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Homework Help: Circuit analysis, problems with Laplace?

  1. Feb 23, 2009 #1
    1. The problem statement, all variables and given/known data

    The switch k is turned at t=0. Calculate [tex]i_k(t)[/tex], when t starts from 0. You can find the circuit in my link.

    2. Relevant equations

    All of them, basically. :tongue:

    3. The attempt at a solution

    http://www.filefactory.com/file/af19a7h/n/index_mht You can find my attempt there. It's in .mhtml format, but any basic browser should open it. I'm not quite sure, if my Laplace to time transformation is correct. The factor before sin is so small. I'd appreciate it, if someone could take a look.

    My symbols might differ from your's. But j is imaginary unit (i.e. 1+j), small letters are in time plane, captiol letters are in complex plane an captiol letters with (s) are in Laplace plane.
  2. jcsd
  3. Feb 23, 2009 #2

    Tom Mattson

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    When I click on your link all I get is...

    ...and a login prompt. Do I have to join FileFactory to read this?
  4. Feb 23, 2009 #3
    There should be free to download link a bit farther down, if not I'll have to upload somewhere else.

    Yep, the second option is free and without registering.
  5. Feb 24, 2009 #4
    I just realized, I might have messed up even more than I thought. I'm missing [tex]e^{-t}[/tex] from my transformation. I'd greatly appreciate it, if someone can locate my mistake. I've been banging my head to the wall and I really wouldn't like to start from scratch.
  6. Feb 24, 2009 #5


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    For t>0 you are using only the capacitor and the inductor. What about the resistor, that is in parallel with the inductor?
    It is the resistor thet will provide the attenuation (the [tex]e^{-\alpha t}[/tex] that is missing).
  7. Feb 24, 2009 #6
    Oh yeah, you're right! So [tex]I_L(s)= \frac {U_{C0}}{L(s^2+ \frac{s}{RC} + \frac {1}{LC}}= \frac {2000 \sqrt{2}}{5}*\frac {1}{s^2+1000s+1000}=\frac {2000 \sqrt{2}}{5*500 \sqrt {3}}* \frac {500 \sqrt {3}}{(s+500)^2+(500 \sqrt {3})^2}[/tex]
    Then [tex]i_k(t)=\frac {4 \sqrt{2}}{5 \sqrt {3}}e^{-500t}sin(500 \sqrt{3}t)[/tex].

    Now it should make sense! Thank you!
    Last edited: Feb 24, 2009
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