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Homework Help: Electrostatic Force Between Conductor and Insulator

  1. Feb 1, 2006 #1

    I have the following question: An uncharged wooden stick is balance on a pivot so that it can rotate freely. If a charged rod is brought close to one end of the stick, the stick will be....

    Attracted by the charged rod is the answer.

    This has got me confused for a while. Here is my logic, which is evidentley wrong. The wooden stick is an insulator, meaning the electrons are not free to move. If a charged rod, be it postive or negative, is brought near the neutral uncharged wooden stick, the result is a neutral charge compared to a negative charge. So with coulombs law, q1 is 0 and q2 is -x lets say. Therefore the product is 0 and the electrostatic force is 0.

    Where am I going wrong???

    Last edited: Feb 1, 2006
  2. jcsd
  3. Feb 1, 2006 #2
    It is true that the electrons are bound tightly but not that tightly that they can not move. Why don't you perform a similar experiment? Take a comb and a few bits of paper. Rub the comb and the bits will be attracted towards it although paper is also an insulator.
    The charge generated thus on the wooden rod is thus negative and the positive charge is slightly repelled. So attraction is there.
  4. Feb 1, 2006 #3

    Doc Al

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    Staff: Mentor

    As hellraiser stated, the electrons are not totally fixed in the insulator. What happens is that the electrons are attracted to the positively charged rod and get shifted slightly creating a polarization charge on the insulator. Since the electrons are a bit closer to the charged rod than the positive charges (which are not so free to move), there is a net attractive force.

    The net charge on the insulator remains zero, however.
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