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Electrostatics and Coulombines: Diagram Included.

  1. Feb 15, 2007 #1
    1. The problem statement, all variables and given/known data
    2 small spheres each having a mass of 0.1g are suspended from the same point on silk threads 20cm long. When given equal charges, they repel, resting 24cm apart. Calculate charge of each.


    2. Relevant equations
    F(w)=mg(a)
    F=kQ(1)Q(2)/d^2
    k=8.987x10^9 N*m^2 / c^2

    3. The attempt at a solution
    I converted grams to kilograms.. 1 x 10^-4 kg
    kilograms to newtons/weight..9.8*10^-4N

    blah blah..
    F=kQ(1)Q(2)/d^2
    (7.35*10^-4N)(.24^2)(8.987*10^9 N*m^2/c^2)=Q^2
    Q=6.86*10^-8C

    http://i80.photobucket.com/albums/j199/pugfug90/physucks.gif
    Here's some work and a diagram. I am 99% sure that the answer is 6.9*10^-8 coulombs. However! I don't understand.. If I used trig to find half of the total force (in the triangle diagram).. why do I use the full distance?
     
  2. jcsd
  3. Feb 15, 2007 #2

    hage567

    User Avatar
    Homework Helper

    If I understand what you're asking: You aren't finding just half the force. Coulomb's law obeys Newton's third law, so the force exerted on Q1 by Q2 has to have the same magnitude as the force of Q2 on Q1, it is just in the opposite direction.
     
  4. Feb 15, 2007 #3
    AHHHHHHH

    But mah physical teacher drew this line between both balls (for notes) and labeled it force.. Hage, that kind of makes sense. Thanks for help. So force isn't really "half", therefore just because I'm using "half force" doesn't mean I use half distance..?

    Anyone wanna do the problem, starting with..

    "2 small spheres each having a mass of 0.1g are suspended from the same point on silk threads 20cm long. When given equal charges, they repel, resting 24cm apart. Calculate charge of each."

    Also, any online links with similar problems?
    ----
    [​IMG]
    http://i80.photobucket.com/albums/j199/pugfug90/physicismean.gif
    Because of that image, and a nice guy that kind of explained it.. I made this graphic and I hope it shows that I understand:) What do you guys think of it?
     
    Last edited by a moderator: Feb 28, 2007
  5. Feb 16, 2007 #4
    Hey hey can anyone confirm my work of art?
     
  6. Feb 16, 2007 #5
    heyheyheyheyhey
     
  7. Feb 17, 2007 #6
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