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Homework Help: Coulomb's Law/Charge Is Quantized Problem.

  1. Jan 20, 2009 #1
    1. The problem statement, all variables and given/known data

    In Fig. 21-28 (http://courses.wcupa.edu/mwaite/phy180/ch21to23gifs/ch21p32a.gif [Broken]), particles 1 and 2 are fixed in place on an x axis, at a separation of L=8.00 cm. Their charges are q1=+e and q2= -27e. Particle 3 with charge q3=+4e is to be placed on the line between particles 1 and 2, so that they produce a net electrostatic force on it. (a) At what coordinate should particle 3 be placed to minimize the magnitude of that force? (b) What is that minimum magnitude?

    2. Relevant equations

    Coulomb's Law: F = (k*(q1q2)/(r^2))
    And what can be derived from what's given: x + y = 0.08 m.
    Known: 1e = 1.602e-19 C, k = 8.99e9 N(m^2)/(C^2)

    3. The attempt at a solution

    Initially I assumed that the minimal force possible was zero, so I set up Coulomb's Law for F31 and F32, using x as the distance between 1 and 3 and y as the distance between 2 and 3. I then set the equations equal to one another and solved the complex quadratic and got an answer that was incorrect. After checking the answer, the distance is SUPPOSED to be 2.00cm from particle 1 and the net force should be 9.21e-24. I've tried coming up with a way to incorporated both forces into one equation and solve for the minimum distance, but I keep getting the wrong answer.

    Any help would be really appreciated. I get the concept, but I don't know how to follow through with it all. Thanks in advance.
    Last edited by a moderator: May 3, 2017
  2. jcsd
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