# KVL



## jdd18vm (Jan 27, 2007)

Okay, just when I think I have something I read it elsewhere and am more confused. See the attached pdfs, Schaums problem 1.15 and even 1.16

Compare to the NCEES problem #4 which I thought I finally got. They seem opposite? Maybe its because of the rearranging?

I posted to the passing zone based on this solution, thought I understood. I don’t think you need the thread but here is the last response.


“First go around the loop and determine the voltage gains and losses by the rules above and find the net voltage source in the loop. Put that voltage on the left side of the equal sign. Next, go around the loop in the same direction. At each resistor, if the current is in the same direction you are going around the loop, add the current times the resistance. If the current is in the opposite direction you are going around the loop, subtract the current times the resistance. Put the current times resistance on the right side of the equal sign.”

You guys have a way of making things clear. How do I assign the polarities at the resistors when none are assigned? Maybe ive been staring at this too long..

Again any help is appreciated.


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## benbo (Jan 27, 2007)

Never mind- I was going to post that it was easier to do with KCL and then I saw the problem above. It is almost always easier to do problems with the nodes rather than the loops IMHO..


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## Frontier05 (Jan 27, 2007)

The first thing to learn is energy transfer. Current through a resistor going from positive to negative is "absorbed" and current going from negative to positive is "supplied". This is important to know that. It also helps with notation with reference polarities (Vab or Vba)

I went through that "the other board" class and didn't find it all too helpful, for me anyway. In fairness though, I was working a lot while taking that too ... Anyways ...

In circuit problems, many times we don't know the actual current path so thats why you arbitrily assign "+" and "-" on each resistor and keep it consistant throughout - schalms. I always assign them as the the "+" closest to the voltage "+". Just assign resistor polarities (will be absorbed or supplied depending on the current loop), and work the problem through assigning your mesh current loops too. If the current comes out negative, the resistor just acts the opposite function then assigned.

Wait till you get to supernodes - those are fun.



jdd18vm said:


> Okay, just when I think I have something I read it elsewhere and am more confused. See the attached pdfs, Schaums problem 1.15 and even 1.16
> Compare to the NCEES problem #4 which I thought I finally got. They seem opposite? Maybe its because of the rearranging?
> 
> I posted to the passing zone based on this solution, thought I understood. I don’t think you need the thread but here is the last response.
> ...


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## singlespeed (Jan 29, 2007)

The first thing to do is to assign the direction of the current! As Frontier says, it may be wrong (will come out negative), but you must do this first.

Then, assign polarity accordingly - current enters the positive terminal of passive components.


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## Dark Knight (Jan 29, 2007)

Once you assigned or assumed the direction of the current, following SSpeed idea, be consistent and stick to that. If not you will have a mess. I think I found a good problem to practice KVL and another to practice KCL. Will scan them and post it tonight. :waitwall:


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## Dark Knight (Jan 29, 2007)

Have some very good problems to pratice KVL, KCL, Nodes and SuperNodes. Involve independant and dependant sources. If interested please PM me. The files are a little bigger than the max allowed here.


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## Frontier05 (Jan 30, 2007)

Here is a quick problem just done. It's an example of a basic circuits problem.

It's not mesh, but still good to know power absorbed and supplied. Hope it helps ..

man ... sorry, it's not letting me upload a 34K jpg. I'll try another time if wanted.


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