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Homework Help: Electrostatic force lab debate

  1. Jan 29, 2012 #1
    (plz help-still not solved) electrostatic force lab debate

    For lab, we had to do the following experiment, and me and my friend are not agreeing as to whether we have correct data:

    We had to charge a ball, then while holding that ball, bring it to a similarly charged ball that was hanging and free to swing. The ball that was held was always placed at the same position. For five trials, we measured how far the swinging ball swung away from the fixed ball.

    We found the electrostatic force on the hanging ball by equating it with the force of tension from the string and the force of gravity, resulting in F=(mgd)/L, where m is the mass of the ball, d is the displacement from equilibrium, and L is the length of the string. Therefore, for greater d, the electrostatic force was greater.

    For each trial we also measured the distance r between the two balls. Since the fixed ball was always placed at the same position, then r was greater only when d was greater, and, since F was greater when d was greater, then F must also be greater when r is greater.

    My friend is saying that this contradicts the fact that F=kq(1)q(2)/r^2 ...ie, force should be greater with lower r.

    I am saying that in this case, since the swinging ball was free to move, it was free to react to the fixed ball in accordance with the amount of force it felt. ie, if it felt greater force, it moved further from the fixed ball (resulting in greater r and d)

    Who is right?
    Last edited: Jan 29, 2012
  2. jcsd
  3. Jan 29, 2012 #2


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    I'm guessing that in each trial, the charge would be different. So you can't treat the charge as the same in each trial.
  4. Jan 29, 2012 #3
    What do you mean?
    We weren't treating the charge the same in each trial.
    In each trial, we measured the distance between the balls and the displacement of the hanging ball. So if the charge in one trial was greater, there would be a greater r, d, and force for that trial.
  5. Jan 29, 2012 #4


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    I mean that generally, the force doesn't rely on the distance only.
  6. Jan 29, 2012 #5
    Ohhhh so in this case, if you plot Force vs 1/(r^2) using our data points from the trials, you shouldn't get a straight line, because then you are assuming that the slope, kq1q2, is constant throughout all of the trials, but in reality q1 and q2 vary....right?

    Becuase see what actually happened is that we plotted F vs 1/r^2 and got a negative slope...ie, the force appeared to have increased as r increased
  7. Jan 29, 2012 #6
    but wait no...either way, q was really small as compared to force and r....it was a couple of nanocoulombs...so I think it could be considered constant for practical purposes? So that can't explain our result that Force increased with increasing distance..
  8. Jan 29, 2012 #7
    but also either way, the charge does not depend on the distance, so when you plot F vs 1/r^2, you should get straight line with positive slope, regardless of the charge...
  9. Jan 30, 2012 #8


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    I'm not totally following your reasoning. Maybe you should write down the equations for the coulomb force and for the horizontal tension explicitly, then it might be easier to see what the slope should be (keeping in mind that the charge is different in different tests).
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