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Homework Help: The force between charged and neutral object

  1. Sep 11, 2016 #1
    1. The problem statement, all variables and given/known data
    I did a lab involving Van de Graaff generator. Basically, below the charged sphere, I put a neutral sphere on a scale and measure the difference in mass which then I can calculate the force acting on the neutral sphere. I repeated the experiment with different distance. What is the proportionality between the force and the distance? One of the tutor said the force is proportional to 1/r^5 but he didn't explain much about it.

    2. Relevant equations

    3. The attempt at a solution
    I know that from coulomb law that the force is proportional to 1/r^2. However I can only use this equation to calculate the force between two charged objects, while my experiment is about the force between a charged and a neutral object.
  2. jcsd
  3. Sep 11, 2016 #2


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    Suppose F = k/rn. You can certainly write F1=k/r1n and F2=k/r2n. Can you put these two equations together with the forces and distances that you measured and find k and n? An accurate determination might be difficult with just two measurements, but if that's all you got, it will have to do.
    Last edited: Sep 11, 2016
  4. Sep 11, 2016 #3


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    Don't know if this works, but you could try considering instead two small conductors, lined up with the charged sphere, connected by a short thin wire. These would form a dipole from the induced charges. You might obtain an r-5 relationship in that model.

    Edit: it does indeed produce that relationship.
    Last edited: Sep 12, 2016
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