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Electric Forces Question

  1. Nov 21, 2006 #1
    How do I calculate Electric forces when objects are dangling from insulated wire. For example
    Neutral metal sphere A, of mass 0.10kg hangs from an insulating wire 2.0m long. An identical metal sphere B, with charge -q, is brough into contact with the sphere A. The spheres repel and stelle down as shown in the following figure

    .\ <) = 12
    * <) AB(top) = 90 degrees.
    Calculate the initial charge on B.

    The formula's are Fe =
  2. jcsd
  3. Nov 21, 2006 #2
    Ok Cory,a are you able to draw a FBD of the forces acting on A?
  4. Nov 21, 2006 #3
    I did with electric force outward, Force of tension angled at 12degrees, and force of gravity. Then I rearranged them to make a triangle.
    .\ =Ft
    ......| Fg
  5. Nov 21, 2006 #4
    That's perfectly fine, what's your problem with solving the question then?
  6. Nov 21, 2006 #5
    I can't seem to understand how to solve the FBD for fe. I tried using
    tan(12) = Fe
    but that doesn't seem to work, and im really just not sure how to go about this.
    Last edited: Nov 21, 2006
  7. Nov 21, 2006 #6
    That doesnt seem to be right, why would you use sin(12)=Fe?


    sin(12)=Fe/Ft isn't it? Think of a triangle. Thus Fe=Ft*sin(12) You can find Ft using Fg
  8. Nov 21, 2006 #7
    Sorry, I edited my last post, I typed the wrong thing. Should I still try Fe = ft*sin12? I don't really understand
  9. Nov 21, 2006 #8
    Ft*cos12=Fg help you?
  10. Nov 21, 2006 #9
    I don't think so.. I'm trying to find the initial charge on A
  11. Nov 21, 2006 #10
    Nm I see your problem, you do know Fe.

    The spheres are equal so q1 = q2

    Fe = Fg*tan12
    Fe =k*q^2*r^2
  12. Nov 21, 2006 #11
    so find Fe with mg*tan12 (which is: 0.20830543) then use that to solve for q in Fe =k*q^2*r^2? why does it change from divided by r^2 to multiplied?
  13. Nov 21, 2006 #12
    Oh it doesn't, misread your equation sorry ^^
  14. Nov 21, 2006 #13
    I seem to still be missing radius
  15. Nov 21, 2006 #14
    Heh sorry, it does not mean radius, r means the distance between the two particles. Are you able to find it using trigonometrics?
  16. Nov 21, 2006 #15
    well yea, but then it's the same as Fe = Fg*tan12 which doesn't work. Maybe I should use the length of the rope?
  17. Nov 21, 2006 #16
    What do you mean?


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