# Two methods -- neither works, thevenin

• potatowhisperer
In summary, GNEILL attempted to solve the homework using the second method, but was confused because it was unfinished calculations.
potatowhisperer

## Homework Statement

we are asked to find the tension between the R6 = 20 ohms resistor, using thevenin[/B]

## 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 .

i tried a different approach and got this but still wrong
thank you again.

#### Attachments

• 2015-05-27 19.52.02.jpg
32.2 KB · Views: 405
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:

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.

You must have made some such error.

#### Attachments

• tmpoA.png
8.2 KB · Views: 591
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 .

## What is the Thevenin theorem?

The Thevenin theorem states that any linear electrical network can be represented by an equivalent circuit consisting of a voltage source in series with a resistor. This equivalent circuit is known as the Thevenin equivalent.

## What are the two methods for finding the Thevenin equivalent?

The two methods for finding the Thevenin equivalent are the open-circuit voltage method and the short-circuit current method.

## Why may neither of these methods work?

There are certain cases where neither of these methods may work, such as in nonlinear circuits or circuits with dependent sources. Additionally, if the network has no steady state, these methods may not yield accurate results.

## How can I determine if the Thevenin equivalent is accurate?

The accuracy of the Thevenin equivalent can be determined by comparing the behavior of the original circuit and the equivalent circuit under different load conditions. If the results are similar, then the Thevenin equivalent is accurate.

## Can the Thevenin equivalent be used for AC circuits?

Yes, the Thevenin equivalent can be used for both DC and AC circuits. However, the calculations may differ for AC circuits due to the presence of reactance.

• Engineering and Comp Sci Homework Help
Replies
10
Views
2K
• Engineering and Comp Sci Homework Help
Replies
17
Views
2K
• Engineering and Comp Sci Homework Help
Replies
6
Views
1K
• Engineering and Comp Sci Homework Help
Replies
3
Views
1K
• Engineering and Comp Sci Homework Help
Replies
5
Views
1K
• Engineering and Comp Sci Homework Help
Replies
7
Views
2K
• Engineering and Comp Sci Homework Help
Replies
9
Views
4K
• Engineering and Comp Sci Homework Help
Replies
9
Views
4K
• Engineering and Comp Sci Homework Help
Replies
7
Views
2K
• Engineering and Comp Sci Homework Help
Replies
8
Views
1K