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Homework Help: Another Electric Charges Q about Leaves

  1. Nov 9, 2008 #1
    1. The problem statement, all variables and given/known data
    set up:(they three are all connected)

    l (referred to as top)
    /\ (the bottom two called "leaves")

    - two leaves are separated when negatively charged rod touches the top of the (vertical thing that connects the two leaves at the bottom)

    - when rod is taken away, and a finger is put on the top, the leaves collapse and touch each other.

    - when hand is taken away, the leaves separate a little (not as much as the first step)

    Q: what are the charges on the two leaves?

    2. Relevant equations

    3. The attempt at a solution
    I guessed that that both leaves are negatively charged.. because they repel at the last step. the finger charges by contact? providing positive charges.
  2. jcsd
  3. Nov 9, 2008 #2

    Chi Meson

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    Science Advisor
    Homework Helper

    There is something wrong with this question. Are you sure you are correct with the transcription? Usually the order is this:

    rod approaches top but does not touch, leaves separate
    finger touches, leaves drop
    finger is removed, leaves still together
    rod is taken away, leaves separate.

    It's possible that the person whose finger touched the electroscope had a net charge on him, but it is usually considered that the touching finger is "grounded." If the question is stated correctly, then the finger is not grounded (the leaves would remain at net zero charge).

    But if the finger had a net charge, positive or negative, and it was touching the electroscope, then the leaves would not have dropped fully as long as the finger was in contact with the scope.

    This scenario actually does happen often in the real world, but this is due to the influence of other net charges nearby (thank you Van der Graaff).
  4. Nov 9, 2008 #3
    Sorry, you were right with the rod not touching. And the rod is still there when the finger touches the electroscope. But the question does not say that the leaves were still together when the finger was put on.. (because i was describing the three pictures that the question gave me to describee the process)
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