Laplace transforms for transient analysis

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Discussion Overview

The discussion revolves around using Laplace transforms for transient analysis in an RC circuit involving a capacitor and a resistor connected in series to a step voltage source. Participants explore how to derive expressions for current and voltage across the capacitor in both the Laplace domain and time domain.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes using the formula for voltage across the capacitor as Vc = V(1-e-t/RC) but later acknowledges a calculation error in determining the time constant.
  • Another participant emphasizes the need to use Laplace transforms to solve the problem, suggesting that the Laplace domain can model the transient response of the circuit.
  • Participants discuss the appropriate circuit equations and the use of component impedances in the Laplace domain, with some suggesting that the variable 's' should replace 'jω' used in AC circuit analysis.
  • There are inquiries about the correct expressions for current and voltage drops across circuit components, with requests for clarification on taking Laplace transforms of these terms.
  • One participant expresses confusion about the algebra involved in deriving the output voltage and seeks assistance in simplifying expressions.
  • Another participant suggests using partial fractions to simplify the Laplace domain expression for voltage, indicating that this method is necessary for finding the time domain solution.
  • Discussions include corrections on algebraic manipulations and the need for clarity in the use of parentheses to maintain accuracy in calculations.
  • Participants share their findings regarding the inverse Laplace transform and express uncertainty about the correctness of their derived expressions.

Areas of Agreement / Disagreement

Participants generally agree on the necessity of using Laplace transforms for the analysis, but there are multiple competing views on the correct algebraic manipulations and methods for deriving the expressions. The discussion remains unresolved with various interpretations and approaches being explored.

Contextual Notes

Some participants express uncertainty about specific algebraic steps and the application of Laplace transforms, indicating that there may be missing assumptions or dependencies on definitions that are not fully addressed in the discussion.

Who May Find This Useful

This discussion may be useful for students and practitioners interested in circuit analysis, particularly those looking to understand the application of Laplace transforms in transient analysis of electrical circuits.

  • #31
agata78 said:
I used this website to educate myself a bit more then i remembered from school www.mash.dept.shef.ac.uk/Resources/web-partialfractions.pdf

But does it matter which way i used to calculate A and B.

i(t)= 20e 1 + (-20) -2t

But what next?

Your transformation of the Laplace terms does not look right! You have found the that in the Laplace domain:
$$I(s) = \frac{20}{s} - \frac{20}{s + 2}$$
Find the forms of those two terms in your Laplace Transform tables and convert them to time domain terms. The result should look familiar!
 
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  • #32
the only way it could be is:

20-20e(-2t)
 
  • #33
agata78 said:
the only way it could be is:

20-20e(-2t)

Right. Technically each term is multiplied by a unit step function to indicate the the voltage is "switched on" at time t = 0. Mathematically it means that this solution applies only for t ≥ 0.

So, writing it in proper form for a function:

[STRIKE]I(t)[/STRIKE]Vc(t) = 20-20e(-2t) = 20(1 - e(-2t))
[STRIKE]
Now you can move on to finding Vc(t). Use the same methods.[/STRIKE]

EDIT: Sorry about that. I misremembered where we were at in the problem sequence! I think you're done with this one!
 
Last edited:
  • #34
Yes i was actually trying to ask you why would i have to calculate it again.

Hurrayyyyy!

Thank you so much for help all the way! One to go!
 

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