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Homework Help: Coulumbs Force question

  1. Feb 14, 2009 #1
    1. The problem statement, all variables and given/known data
    A charged particle A exerts a force of 2.62microNewtons to the right on a charged particle B when the particles are 13.7 mm apart. Particle B moves straight away from A to make the distance between them 17.7mm. What vector force does it then exert on A?


    2. Relevant equations
    I know its easy, very, but for the love of god I can't figure this out.


    3. The attempt at a solution
    I know that they are both positive particles, so im assuming both are +1.60x10^-19c.
    Also I believe we use F= (K q1 q2/r^2). Ive tried inserting different things in to the formula but none of it gives me the answer im looking for which is 1.57micronewtons
     
  2. jcsd
  3. Feb 14, 2009 #2

    Delphi51

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    Yes, use use F= (K q1 q2/r^2).
    No need to assume a charge. Put in the force and distance for the first position, and calculate the value of K q1 q2. This will carry over to the new distance.
    Knowing that and the new distance, you can calculate the new force.
     
  4. Feb 14, 2009 #3

    Redbelly98

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    I wouldn't make that assumption ... I see nothing in the problem statement that would allow this.
    Yes, that is the equation for this problem.
    Here's a question: what must the product (q1 q2) be equal to, so that the force is 2.62 μN when r=13.7mm?

    EDIT: Delphi gave even better advice. What is (K q1 q2) when the force is 2.62 μN and r=13.7mm?
     
  5. Feb 14, 2009 #4
    alright, well K=8.99 x 10^9

    so the formula should look like this now. r x [square root of F/k] = q1 q2
    inputting everything it looks like this
    (0.0137m) times square root of (2.62 x 10^-7 N )/(8.99 x 10^9) = 7.39 x 10^-11
    equaling that to q1 q2. this is one part im stuck on, I'll be able to get the rest but I don't know what to do with both unknowns.
     
  6. Feb 14, 2009 #5

    Delphi51

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    If q1 q2 is bothering you, make this change in the formula: C = q1 q2.
    Then you will have only the one unknown!
    The important thing is that C is a constant in this problem.
    Even better: let C = kq1 q2. Saves wear on the calculator!
     
  7. Feb 14, 2009 #6

    Redbelly98

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    Uh, wait, that should be
    r^2 x F/k = q1 q2​
     
  8. Feb 14, 2009 #7
    there we go, again thanks everyone. that was what was screwing up my answer, I saw another equation like the one I put up for a similar question so I thought I could apply it here. I see now that I can't. again thanks.
     
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