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Electrostatic force and # of elementary particles

  1. Mar 28, 2007 #1
    I need to find the electrostatic force & the number of elementary particles between two charged balloons held in the center of their strings (Think of an Isosceles Triangle).

    1. The problem statement, all variables and given/known data
    Mpurple(balloon) - .00354 kg
    Mred - .00405 kg
    Lpurple - .725 m
    Lred - .68 m
    θ = 16°
    T = Tension

    2. Relevant equations & The attempt at a solution

    Fnet1 = Mpurple*g - Tcosθ = 0 T2 = (Mred*g)/cosθ
    Tcosθ = Mpurple*g
    T1 = (Mpurple*g)/cosθ

    Fnetx = F-Tsinθ
    Tsinθ = F
    (M*g)/cosθ * sinθ = F
    M*g*tanθ = (k*q²)/r²
    M*g*tanθ = (k*q²)/(Lpurple*sinθ + Lred*sinθ)
    (Lpurple*sinθ + Lred*sinθ)*( M*g*tanθ) = k*q²
    q² = ((Lpurple*sinθ + Lred*sinθ)*( M*g*tanθ))/k
    q = √(((Lpurple*sinθ + Lred*sinθ)*( M*g*tanθ))/k)
    qPurple = √(((.725*sin16° + .68*sin16°)*( .00354*9.81*tan16°))/8.99x109)
    qRed = √(((.725*sin16° + .68*sin16°)*( .00405*9.81*tan16°))/8.99x109)

    Tpurple = (.00354 kg * 9.81m/s²)/cos16° = 0.036 N
    Tred = (.00405 kg * 9.81m/s²)/cos16° = 0.041 N
    qPurple = 6.54 x 10-7 C
    qRed = 7.01 x 10-7 C

    I don't know how I'd go about finding the Electrostatic force or the number of elementary particles. Any help is appreciated.
  2. jcsd
  3. Mar 28, 2007 #2
    anyway you can post a diagram--or at least better describe the physical situation. You've done a lot of work and we would like to help..
  4. Mar 29, 2007 #3
    OK, diagram coming up (If you can read my handwriting) Also, I misread the handout. I was looking for the number of elementary charges, not particles. I know how to do that, so I only need help with the electrostatic force.

  5. Mar 29, 2007 #4


    User Avatar

    For electrostatic force, use the equation:


    where e0 is the permittivity of free space.

    and r is the distance between the charges. Just find the distance between the balloons (which can be solved with some elementary trig) and you have your answer.
  6. Mar 29, 2007 #5
    Ok, thanks.
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