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Homework Help: Problem from Electric Forces: Coulomb's Law section

  1. Apr 7, 2007 #1
    I'm having problems with the following question:
    Neutral metal sphere A, of mass 0.10 kg, hangs from an insulating wire 2.0 m long. An identical metal sphere B, with charge -q, is brought into contact with sphere A. The spheres repel and settle as shown. Calculate the initial Charge on B.

    |\ <--- The angle inside the triangle is 12 degrees.
    ||| \
    |||| \
    ||||| \
    |90'|| \
    B----- A

    The formula for Coulomb's law is F(e)= (kq1q2)/r^2, where k is a constant of 9.0 X 10^9 Nm^2/C^2

    I tried finding the force of gravity for A, using Fg=mg, and i ended up getting 0.98 N (down). Using, tan, I found the force between BA to be 0.2083 N, and i assumed that this is the F(e). I used sin to find the distance (separation) between BA, and i got 0.4158 m. I plugged this information into Coulomb's formula and i got q= 2.0 X 10^-6 C, which is wrong. Please tell me what I did wrong.
  2. jcsd
  3. Apr 7, 2007 #2
    Err... I didnt really follow you, but the charge will get transferred equally among both the spheres. So, A will have -q/2 and B will have -q/2. F=[kq^2]/[4r^2]. That should help you out.
  4. Apr 7, 2007 #3
    hmm.. i got the right answer with the formula that you gave me... but i don't understand why the charges are halved.
  5. Nov 24, 2007 #4
    You have the stupid Nelson book that doesn't give an explanation right?
    I recommend you to get the David Halliday and Resnick Textbook. I could give you the ISBN if you want it. I could also assume that you're in Ontario somewhere, because they system there is messed up for education. In the chapter it doesn't even explain that problem. I don't understand this question either. How did you get that formula?
  6. Mar 4, 2009 #5
    hello can anybody tell me .............there are two point charges placed at d distance and there are two diffrent mediums filled between the space one above the other. the lengh of each medium each also d.dielectric constant of mediums is k1 and k2 .what is the force between the forces.
  7. Mar 4, 2009 #6
    i have the problem ..........there are two point charges placed at d distance .the space between them is fillid with two different mediums of dielectric constants k1 and k2.one medium is filled up to d1 distance and other medium up to d2 distance .d1+d2=d.what is the force acting between the two charges?
  8. Mar 4, 2009 #7
    what is the force acting between the charges.if distance between charges is d.and two different mediums are filled between the chages.as shown in figure.k1 and k2 are dielectric constant of two mediums.

    q1 _______________________________ q2
  9. Mar 4, 2009 #8
    the figure for above question is as .....

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