I think I made a crucial mistake at the electronics test, please take a look

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Homework Help Overview

The discussion revolves around an electronics test problem involving a circuit where the original poster is tasked with finding the potential difference between two points, labeled a and b. The circuit includes multiple voltage sources, and the poster expresses confusion about how to approach combining these voltages and applying Kirchhoff's laws.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the application of Kirchhoff's Voltage Law (KVL) and Kirchhoff's Current Law (KCL) to analyze the circuit. The original poster questions whether the voltage sources can be combined and expresses uncertainty about the correct approach to take. Some participants suggest writing equations based on KCL and KVL, while others mention the complexity of the problem and the number of unknowns involved.

Discussion Status

The discussion is ongoing, with various participants offering insights into potential methods for solving the problem. There is a recognition of the difficulty of the problem, and some participants provide encouragement while others express their own uncertainties. No consensus has been reached, and multiple interpretations of the problem are being explored.

Contextual Notes

The original poster mentions feeling overwhelmed by the problem and regrets not choosing an easier one. There is also a reference to a clause in another problem that may have influenced the choice of this particular question. The discussion reflects a mix of emotions, including frustration and a desire for clarification.

  • #61
I redid it. Does I2 = 6.4615 [A] ?
 
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  • #62
Femme_physics said:
I redid it. Does I2 = 6.4615 [A] ?

I get I2 = 5.714 A
(edit: made a mistake here though)

Btw, originally I though R1 was 0.8 ohms, with which you get "nice" round answers.
But I see now that you have R1 to be 0.5 ohms (are you sure?).
 
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  • #64
Femme_physics said:
I made up the figures so yes :)

So what is it this time?

The correct answer is I2 = 12.3 A

You made a mistake with the signs when you removed the parentheses. ;)
 
  • #65
If you keep I2 in fraction form, I2=160/13, you get a nice simple number for the voltage.

ehild
 
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  • #66
I like Serena said:
The correct answer is I2 = 12.3 A

You made a mistake with the signs when you removed the parentheses. ;)
If you keep I2 in fraction form, I2=160/13, you get a nice round number for the voltage.

ehild

Wait a second now, that means my calculator was correct!^^ Except the answer is in plus. You said my answers weren't correct. You mean the sign wasn't correct but the figures were, yes?
 
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  • #67
Femme_physics said:
Wait a second now, that means my calculator was correct!^^ Except the answer is in plus. You said my answers weren't correct. You mean the sign wasn't correct but the figures were, yes?

Yes. :)
 
  • #68
Yes, you calculator was correct, only you input wrong data. The sign was negative because the constants should be on the other side as the unknowns.

ehild
 
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  • #69
Yes, you calculator was correct, only you input wrong data. The sign was negative because the constants should be on the other side as the unknowns.

Aha! How curious :) Thanks. So here I'll go solving the actual problem:

, http://img32.imageshack.us/img32/1391/198dq.jpg

And...I have a feeling I'm off.
 
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  • #70
Femme_physics said:
Aha! How curious :) Thanks. So here I'll go solving the actual problem:

And...I have a feeling I'm off.

Almost. ;)

But your final Vab is not right.

How did you get Va and Vb?


Actually, what you should have done is calculate the voltage drop across the branch from a to b.
 
  • #72
Femme_physics said:
Start with Va. That's how I got Va ->

Sorry, but you can't do that.
These voltages do not add up like that.


Can you calculate the voltage drop across 1 branch from a to b?
 
  • #73
Femme_physics said:
Start with Va. That's how I got Va ->

http://img96.imageshack.us/img96/5859/bekeif.jpg

Voltages need a reference point. You are free to say that Va is 16 V, but not by using the method you used to combine voltages which has no basis at all. This Va of 16 V would be relative to some other reference point, which you don't know about yet. The value of Va relative to arbitrary points is completely irrelevant to the problem. What matters is the voltage Va relative to Vb, which we call Vab. Vab is the only quantity that matters here. So, it's best to not even introduce Va and Vb in this problem.

Remember what I posted earlier?

I1=(Vab-10V)/R1
I2=(Vab+9V)/R2
I3=(Vab-6V)/R3

and I1+I2+I3=0

Nowhere do you see Va or Vb, but you do see Vab.

You are really overcomplicating the process here. A simple substitution using the above equations would allow you to calculate Vab very simply. Then once Vab is known, the three current formulas above would give you I1, I2 and I3 instantly.

