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Thread: Coulombs Law question

  1. #1 Coulombs Law question 
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    A negative charge Q is located between two positive charges, +4Q and
    +16Q. The distance between +4Q and Q is d1 and the distance between
    +16Q and Q is d2.
    All charges lie along the same line and d1+d2 = 9 cm.

    Calculate d1 and d2 which will provide a stable equilibrium position for
    the charge Q, i.e., the location where the total force acting on Q due
    to +4Q and +16Q is zero.


    So far I have F1 + F2 = Ftotal = 0 since it specifies a stable equilibrium position. However, I can't seem to get the next step.

    Any help would be much appreciated.
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  2. #2  
    KJW
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    Quote Originally Posted by WilsonWilson View Post
    However, I can't seem to get the next step.

    Any help would be much appreciated.
    The next step would be to express the forces in terms of the charges and distances.
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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  3. #3  
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    Quote Originally Posted by KJW View Post
    The next step would be to express the forces in terms of the charges and distances.
    Hey,

    I have them both in the forms Q1Q2/4.pi.epsilon.r^2 and then tried to work r out from them both. The problem is doing this step.
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  4. #4  
    KJW
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    Quote Originally Posted by WilsonWilson View Post
    Hey,

    I have them both in the forms Q1Q2/4.pi.epsilon.r^2 and then tried to work r out from them both. The problem is doing this step.
    Why is this a problem? (Show us where the problem is)
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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  5. #5  
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    Quote Originally Posted by KJW View Post
    Why is this a problem? (Show us where the problem is)
    http://i.imgur.com/akL45re.jpg

    I have made various attempts at working it out from here. All with ridiculous answers. I have either done it wrong so far or am missing something very obvious right now. Any ideas?

    Thank's.

    Also ignore the random numbers on the right.
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  6. #6  
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    Your third lines with the Qs in is nearly right but there should be a minus sign in the middle. Also the r's should be different variables. R1 + R2 = 9.
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  7. #7  
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    Hey, yeah so my problem with it after that is just getting an R on its own. If I get one R on its own it will still be in terms of the other R if you get me?
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  8. #8  
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    You have two equations and two unknowns so you can solve for both of them by substitution. R1= 9 - R2.
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