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Relationship: Force on a charge at some distance from capacitor Plates

  1. Mar 2, 2013 #1
    1. The problem statement, all variables and given/known data

    Two capacitor plates (infinite plane) with uniform charge distribution but different charges (one positive, one negative) are distance "d" apart. A charge is place at d/2 and feels force |F1|. A charge is placed at d/4 and feels force |F2|. How does |F1| compare to |F2|?

    2. Relevant equations

    F=qE (Force on a test charge is proportional to the electric field at the point charge)
    E = η/ε0 (η = Q/A, charge distribution); Electrical field between two parallel plate capacitors

    3. The attempt at a solution

    The electrical field between two parallel plate capacitors is constant and uniform in direction at any point. Since F=qE, E is constant and 'q' is negligible, F≈E everywhere. Thus, |F1|=|F2|.
    Last edited: Mar 2, 2013
  2. jcsd
  3. Mar 3, 2013 #2


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    That looks good except for the statement F≈E. q is assumed negligible in the sense that it is assumed q does not disturb the uniform charge distributions on the plates. But that doesn't mean that you can ignore q in the equation F = qE. Do you see that your answer is still valid even for F = qE?
    Last edited: Mar 3, 2013
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