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Homework Help: Simple electrostatic field question

  1. Dec 2, 2013 #1
    Hey everyone, so I think my question will be relatively simple compared to what I've viewed elsewhere here. Basically, I am an adult taking grade 12 physics as self study, and for the most part I can work things out ok. I am planning on taking the diploma exam for my province in January.

    Anyways, I am on the unit of electrostatics, and one question kind of has me confused. I am wondering if there is a problem with my textbook. I'll try to write it out.

    (-) ------- A)----------B)

    "Compare the electric potential energy of a positive test charge at points A and B near a charged sphere, as shown below"

    Now the answer states that the Potential is higher at Point A, then at Point B, and goes on to reason that because we choose infinity as our "zero" point, that A) will be higher.

    The way I understand it, the more "work" it takes to get a positive charge to either point, the higher the potential energy, so that would be point B isn't it? Or, if all other points other then infinity are considered negative potential, then doesn't the point closer to infinity have higher potential (although it's still negative). Finally, the last reason I believe it is B, is because of the conservation of energy. If I were to release a positive charge at Point B and it were to accelerate towards negative charge, it would have more velocity at the moment it were to contact the negative source charge. This would be a higher kinetic energy, which must mean a higher potential energy at point B. Am I going wrong somewhere!?

  2. jcsd
  3. Dec 2, 2013 #2


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    hey, welcome to physicsforums :)
    Are you sure the charged sphere is negatively charged? Why not positively charged?
  4. Dec 2, 2013 #3
    Yeah, It's definitely a negative charge. It's blue and it's got a (-) symbol inside.

    It's frustrating because It's as if my understanding is a house of cards just ready to topple over.

    I can understand how the field would be stronger in between point A and the source, vs point B and the source... right??!
  5. Dec 2, 2013 #4


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    ok. interesting. I definitely think your answer is correct for a negatively charged sphere. So yeah, there might be a mistake in the book.
  6. Dec 2, 2013 #5
    Awesome, well, thanks. I know it's hard for them to be 100% correct but man is it frustrating when the book is wrong and you question your basic understanding for hours.
  7. Dec 2, 2013 #6


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    yeah, it is really annoying when that happens. I feel like it can't be that hard to make a book with no errors in it. But it happens so often, that I guess it is more difficult than it seems. Maybe it's because when they make changes at the last minute, this introduces more errors... I dunno. Anyway, all the best. Hope there aren't too many more errors.
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