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Homework Help: Coulomb's law/Spring constant & Electric field problems

  1. Sep 6, 2009 #1
    1. The problem statement, all variables and given/known data
    1. A tiny sphere with a charge of q = +8.2 µC is attached to a spring. Two other tiny charged spheres, each with a charge of -4.0 µC, are placed in the positions shown in the figure, in which b = 4.2 cm. The spring stretches 5.0 cm from its previous equilibrium position toward the two spheres. Calculate the spring constant (diagram: http://imgur.com/FKcPp.gif ).

    2.Two equal charges (Q = +0.95 nC) are situated at the diagonal corners A and B of a square of side x = 1.0 m as shown in the diagram. What is the magnitude of the electric field at point D (diagram: http://imgur.com/eervv.gif )?

    2. Relevant equations
    1. Coulomb's law:
    F = Kq1q2/r^2

    Spring Constant:
    F = kx

    2. Electric field:

    3. The attempt at a solution
    1. q1= 8.2*10^-6 q2= 4.0*10^-6 r= .027?? (I found this using Pythagorean theorem, but not sure if it's right.
    So then, with that I plug it all in in coulumb's law and get 404.49N for one of the spheres and for my vertical component 404.49Nsin(65) (I got 65 from tantheta = .042/.02) = 366.59N
    Because there's 2 of them I multiplied 366.59*2 = 733.18
    733.19 = kx
    733.19 = k(.05)
    k= 14663 N/m But apparently that's not right.

    2. I couldn't find a clear similar model for problem two, so this is all I have so far.
    q= .95*10^-6 r= 1m
    plug those variables into E=kq/r2 and I got 8540.5 N/C. I'm pretty sure there's a lot more to this, but I don't know where to go on from there.

    *EDIT* ok I found what I did wrong with the first problem. I redid the whole problem and my 'r' was incorrect, it was supposed to be .0465m. After I got that, everything else was smooth sailing. Now all I need to do is learn how to do problem 2, which I have no clue. Hopefully someone could help me.
    Last edited: Sep 6, 2009
  2. jcsd
  3. Sep 6, 2009 #2
    ok I finally figured out the second question two, it took me a few attempts because I was confused with the SI units. I pretty much used this equation: sqrt (EA^2+EB^2)
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