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Charge concept problem

  1. Jan 13, 2012 #1
    1. The problem statement, all variables and given/known data Hi so i am having a problem understanding charge. The problem is
    Two identical metal spheres on insulating stands are placed some distance apart. On one sphere is some amount of excess negative charge. On the other sphere is twice as much excess positive charge. Hanging midway between the two spheres on an insulating thread is a metal ball (a pendulum in effect). The ball of the pendulum is pushed gently to the side (with an insulating rod) until it contacts the positively charged sphere and then it is released.
    This is not a calculation problem but more of a conceptual problem.

    2. Relevant equations
    no equations


    3. The attempt at a solution Im thinking that the metal ball would first start out postively charged as its already touching the positve sphere and then as it swings over the postive charge would attract the negative charge and then lose the positve charge as it swings back over and finally lose its negative charge once it swings back over again, and the process would continue until someone stopped the medal ball. But im not sure. There could also be other questions related to this but not so sure what those could be as well.
    please help
     

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  3. Jan 13, 2012 #2

    SammyS

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    So, what is your question?
     
  4. Jan 13, 2012 #3

    ehild

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    You think it well, the small ball will swing between the spheres and carry charge from one to the other. What will stop the process? What is the final charge on the spheres?

    ehild
     
  5. Jan 13, 2012 #4
    my question is am i right/on the right path or ya. The question doesn't state the process will stop or what the final charge would be. I just wanted to know if i had the concept down.
     
  6. Jan 13, 2012 #5

    ehild

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    The concept of the small ball swinging and carrying charge is correct, but it is not sure that the small ball will swing forever.

    ehild
     
  7. Jan 13, 2012 #6
    okay thanks, so would it go in cycles as well. for ex. start out positve, then go to positve and negative then just negative and to eventually no charge where it starts over again. This was my main concern
     
  8. Jan 13, 2012 #7

    ehild

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    The small ball will always carry some charge. When it touches the negative sphere it becomes negatively charged and it becomes positive when touching the positive sphere. The electrons will added to the ball or removed from it till there is potential difference between sphere and ball.

    ehild
     
  9. Jan 13, 2012 #8
    So now i have a couple more quesitons. after the small ball makes contact with the negative sphere the first time what charge will it have. Will it be slightly more positive, negative? and besides gravity what forces is will the ball feel from the larger spheres. will it be repelled by the positive and attracted to the negative?
     
  10. Jan 13, 2012 #9

    ehild

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    It will have negative charge and repelled by the negative sphere and attracted by the positive one. The ball feels the Coulomb force from the spheres in addition to gravity.

    ehild
     
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