge?"
  6. Apr 18, 2015 #5
    I see what your saying. I think I misunderstood the question then. I am only thinking of the cases where charge movements would be spontaneous so there will always be a lose of potential energy (turned into KE to accelerate the particle). The cases that you introduced will require an external work to be done where delta V = Work external over q (non spontaneous). If that is the case, the there will be a gain of potential energy as work is done by an external source that is "forcing" the charge to go against its spontaneous pathway.
  7. Apr 18, 2015 #6
    Correct. i and f represent the end points of a specific path, which does not depend on the charge. The question is asking you to recognize that different things would happen to different charges in this single arbitrary path.
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