Circuit analysis, problems with Laplace?

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Discussion Overview

The discussion revolves around a circuit analysis problem involving the calculation of current i_k(t) after a switch is turned on at t=0. Participants are exploring the application of Laplace transforms in this context, addressing potential errors in the transformation process and the role of circuit components such as capacitors, inductors, and resistors.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses uncertainty about their Laplace to time transformation, specifically mentioning a small factor before a sine function.
  • Another participant points out that a login prompt appears when attempting to access the provided link, questioning the accessibility of the shared attempt.
  • A later reply suggests that a free download option exists for the file, indicating a possible solution to the access issue.
  • One participant acknowledges a potential error in their transformation, noting the absence of an exponential decay factor e^{-t} and requests assistance in identifying their mistake.
  • Another participant emphasizes the importance of including the resistor in parallel with the inductor, stating that it contributes to the attenuation represented by the missing e^{-\alpha t} term.
  • A subsequent response confirms the need to include the resistor, providing a corrected expression for the Laplace transform and the resulting time-domain current i_k(t).

Areas of Agreement / Disagreement

Participants exhibit some agreement on the necessity of including the resistor in the analysis, but the discussion includes unresolved uncertainties regarding the initial transformation and the specific calculations involved.

Contextual Notes

There are indications of missing assumptions related to the circuit configuration and the definitions of variables used in the Laplace transformation process. The discussion reflects a reliance on specific mathematical steps that have not been fully resolved.

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Homework Statement



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.

Homework Equations



All of them, basically. :-p

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

index​.mht
Size: 251.71 KB
Description: No description

...and a login prompt. Do I have to join FileFactory to read this?
 
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.
 
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.
 
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).
 
CEL said:
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).

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:

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