Why Is My Thevenin Resistance Calculation Different from the Textbook's?

In summary, the conversation involves finding the equivalent Thevenin resistance in a circuit with two power sources and multiple resistors. The result in the book is 8 ohms, but the person is getting 8/6. They use the Thevenin theorem to find the equivalent resistance, but are unsure if R4 should be included in the analysis. They then discuss using other methods, such as applying 1V to the input, to find the Thevenin resistance. The conversation ends with one person offering to send their analysis privately.
  • #1
Questions999
151
0

Homework Statement



We have the circuit.http://i.imgur.com/X4IYi4j.jpg
Find the equivalent thevenin resistance ..the result in my book is 8 ohm but I alway get 8/6 ..how so?

Homework Equations


Thevenin Teorem


The Attempt at a Solution



1/Re=1/4+1/2 and so Re=8/6 ..
 
Physics news on Phys.org
  • #2
You'll have to show your work so we can see where you are going wrong. Is R4 part of the circuit for which you want to find the Thevenin equivalent, or does it represent the load?
 
  • #3
I don't calculate R4 because I have to use the Tevenin's transformation between the points a and b..to find equivalent resistance, I took the R3 and R2 in parallel and found 1/Re=1/R3+ 1/R2
 
  • #4
I get R thevenin = 80/21 ohms. I included R4.

Without R4 I get R thevenin = 5 ohms.
 
Last edited:
  • #5
Elaia06 said:
I don't calculate R4 because I have to use the Tevenin's transformation between the points a and b..to find equivalent resistance, I took the R3 and R2 in parallel and found 1/Re=1/R3+ 1/R2

So R4 is not part of the circuit under analysis. Fine.

What makes you think that R2 and R3 are in parallel? Even if you could suppress the two current sources (which you can't because one of them is an active source), wouldn't that leave the resistors in series?

You'll have to use another method to find the Thevenin equivalent since you have an active source in the circuit.
 
  • #6
Rude man,how about without R4?
I they are in series,than the result is 6 ,still different from my book..
 
  • #7
Elaia06 said:
Rude man,how about without R4?
I they are in series,than the result is 6 ,still different from my book..

The controlled source is going to muck with the Thevenin resistance. You need another analysis method.

@rude man: I'm seeing the book's answer of 8Ω.
 
  • #8
Elaia06 said:
Rude man,how about without R4?
I they are in series,than the result is 6 ,still different from my book..

As gneill pointed out, you have to include the effects of the voltage-dependent voltage source. You can't just parallel resistors etc.

The way I do it is to apply 1V to the input, then compute the input current i using KVL or whatever, then R thevenin = 1/i.
 
  • #9
gneill said:
The controlled source is going to muck with the Thevenin resistance. You need another analysis method.

@rude man: I'm seeing the book's answer of 8Ω.

Looks like I struck out on this one! :blushing:
 
  • #10
rude man said:
Looks like I struck out on this one! :blushing:

Hey, you do good work. A small slip every now and then can't be avoided. Besides, they tend to rejuvenate concentration :smile:
 
  • #11
gneill said:
Hey, you do good work. A small slip every now and then can't be avoided. Besides, they tend to rejuvenate concentration :smile:

Still bugs me. Want to send me your analysis privately?
 
  • #12
rude man said:
Still bugs me. Want to send me your analysis privately?

Sure, I'll see if I can slip it into a PM.
 
  • #13
Elaia06 said:

Homework Statement

i'm disappointed that you appear to have given up on this question. I seem to have worked it out needlessly, then. :smile:

Perhaps someone else would like clues on how to do it? It's a good exam-type question. :wink:
 
  • #14
The circuit isn't very clear but if there's 2 power sources then Kirchhoff's voltage law and the use of simultaneous equations would be helpful
 

FAQ: Why Is My Thevenin Resistance Calculation Different from the Textbook's?

What is circuit analysis?

Circuit analysis is the process of studying and understanding the behavior of electrical circuits. It involves using mathematical and analytical techniques to determine the voltage, current, and power in a circuit.

What are the different methods of circuit analysis?

The two main methods of circuit analysis are nodal analysis and mesh analysis. Nodal analysis uses Kirchhoff's current law to determine the voltage at each node in a circuit, while mesh analysis uses Kirchhoff's voltage law to determine the current in each loop of a circuit.

What is the purpose of circuit analysis?

The purpose of circuit analysis is to understand how a circuit works and to predict its behavior. This can help in designing and troubleshooting circuits, as well as determining the most efficient and cost-effective ways to build them.

What are some common components in circuit analysis problems?

Some common components in circuit analysis problems include resistors, capacitors, inductors, voltage and current sources, and switches. These components are used to build more complex circuits and their behavior can be analyzed using circuit analysis techniques.

What are some real-world applications of circuit analysis?

Circuit analysis is used in a wide range of real-world applications, including designing and analyzing electronic circuits for computers, communication systems, medical equipment, power generation and distribution, and many other devices and systems that use electricity.

Similar threads

Calculate voltage over capacitor and resistance
Replies
7
Views
594
Determine Thevenin equivalent circuit.
Replies
1
Views
636
Two networks described by v-i graph: Find voltages when interconnected
Replies
1
Views
836
Thevenin's circuit analysis. Multiple voltage sources
Replies
6
Views
2K
Thevenin resistance in an opamp RC circuit
Replies
2
Views
2K
What is the Thevenin and Norton equivalent for the given circuit in Fig. 4.125?
Replies
10
Views
2K
Thevenin equivalent (symbolic circuit calculation)
Replies
8
Views
1K
The equivalent resistance for the resistors in this circuit?
Replies
5
Views
2K
Why is My LTSpice Thevenin Resistance 7.6 Ohms?
Replies
8
Views
1K
What is the Thevenin's Equivalent Circuit for a Given Linear Circuit?
Replies
6
Views
2K

Hot Threads

Recent Insights

Back
Top