Two methods -- neither works, thevenin

Click For Summary

Discussion Overview

The discussion revolves around the application of Thevenin's theorem to find the voltage across a specific resistor (R6 = 20 ohms) in a circuit. Participants explore different methods of analysis, including nodal analysis and circuit simplification techniques.

Discussion Character

  • Homework-related
  • Technical explanation
  • Exploratory

Main Points Raised

  • One participant presents a problem statement and expresses frustration over incorrect results when applying Thevenin's theorem.
  • Another participant suggests a step-wise approach to applying Thevenin's theorem, recommending breaking the circuit at various points to simplify the analysis.
  • A different participant critiques the initial nodal analysis, indicating an error in the matrix solver used, suggesting that the solution was nearly correct.
  • There is acknowledgment of a calculation mistake in the first method, which was identified as a sign error, leading to a sense of relief from the participant.
  • Participants discuss the need to complete calculations in the second method, indicating that unfinished calculations contributed to confusion.
  • One participant expresses appreciation for the guidance received, noting that it has helped them think more logically about their approach to the problem.

Areas of Agreement / Disagreement

Participants generally agree on the utility of Thevenin's theorem and the importance of careful calculation. However, there are differing views on the effectiveness of the methods used, with some participants suggesting alternative approaches while others focus on correcting specific errors.

Contextual Notes

Some limitations in the discussion include unresolved errors in calculations, dependence on specific circuit configurations, and the need for further clarification on the application of Thevenin's theorem in this context.

Who May Find This Useful

This discussion may be useful for students learning about circuit analysis, particularly those struggling with Thevenin's theorem and nodal analysis techniques.

potatowhisperer
Messages
31
Reaction score
1

Homework Statement


we are asked to find the tension between the R6 = 20 ohms resistor, using thevenin[/B]
2015-05-27 19.52.02.jpg

Homework Equations


the given solution for Ethev is 27.78

The Attempt at a Solution


is seen in the pictures below

thank i really appreciate your help as it gets frustrating not to know what is wrong .
2015-05-27 19.52.41.jpg

i tried a different approach and got this but still wrong
thank you again.
2015-05-27 20.19.10.jpg
 

Attachments

  • 2015-05-27 19.52.02.jpg
    2015-05-27 19.52.02.jpg
    32.2 KB · Views: 451
Physics news on Phys.org
Hi potatowhisperer, Welcome to Physics Forums.

One of the nice things about Thevenenin's and Norton's theorems is that you can apply them step-wise. You can "break" a circuit at successive locations, incorporating components into an evolving Thevenin or Norton model as you go. Often this can be easier than performing a mesh or nodal analysis of the entire circuit. Consider the suggested circuit "breaks" here:

Fig1.gif


At break (A) you have a simple voltage divider to the left. So a Thevenin equivalent is trivial to find. At (B) an "open" resistance R3 is tacked on, modifying the Thevenin resistance. Simple! Then at (C) the then-current Thevenin model is part of a voltage divider with R5, so again a trivial reduction...

Give it a try!
 
Bon jour, potatowhisperer,

Your first solution, the nodal analysis, is ok until the very end. You did something wrong when you used your matrix solver.

?temp_hash=852ab311d37be5847cf92a2503f323ba.png


You must have made some such error.
 

Attachments

  • tmpoA.png
    tmpoA.png
    8.2 KB · Views: 648
i tried a different approach and got this but still wrong
Your answer is right, but that's for VA. You need to go further and determine V2.
 
I love y'all
i m glad that the first method was just a calculation mistake , though i am embarrassed to have taken your time just because of a sign, i really appreciate your help again.
about the second method , i can not thank you enough for telling that it was unfinished calculations , as i was confused cause now i can think more logically about what i m doing , i have had many instances when i got so frustrated with my mistakes that i would just stop ... so thank you very much.
 
GNEILL,I am definitely going to try that method, it does simplify things , A LOT .
 

Similar threads

Engineering How do I find Vth with the node voltage method?
  • · Replies 10 ·
Replies
10
Views
4K
Why is the Thevenin resistance in this problem 3.2 kΩ?
  • · Replies 17 ·
Replies
17
Views
3K
Engineering Converting to a circuit's Thevenin equivalent?
  • · Replies 7 ·
Replies
7
Views
3K
Why do I get different answers with different methods - Thevenin
  • · Replies 3 ·
Replies
3
Views
2K
Three-part Problem: Thevenin + Dependent current source
  • · Replies 6 ·
Replies
6
Views
2K
Engineering Finding Current in a Thevenin Circuit
  • · Replies 5 ·
Replies
5
Views
2K
Engineering How do I find the Thevenin Equivalent of this circuit?
  • · Replies 10 ·
Replies
10
Views
3K
Engineering Norton/Thevenin Equivalent circuits with a dependent voltage source
  • · Replies 9 ·
Replies
9
Views
6K
Thevenin Resistance and the Time Constant
  • · Replies 12 ·
Replies
12
Views
4K
Thevenin Equivalent : Wrong value for Rth
  • · Replies 7 ·
Replies
7
Views
3K