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Coulombs Law Problem - Find the Force

  1. Jul 6, 2009 #1
    Coulombs Law Problem -- Find the Force

    1. The problem statement, all variables and given/known data
    Two small objects, A and B, are fixed in place and separated by 3 cm in vacuum. Object A has a charge of 2 uC and object B has a change of -2 uC. How many electrons must be removed from A and put onto B to make the electrostatic force that acts on each object an attractive force whose magnitude is 68 N?

    We know an electrons charge is 1.60*10^-19 C

    2. Relevant equations
    F=kq1q2/r^2 (coulombs law)



    3. The attempt at a solution

    I tried to solve for q (the charge). Since they are both the same I took the square root. Then I divided by the charge of e-. I am out of ideas :(

    I can do simple coulombs law problems in this chapter, but the harder ones like this stump me. :((


    Thanks,
    Katie
     
  2. jcsd
  3. Jul 6, 2009 #2
    Re: Coulombs Law Problem -- Find the Force

    So after doing this, what is the quantity that you have arrived at? What is q/e?
     
  4. Jul 6, 2009 #3
    Re: Coulombs Law Problem -- Find the Force

    F = K*Q^2/r^2
    68 = (8.99 * 10^9 N * m^2/C^2)(q^2)/(3*10^-2)^2
    q=2.6*10^-6 C
    e=1.6*10^-19
    (2.6*10^-6C)/(1.6*10^-19)=1.63*10^13

    the answer in the back is 3.8*10^12
     
  5. Jul 6, 2009 #4
    Re: Coulombs Law Problem -- Find the Force

    I'm not asking for the number.

    I'm saying: if q is the charge, and e is the charge per electron, what does the quantity q/e mean? How does that relate back to what the question wants?
     
  6. Jul 6, 2009 #5
    Re: Coulombs Law Problem -- Find the Force

    q/e is going to be the number of e. But I guess that would be the number of e- for that charge. So they ask for the e- to change the charge. Would I then find the difference by subtracting from 2 and then dividing by e-?
     
  7. Jul 6, 2009 #6
    Re: Coulombs Law Problem -- Find the Force

    The question wants x electrons per object.
    You just calculated the no. of electrons you need
    From the question you can find out how many electrons you had at the start.

    Hopefully its obvious enough now?
     
  8. Jul 6, 2009 #7
    Re: Coulombs Law Problem -- Find the Force

    okay got ya, I just need to take it into consideration with the initial formula
    instead of solving for just q I am solving for q+ex
     
  9. Jul 6, 2009 #8
    Re: Coulombs Law Problem -- Find the Force

    Ok, I can't seem to get the right answer and I don't know where I'm going wrong.

    I set the equation up as

    F = k(q1 + ex)(q2 - ex) / r^2

    where q1 is the positive charge and q2 is the negative charge.

    So this works out to

    68 = (8.99*10^9)(2*10^-6 + x*1.6*10^-19)(-2*10^-6 - x*1.6*10^-19) / (3*10^-2)^2

    but this doesn't lead to the right answer. Where is my set up going wrong?
     
  10. Jul 6, 2009 #9
    Re: Coulombs Law Problem -- Find the Force

    It does, I've checked. Calculator error somewhere?
     
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