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Homework Help: Work done by electrostatic force

  1. May 3, 2013 #1
    1. The problem statement, all variables and given/known data
    A small, positively charged sphere is released from rest and moves directly away from a larger, positively charged sphere. During this process, the electrostatic force:

    a) does positive work and increases the kinetic energy of the small sphere
    b) does negative work and increases the kinetic energy of the small sphere

    2. Relevant equations

    3. The attempt at a solution

    ( + ) ----> (+)

    So the electrostatic force is pushing the smaller charged force away. It looses potential energy and converts it into kinetic energy. Since W = (delta) (Kinetic energy) and kinetic energy increases, Ek > 0

    Movement is also in the direction of the electric field (cos0) = 1 therefore it should be positive (as far as I know)

    Work should be positive, the answer should be A)

    However, the answer is B). Why?

    It doesn't mathematically make sense to me. I've proven that W = change in kinetic energy.

    However, when I conceptually think about it it makes sense that work is negative- because it's the same thing as a mass falling from a height- negative work is done on it.

    Is this physics? Will it be full of exceptions and mathematical fallacies?

    It seems like physics is turning more and more into a computer script

    if case 1: do this algorithm
    if case 2: do that algorithm

    Is that it? Will I just have to just memorize these exceptions as I go along in physics, or is there another way of looking at it?
    Last edited: May 3, 2013
  2. jcsd
  3. May 3, 2013 #2


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    Science Advisor
    Homework Helper

    No, I think you have the right way of looking at it. The force does positive work on the kinetic energy of the system. I think it's more likely to be a typo in the answer than any need to reevaluate your basic concepts.
  4. May 3, 2013 #3
    Thank you for clearing my confusion
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