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Coulomb's Law question

  1. Sep 25, 2009 #1

    Suy

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    1. The problem statement, all variables and given/known data
    I just have a quiz, but i don't get this question
    this is all i can remember
    A=-4.0*10^-6C
    B=+2.5*10^-6C
    mass of sphere: 51g
    initial separated distance:10.0cm
    If A and B touch and then repel, what is the new separated distance
    After they touch,
    Tension force on the string is 0.578N
    this is the picture after they touch
    http://img10.imageshack.us/img10/5527/26792246.jpg [Broken]

    2. Relevant equations

    Fe=(kq1q2)/r2
    F=mg?
    3. The attempt at a solution
    I don't get this question at all, but i know i need to find the force first...
    After they touch, the electron divide equally, so +2.5*10^-6 neutralize -2.5*10^-6, 1.5*10^-6 left, and divide by 2 is 0.75*10^-6
    am i right?
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 25, 2009 #2

    Delphi51

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    Homework Helper

    Yes, I think you have correctly averaged the charges.
    The next step is to make a careful diagram of the forces involved. On each charge, there is an electric force, gravitational force and the pull of the string. Sketch these vectors for one of the charges and then get expressions for their horizontal and vertical components.

    After that, you use the fact that the total force on the charge must be zero in both the vertical and horizontal directions because the charge is not accelerating. This should give you two equations to solve and find the angle.
     
  4. Sep 26, 2009 #3

    Suy

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    How do you sketch electric force, can you show me?
    http://img43.imageshack.us/img43/962/72025656.jpg [Broken]
     

    Attached Files:

    Last edited by a moderator: May 4, 2017
  5. Sep 26, 2009 #4

    Suy

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    0=tan(30)mg+sin(30)*0.578N+(-Fe/cos(30))
    solve for radius,
    the answer is 0.1m, 10 cm
    am i right?
     
  6. Sep 26, 2009 #5

    Delphi51

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    I don't follow your solution. The force diagram for the charge on the right should show the electric force going to the right. The gravity force down. The tension force along the string. The equation for the horizontal forces would have only two forces in it.
    The equation for the vertical forces would have only two forces.
    Your equation has three forces.
     
  7. Sep 26, 2009 #6

    Suy

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    Why only have two force acting on it,? but isn't there are 3 force(tension,gravity,electric force)
    can you show me the equation?
    ty!
     
  8. Sep 26, 2009 #7

    Delphi51

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    Homework Helper

    You must write that the sum of the forces is zero in the horizontal direction. And in the vertical direction. For the horizontal direction, there is no force of gravity so only two forces. I don't think I'm supposed to spoil your experience by giving away the equations before you attempt them.
     
  9. Sep 26, 2009 #8

    Suy

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    ok,but do i really need vertical direction ?
     
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