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Homework Help: Electrical forces and energy: Electric Potential help

  1. Mar 15, 2013 #1
    1. The problem statement, all variables and given/known data
    Hi, I have a couple different problems that I did not understand.
    I have the images here.
    The problems which I did not understand were: 12, 13, 17. [EDIT: I calculated answer to 12 using conservation of energy.]
    For 13, I know how the graph looks; however, I'm not sure how to calculate the two zeros as well as the local minimum on the left.

    tDUPtKT.jpg

    ImpBgrD.png

    2. Relevant equations

    WKXhHDD.png


    3. The attempt at a solution

    12. No idea whatsoever. I tried using the electrostatic forces equation, however there is no specific "r" value between where the Q2 and the other Q's are, so you can't do that. I think it might have something to do with momentum, but I'm not sure how to apply that.

    EDIT: Uo + Ko = Uf + Kf. Using this, I got the right answer Vo = 6.19 m/s.

    13. I already have the graph which looks like this:

    XUOgaCj.png

    The black spots are the values I need to find. I tried using the field graph to find the values, but it failed. I also tried using kq/r = kq/r to find the zeros, but I didn't get the right answers again.

    17. This one I know how to do, I'm just confused on what to plug in.

    The ΔV = -EΔx, so then you do Delta V (125 > 100) + Delta V (100 > 50) + Delta V (50>25) and that should be the answer. However I'm not sure what to plug in for the "q" in each case.

    CORRECT ANSWERS
    12. 6.19 m/s
    13. The values of the two zeros are -5.46 m and 1.0 m. The y-value of the local min is -2.00 m.
    17. 3.42 * 10^4 V

    I'd appreciate it for any help. Thank you!
     
    Last edited: Mar 15, 2013
  2. jcsd
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