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Charging a Conducting Rod

  1. Sep 8, 2009 #1
    1. The problem statement, all variables and given/known data

    Here is a pretty straight forward question, i just wanted to double check my answers (in bold!)

    This problem explores the behavior of charge on conductors. We take as an example a long conducting rod suspended by insulating strings. Assume that the rod is initially electrically neutral. For convenience we will refer to the left end of the rod as end A, and the right end of the rod as end B. In the answer options for this problem, "strongly attracted/repelled" means "attracted/repelled with a force of magnitude similar to that which would exist between two charged balls.

    1) A small metal ball is given a negative charge, then brought near (i.e., within about 1/10 the length of the rod) to end A of the rod . What happens to end A of the rod when the ball approaches it closely this first time? It is strongly attracted

    It is strongly repelled.
    It is strongly attracted.
    It is weakly attracted.
    It is weakly repelled.
    It is neither attracted nor repelled.

    2) After a great many contacts with the charged ball, how is the charge on the rod arranged (when the charged ball is far away)? There is positive charge on end A with end B remaining neutral.

    There is positive charge on end B and negative charge on end A.
    There is negative charge spread evenly on both ends.
    There is negative charge on end A with end B remaining neutral.
    There is positive charge on end A with end B remaining neutral.

    3)How does end A of the rod react when the charged ball approaches it after a great many previous contacts with end A? Assume that the phrase "a great many" means that the total charge on the rod dominates any charge movement induced by the near presence of the charged ball. I am not positive.. are they weakly attracted?

    t is strongly repelled.
    It is strongly attracted.
    It is weakly attracted.
    It is weakly repelled.
    It is neither attracted nor repelled.

    4)How does end B of the rod react when the charged ball approaches it after a great many previous contacts with end A?neither
    It is strongly repelled.
    It is strongly attracted.
    It is weakly attracted.
    It is weakly repelled.
    It is neither attracted nor repelled.

    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 8, 2009 #2

    rl.bhat

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    2) In the conducting rod charges distribute through the conductor. In each contact some negative charge is transferred to the rod. So the choice is.......

    3) Now the rod is negatively charged. Some traces of negative charge may left out in the ball. Then the choice is........
     
  4. Sep 8, 2009 #3
    2) It is weakly repelled (since side B is negative)
    3) It is strongly repelled
     
  5. Sep 8, 2009 #4

    rl.bhat

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    In 2) go through the choices.
    In 3) The left out charge in the ball is very small. So the repulsion is..?
     
  6. Sep 8, 2009 #5
    OOPS! I am sorry I read the wrong question..

    2) there is negative charge on end A with end B remaining neutral.
    3) weak repellsion
     
  7. Sep 8, 2009 #6

    rl.bhat

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    2) is not correct. In the conductor charges cannot remain at one end.
     
  8. Sep 9, 2009 #7

    ideasrule

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    If they did, the electrons at the charged end would repel each other and distribute themselves evenly in a tiny fraction of a second.
     
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