Solving circuit using Laplace Transforms

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Discussion Overview

The discussion revolves around solving a circuit to find the voltage across an inductor using both differential equations and Laplace transforms. Participants share their approaches and seek assistance in resolving discrepancies between their results.

Discussion Character

  • Exploratory, Technical explanation, Homework-related

Main Points Raised

  • One participant presents their solution for the current in the inductor using differential equations, yielding an expression for iL(t) but struggles to match this result when applying Laplace transforms.
  • Another participant points out a potential error in the original poster's loop 2 equation regarding the voltage drop across the inductor, suggesting it should be expressed as (i2)sL.
  • The original poster acknowledges the correction and expresses gratitude, indicating that the fix led to the correct answer.
  • A later reply mentions the original poster's intention to solve the circuit by hand using differential equations again, noting previous attempts were close but not correct.

Areas of Agreement / Disagreement

Participants generally agree on the correction regarding the voltage drop across the inductor, but the overall discussion remains unresolved as the original poster continues to seek help with their differential equation approach.

Contextual Notes

The discussion includes unresolved mathematical steps and assumptions related to the application of Laplace transforms and the setup of the circuit equations.

perplexabot
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Hi all. I need to find the voltage across the inductor. I begin by solving for the current in the inductor. I first solved a circuit using differential equations (with mathematica) and achieved an answer for current in inductor of: iL(t) = E^(-5 t) (-0.75 + 1.25 E^(4 t)

Later, I tried to solve the same circuit using Laplace transforms and was not able to yield the same answer (or anything close). The original circuit and my approach for solving the circuit using Laplace transforms is attached as a pdf file. In the pdf file, I am attempting to solve for the current in the inductor. I get an equation for iL(s), however, when I take the inverse laplace, the answer looks nothing like my diff-eq iL(t). Please let me know where I am going wrong. Hints are preferred over answers. Thank you.
 

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In your loop 2 equation the voltage drop across inductor should be (i2)sL.
 
the_emi_guy said:
In your loop 2 equation the voltage drop across inductor should be (i2)sL.

WOW, you are so right. I can't believe I didn't see that. Thank you so much. I will give it another go. Hopefully I will not run into another error. Thanks again.
 
Thanks the_emi_guy. That simple fix gave me the exact answer.
 
You're welcome
 
I am now trying to solve the circuit by hand using differential equations. I have attempted it twice and went over it many times but have not achieved the correct answer (i am close though). I may upload my attempt tonight and hopefully you (or someone else) may help me with that too.
 

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