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Homework Help: Determine Charge with Spring between Particles

  1. Feb 5, 2012 #1
    1. The problem statement, all variables and given/known data
    Two initially uncharged identical metal spheres, 1 and 2, are connected by an insulating spring (unstreched length L0 = 1.55 m, spring constant k=30.0 N/m).

    Charges +q and -q are then placed on the spheres and the spring contracts to length L = 1.25 m.

    a. Determine the charge q.
    b. If the spring is coated with metal to make it conducting, what is the new length of the spring?

    2. Relevant equations

    Fs = -kΔx
    Fe = 1 / 4∏ε0 * (+q)(-q) / r^2
    m1 = m2

    3. The attempt at a solution

    I attached my work.
    for a. what I basically did was set Fe = Fs and solved for q.
    I got 3.16*10^-5 C, but the answer should be 3.96*10^-5 C. Could it be because of truncated significant figures?

    for b. How can I show that coating the spring in metal cause the metal ball to return to L0?

    Attached Files:

  2. jcsd
  3. Feb 5, 2012 #2
    You dropped r^2.

    1.25^2 * your result is what you want?
  4. Feb 5, 2012 #3
    Your right!
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