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Electric Force II

  1. Jan 18, 2008 #1
    [SOLVED] Electric Force II

    1. The problem statement, all variables and given/known data
    You have a lightweight spring whose unstretched length is 4.0 cm. You're curious to see if you can use this spring to measure charge. First, you attach one end of the spring to the ceiling and hang a 1.0 g mass from it. This stretches the spring to a length of 5.0 cm. You then attach two small plastic beads to the opposite ends of the spring, lay the spring on a frictionless table, and give each plastic bead the same charge. This stretches the spring to a length of 4.5 cm.

    What is the magnitude of the charge (in nC) on each bead?


    2. Relevant equations
    F = -k1 * x
    F = k2 * q1q2 / r^2

    g = a = -9.81
    k1 = Spring constant. Unknown.
    k2 = Electrostatic constant = 9*10^9


    3. The attempt at a solution
    In the first experiment
    F = ma = -k1*x
    0.001*-9.81 = -k1*(0.05 - 0.04)
    Therefore the spring constant k1 = 0.981

    Now the second experiment:
    F = -k1*x
    F = -(0.981)(0.045 - 0.04)
    F = -0.004905
    This is the force applied on a spring of k1 = 0.981, to stretch it 0.5cm

    Similarily, this should also be the force between the two beads:
    F = k2*q1q2 / r^2, however q1 = q2 according to the question.
    F = k2*(2q) / r^2, solving for q.
    Fr^2/(2*k2) = q
    (-0.004905)(0.045)^2/(2*9*10^9) = q

    q = -0.0000000000000005518125 C
    q = -0.0000005518125 nC

    This answer is incorrect.
    Where is my mistake?
     
    Last edited: Jan 18, 2008
  2. jcsd
  3. Jan 18, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    OK.
    That should be q^2, not 2q.
     
  4. Jan 18, 2008 #3
    I almost feel like I'm wasting your time, as my mistakes seem so trivial..

    F = k2*q1q2 / r^2, however q1 = q2 according to the question.
    F = k2*(q^2) / r^2, solving for q.
    Sqrt((Fr^2)/k2) = q
    Sqrt((-0.004905 * 0.045^2)/(9*10^9)) = q
    Sqrt(-0.000009932625 / 9000000000) = q
    q = -0.000000000000001103625 C
    q = -0.000001103625 nC
    q = -1.103625 * 10^-6 nC

    This answer is also incorrect. While what you pointed out was indeed a mistake I made, there appears to be more >.<;
    Reading over it again.
     
  5. Jan 18, 2008 #4

    Doc Al

    User Avatar

    Staff: Mentor

    This is OK. (But get rid of that minus sign!)
    Looks like you didn't take the square root.

    (Please use scientific notation. All those zeroes are opportunities for error. And hard to read! :wink:)
     
  6. Jan 18, 2008 #5

    F = k2*q1q2 / r^2, however q1 = q2 according to the question.
    F = k2*(q^2) / r^2, solving for q.
    Sqrt((Fr^2)/k2) = q
    Sqrt((0.004905 * 0.045^2)/(9*10^9)) = q
    Sqrt(9.932625*10^-6 / 9*10^9) = q
    q = Sqrt(1.103625*10^15)
    q = 3.3220851885525151294968428473179*10^-8 C
    q = 33.221 nC

    Checking... correct!

    Doc Al is saving my education.
    <3
     
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