Another hint is that the numbers work out very simply here and Vab, I1, I2 and I3 are all nice integer numbers. Don't even post an answer like Vab=14.5 V or I1=3.45556. It will be wrong. :smile: (EDIT ! Scratch that, I read R1=0.8 Ohms instead of R1=0.5 Ohms)
 
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  • #75
Just take ONE path from b to a and sum the voltage rises/drops along the way.
 
  • #76
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  • #77
Femme_physics said:
Okay okay!

I got it right? o:)

Yes, that's correct if Va is your reference point and you're looking for Vba, the potential at b with respect to a.

For some reason I thought that you wanted Vab, the potential at a with respect to b. In other words, I thought you were to take b as the point of reference for potentials in the circuit. But if it's a, then fine.
 
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  • #78
Victory! But check the sign, how is Vab defined.

ehild
 
  • #79
w00t! You said "victory" that means I don't have to check or do anything more! :-p:approve:

Thank you all sooooooooooooooooooooooooooo much! o:) I wouldn't be able to get this far without the great helpers of this great forum!

*does victory dance!*EDIT: I'll read the Norton Equivalents stuff you posted later... I need a well needed break after this victory lap ;)
 
  • #80
Here's some more light reading... demonstration of three different methods to solve the problem.
 

Attachments

  • #81
Here is a fourth way. (by the way, sorry, but before I wrongly read R1 as 0.8 ohms instead of 0.5 ohms. the math works out much better with 0.8 ohms)

I don't present this 4th method with the intent of teaching it, but instead offer it as a museum piece. Before the days of computers, calculators and other such technology, and back in the stone age when we used slide rules and math tables, the signal flow graph (SFG) method was a very common tool used by circuit designers.

Very complex circuits could be solved by making a pictorial flowgraph and applying Mason's gain formula. The method automatically spit out the formulas for anything of interest in a linear system and also put the formula in a form that allowed rapid approximations, when the formulas got too unweildy. Another advantage is that feedback loops (which are critical to understand in complex systems) are readily seen in the flowgraph as internal loops.

For this relatively simple circuit, the solution for Vab is found without any intermediate steps. Simply draw a properly labeled schematic. Then draw the flowgraph by inspection of the schematic, - step by step starting at the output Vab and working back to the sources V1, V2 and V3. Then Mason's gain formula (known by heart) gives the answer directly, without algebra.
 

Attachments

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  • #82
Oh, my, god, gneill! This is an incredible piece of work! I can't believe you did it. Thank you so much-- this will be cherished for sure! I wish I could give you more than just my mere thanks, but I assure you this will be used wisely. I will tomorrow go on on the business of reading it all. The posts, the attachments, and everything, to understand the other methods of solving the problem. I will definitely reply more.

EDIT: And steven, same thing said to you! I'll read everything tomorrow. You two are incredible! Everyone, really. ILS, ehild, everyone :)!

Incredibly cheered me up. I'll be doing at the end of this month term B of the test, hopefully I'll be acing it! :)
 
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  • #83
Glad to help, FP. I hope it proves useful.
 
  • #84
stevenb said:
Before the days of computers, calculators and other such technology, and back in the stone age when we used slide rules and math tables, the signal flow graph (SFG) method was a very common tool used by circuit designers.

Hi Stevenb,
I really grew up in the stone age using slide rules and math tables, but I did not hear about signal flow graphs - it looks interesting- but I do not understand how to draw such one and how the method works, can you suggest some material about it?


ehild
 
  • #85
ehild said:
Hi Stevenb,
I really grew up in the stone age using slide rules and math tables, but I did not hear about signal flow graphs - it looks interesting- but I do not understand how to draw such one and how the method works, can you suggest some material about it?


ehild

I'm actually post-stone-age myself, but not by much of a margin. I started studying engineering around 1980, so I was taught by some stone-age mentors, which I feel very fortunate about, by the way.

I was lucky to learn electronics from one of the old analog design masters, just before he retired. He was in his eighties at the time, and wearing a hearing-aid bigger than today's cell phones. I'm pretty sure he chiseled out a stone wheel or two in his time. I still have the notes from his class on analog circuit design, which I took as a senior in college. I was further blessed in grad school to be assigned twice as a TA for the lab associated with his lectures. There is no better way to learn than to try and teach others, and doing it under the piercing gaze of such a master just added to the pressure. So this was quite an education.

