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Simple Coulomb's Law Question Please Help

  1. Apr 20, 2013 #1
    Simple Coulomb's Law Question Please Help!!

    1. The problem statement, all variables and given/known data

    Two identical charged objects have charges of 6μC and -2μC. When placed a d distance apart, their forces of attraction is 2N. If the objects are touched together, then moved to a distance of separation of 2d, what will be the new force between them?


    2. Relevant equations

    F= k((q1)(q2))/r^2

    3. The attempt at a solution

    The answer is 0.17 (repulsive) in the back of the book. I think touching the objects together is supposed to have an affect, but not sure what it is.

    Any help is much appreciated folks! :)
     
  2. jcsd
  3. Apr 20, 2013 #2

    Dick

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    Touching the objects together means allowing charge to flow between them. So after touching you will wind up with two objects of the same charge. What will be that final charge?
     
  4. Apr 20, 2013 #3
    Would the final charge of each be 2C ?
     
  5. Apr 20, 2013 #4

    Dick

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    Yes. Well, 2μC anyway. But I know what you mean. The rest of the problem is just ratio and proportion, right?
     
  6. Apr 20, 2013 #5
    Pretty much yea. Thank you very much! :)
     
  7. Apr 20, 2013 #6
    hey sorry, but I'm still not getting the right answer for some reason, here's my solution (see picture)
     

    Attached Files:

  8. Apr 20, 2013 #7
    nevermind, I realized my method was wrong and got the answer. Thanks again!
     
  9. Apr 20, 2013 #8

    Dick

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    It's simpler if you just pay attention to what changes. That's the ratio and proportion trick. k and r^2 and μC and all the other units don't change. Divide the two forces by each other and they all cancel. All that changes is (-6)(2) becomes (2)(2) and you get a 1/4 factor from the change in r. What's the ratio of the two forces?
     
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