Circuit Analysis: Solving the Mystery of V3's Voltage Directionality

Click For Summary

Discussion Overview

The discussion revolves around the labeling of voltage V3 in a circuit diagram, specifically its polarity with respect to a power source and ground. Participants explore concepts related to Kirchhoff's Voltage Law (KVL) and the implications of voltage directionality in circuit analysis.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions why voltage V3 is labeled as negative with respect to the power source and positive with respect to ground, suggesting that the directionality should align with V4.
  • Another participant proposes that the labeling may serve an educational purpose, indicating that V3 will likely be a negative value while V4 will be positive.
  • A third participant recalls a teaching method emphasizing the importance of polarity signs, suggesting that incorrect assumptions about polarity can lead to negative values but do not affect the correctness of the overall analysis.
  • One participant confirms that the same principle applies to Kirchhoff's Current Law (KCL), where choosing current directions can also result in negative values if the initial guess is incorrect.
  • Several participants emphasize the importance of meticulousness in circuit analysis to avoid errors in sign conventions.

Areas of Agreement / Disagreement

Participants generally agree that the labeling of voltage is not intuitive and serves a pedagogical purpose, but there is no consensus on the necessity of such labeling in practical applications. The discussion remains unresolved regarding the implications of these conventions in real-world scenarios.

Contextual Notes

Participants express uncertainty about the practical application of the polarity signs and the potential for confusion in circuit analysis. The discussion highlights the importance of rigor in algebra and attention to detail in electrical engineering.

Who May Find This Useful

This discussion may be useful for students and practitioners in electrical engineering or physics who are grappling with circuit analysis concepts, particularly those related to voltage polarity and Kirchhoff's laws.

squiggles
Messages
4
Reaction score
0
Hi guys. Really elementary question here, I've been kicking myself for ages trying to figure this out, but I suppose everybody needs a little nudge sometimes.

I have been staring at this circuit diagram:

tsVhvmk.png


From this page:

http://www.facstaff.bucknell.edu/mastascu/eLessonsHTML/Basic/Basic5Kv.html

which is basically a rundown of Kirchoff's Voltage Law, and I have been wracking my brain for why voltage V3 is labeled (-) with respect to the power source and (+) with respect to ground.

If I follow a current running from the positive terminal of the power source, then it splits up into the two branches to go through R2 or R3 and R4, respectively. The directionality of the voltage (to my intuition) should be the same for V3 as it is for V4. It is dropping voltage, not adding to it.

Thanks for any help you can provide!
 
Engineering news on Phys.org
I'm guessing it was done to teach a concept. I would never label it that way in real life.

In this case, all it means is that V3 is probably going to be a negative number (and V4 will be positive).
 
Second for post #2.
When sliderules roamed the Earth and i was being taught KVL , teacher made it a major point to write those little polarity + & - signs .
He said emphatically: "If polarity is not intuitive by looking, just guess - if you guess wrong you'll get a negative number for that one. It only means you guessed wrong, your answer will be correct for the polarity you've shown. "
That's why it is so important to be rigorous and neat in your algebra. No shortcuts or you'll start dropping signs.

old jim
 
Okay, so it's just for the sake of making a point. Thanks you guys!

I just want to check that it's the same for KCL: pick your current directions, and if you're wrong, then you'll get a negative sign for that particular branch of current.
 
Also, "when sliderules roamed the earth". lol.
 
Yes, same for KCL.

Meticulous attention to detail will pay off down the road. Form your good habits early.
 
Thank you sirs, I will watch my signs with care.
 

Similar threads

FInding current in parallel and series circuits
  • · Replies 10 ·
Replies
10
Views
3K
Engineering Applying Kirchoff's Voltage Law to a circuit
  • · Replies 3 ·
Replies
3
Views
3K
Finding the Current in a circuit
  • · Replies 6 ·
Replies
6
Views
3K
Engineering Circuit with multiple voltage sources
  • · Replies 11 ·
Replies
11
Views
5K
Engineering Approach for solving voltage across a resistor in circuit?
  • · Replies 4 ·
Replies
4
Views
2K
Calculating Node Voltage using Nodal Analysis
  • · Replies 4 ·
Replies
4
Views
2K
Untangling a Confusing Circuit: Solving Kirchoff's DC Laws
  • · Replies 12 ·
Replies
12
Views
2K
General DC Circuit: Voltage drop different from supply source
  • · Replies 29 ·
Replies
29
Views
3K
Apply Nodal Analysis to the Circuit
  • · Replies 2 ·
Replies
2
Views
3K
Circuit problem involving sources and resistors
  • · Replies 7 ·
Replies
7
Views
3K