Anyway, Prof. H. was a firm believer that the SFG approach is the best for analog circuit design, even after the wider availabiltiy of computers. He felt it provides insight to the inner workings of feedback. Somehow he eventually convinced me of this and I've used this approach throughout my own career. The only thing I've added to the process is checking all formulas derived from Mason's gain rule, using symbolic processors like Mathematica, Maple and Maxima. Hence, I get the confidence of accuracy from the new ways, while keeping the insight of the old ways.

You are right. The SFG approach is one that most people have not heard about, for some reason. Perhaps it's because there is a pretty steep learning curve to get proficient enough to use it to it's full advantages. Hence, it's a hard sell to get people to appreciate it. There are so many ways to solve linear systems that many people think it's just another way, but few give design insight like this technique.

The original paper on this is by S. J. Mason, from which we get Mason's Gain Rule.

S. J. Mason, "Feedback Theory - Some Properties of Signal Flowgraphs", Proc. of the Inst. of Radeo Eng., 41, pp. 1144-1156; September 1953

His follow up paper is available on-line here:

http://dspace.mit.edu/bitstream/handle/1721.1/4778/RLE-TR-303-15342712.pdf

And some on-line documents are here. The second one looks really good.

http://www.ece.tufts.edu/~srout01/ee12-2008/pdfs/lecture14-ogata.pdf

http://www.ives.edu.mx/bibliodigital/Ingenierias/Libros%20Ingenieria/Automatica/the%20engineering%20handbook/Section%2016/Ch96.pdf

If you are interested, you are welcome to PM me your email, and I can send you a PDF of some pages from my electronics notebook (20 MByte file !). I wish I could post it here, but it's just too big.
 
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  • #86
Thank you! I'll send a PM.

ehild
 
  • #87
Sorry it took me a while to reply, the second term of the electronics test is only at the 12/7/11 and I got other tests before that, but I do wish to say a couple of things.

1) With respect to flowchart diagram, how useful is it? Should I really bother studying what appears to be a new complicated method to solve things that doesn't appear to be in our course material?

2) With respect to matrices in the solution. We haven't studied matrices in our math-preparation course, and I doubt we will. I'd hate to start extra math just to pick up on an "alternatve path" to solve this stuff. I am interetsted in understanding Norton Equivalents. Are matrices compulsory?

3)

Trying to understand what you did gneill at the attachement.

Plus, I don't see how you could've turned this:

http://img87.imageshack.us/img87/8149/this1b.jpg

To that!:

http://img232.imageshack.us/img232/3737/this2d.jpg Is it really the same circuit? Because I see different loops I can due in the latter circuit that I can't do in the original. I don't really see where the voltage sources are in here.
 
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  • #88
Femme_physics said:
1) With respect to flowchart diagram, how useful is it? Should I really bother studying what appears to be a new complicated method to solve things that doesn't appear to be in our course material?

I think the flowchart explanation was not intended for you, but for me. :smile:

Actually this thread contains tons of information that falls outside of the scope of what you need to learn for your finals test. :wink:
Femme_physics said:
2) With respect to matrices in the solution. We haven't studied matrices in our math-preparation course, and I doubt we will. I'd hate to start extra math just to pick up on an "alternatve path" to solve this stuff. I am interetsted in understanding Norton Equivalents. Are matrices compulsory?

No, you don't need matrices. It's an aside showing a short hand notation. You can ignore it.
Femme_physics said:
3) Is it really the same circuit? Because I see different loops I can due in the latter circuit that I can't do in the original. I don't really see where the voltage sources are in here.

It's a "Norton equivalent" circuit, but I prefer not to try to explain it.
IMHO it's really better if you learn the other 2 methods first.
 
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  • #89
Where can I find a good source of such problems/questions where I apply KLV and KLC?
 
  • #90
Femme_physics said:
Where can I find a good source of such problems/questions where I apply KLV and KLC?

Google?
For instance google "practice KVL and KCL".

Oh, and you could search in PF I think. There must be tons of similar problems here! :D


Here's a few google hits that you may like:

http://www.facstaff.bucknell.edu/mastascu/elessonshtml/Basic/Basic4Ki.html

http://www.google.com/url?sa=t&sour...dHwx4GC-A&sig2=qig3JELuBhX3j7NIzS8j3g&cad=rja
 

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