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Circuit DE

  1. Apr 14, 2013 #1
    1. The problem statement, all variables and given/known data

    attached

    2. Relevant equations



    3. The attempt at a solution

    Is the correct answer a choice?

    I get Q= .17e-6cos(4000t), every single time.
     

    Attached Files:

  2. jcsd
  3. Apr 14, 2013 #2

    Dick

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    Well, how do you get that?
     
  4. Apr 14, 2013 #3
    Q'' + 4e6Q = 0 =>

    r^2 + 4e6 = 0 (characteristic equation) => r = 0 +/- 4000i

    => Q = c1cos(4000t) + c2sin(4000t)
    => Q' = -4000c1sin(4000t) + 4000c2cos(4000t)

    => Q(0) = c1 = .17e-6
    => Q'(0) = 4000c2 = 0 => c2 = 0

    So

    Q = .17e-6cos(4000t)
     
  5. Apr 14, 2013 #4

    Dick

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    The square root of 4e6 is 2000, not 4000.
     
  6. Apr 14, 2013 #5
    I'm confused. Why would I be taking the square root of 4e6?

    Shouldn't I be taking the square root of b^2 - 4(a)(c)?

    Which is 0 - 4(1)(4e6) = 16000000, and the square root of that is 4000.


    Thanks
     
  7. Apr 14, 2013 #6

    Dick

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    You don't NEED to use the quadratic formula to solve r^2+4e6=0, but if you do you shouldn't forget there is a 2a in the denominator.
     
  8. Apr 14, 2013 #7
    Wow, I was thinking a = 1 and ignored it.

    And I know I don't need to, it just like to for complex since it spits it out in the right form already.

    That's what I get for rushing.

    Thanks again
     
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