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Homework Help: Physics Homework Question on Electrostatic forces

  1. Jan 10, 2016 #1
    1. The problem statement, all variables and given/known data
    Charge is distributed on the surface of a spherical balloon (an insulator). A point particle with charge q is inside. If polarization effects are negligible the electrical force on the particle q is greatest when:

    a. it is near the inside surface of the balloon

    b. it is at the center of the balloon

    c. it is halfway between the balloon center and the inside surface

    d. it is anywhere inside (the force is same everywhere and is not zero)

    e. it is anywhere inside (the force is zero everywhere)

    2. Relevant equations

    F = kqq/r^2
    3. The attempt at a solution
    I thought it would be "a" because their would be more differences in forces the closer the charge is to the surface.
    The correct answer is "e". I do not understand why :(.
  2. jcsd
  3. Jan 11, 2016 #2

    Simon Bridge

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    Difference in what forces?
  4. Jan 11, 2016 #3


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    Due to which thing will the charge q experince a force? What is necessary for a charge to experience force?
  5. Jan 11, 2016 #4
    If you apply Gauss' Law, what electric field is produced inside of the balloon
    due to a uniformly distributed charge on the surface of the balloon?
  6. Jan 11, 2016 #5

    Simon Bridge

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    I suspect OPs reasoning is due to charge being quantized but need feedback to be sure.
  7. Jan 11, 2016 #6
    Ya my reasoning was because I thought charge is quantized.
  8. Jan 14, 2016 #7

    Simon Bridge

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    The question assumes the situation where charge is well approximated as a continuous fluid... i.e. the net charge is very much larger than the charge quanta, and the scale is such that the description of the surface as "a sphere" still makes sense (as opposed to, say, an array of nuclei in a cloud of electrons). In that approximation, the field inside the classical surface is everywhere zero.

    This is usually the case unless you are told otherwise, and holds extremely well for almost every real-life static electricity problem.
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