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Homework Help: Charged pen and aluminium foil

  1. May 17, 2008 #1
    1. The problem statement, all variables and given/known data
    A clear plastic pen of length 14.5 cm is rubbed with wool. The charged plastic pen is held over a small uncharged disk shaped piece of aluminium foil of diameter 0.5cm. How close you must move the pen to the foil in order to pick up the foil?

    3. The attempt at a solution

    Ok I thought I would let the disk be a diople and let the pen be a point charge but nothing worked. Please explain to me how I should do this question.

    Thank you very much
  2. jcsd
  3. May 17, 2008 #2


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    Homework Helper

    You migth want to study the forum rules. You MUST attempt the problem, and tells us what relations and formulas you know.
  4. May 17, 2008 #3
    This is what I did?

    Ok this is what I did but don't know whether it is right or not:
    E = 1/(4 pi epslon) * Q/d^2 is the electric field of the pen

    I said that the aluminium foil works like a capacitor therefore:
    (qA)/epsilon - 1/(4 pi epslon) * Q/d^2 = 0 also Q=10 nanoC the charge of the pen

    then I can figure out the charge of the foil on one side I think(I hope this is right)

    Electricfield of disk = ((Q/A)*(s/R))/2epsilon where s=thickness of the disk
    also m=dV is the formula for density, but is V= pi r^2 s
    then if the above is right we know F = mg
    F = ((Q/A)*(s/R))/2epsilon m = D pi r^2 s
    ((Q/A)*(s/R))/2epsilon = D pi r^2 s g

    But is this even right, I really need you guys to help me please HELP me !!!
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