KVL Exercise: Basic Homework Statement & Solution

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    Exercise Kvl
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Discussion Overview

The discussion revolves around a homework problem involving Kirchhoff's Voltage Law (KVL) applied to a circuit diagram. Participants are analyzing their attempts at solving the problem, addressing potential errors in their calculations and the labeling of voltages in the circuit.

Discussion Character

  • Homework-related
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant presents their equations for two loops in the circuit, leading to a voltage of VR2 = -20V and V2 = -24V, questioning the correctness of their solution.
  • Another participant suggests that the labeling of the voltage drop across a resistor may be confusing, indicating that the -8V should be interpreted as a +8V potential rise.
  • A participant questions the significance of changing the sign of the potential change, prompting a response that emphasizes the importance of sign in KVL equations.
  • There is a request for clarification on how the equation for part (a) would change if the potential change was labeled as -8V, with a participant asserting that the equation should reflect that potential change correctly.
  • Another participant agrees that the issue lies with the labeling of the resistor in the problem, suggesting it should be labeled as "8V" instead of "-8V".

Areas of Agreement / Disagreement

Participants express differing views on the implications of voltage sign changes and the correctness of the circuit diagram's labeling. There is no consensus on the resolution of the initial problem, as participants continue to explore the implications of the voltage signs and their effects on the equations.

Contextual Notes

There are unresolved issues regarding the labeling of voltages in the circuit diagram, which may lead to confusion in the application of KVL. The discussion reflects varying interpretations of the potential changes based on the circuit's labeling.

Ammar w
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Homework Statement


Click on the link
https://www.diigo.com/item/image/2sb3i/yjim


Homework Equations


KVL Law
Similar Example :
https://www.diigo.com/item/image/2sb3i/orwa

The Attempt at a Solution


(a)
In the left loop :
-8-12+VR2 = 0
VR2 = -20 v

(b)
In the big loop :
-8-12+7-9-V2-3+VR1 = 0
-8-12+7-9-V2-3+1 = 0
V2 = -24

Why is my solution wrong?

If I take the right loop for (b) using the right answer for (a) :
-4+7-9-V2-3+1 = 0
V2 = -8 which is true.

Is the wrong with the negative voltage (-8) ??

thanks.
 
Last edited by a moderator:
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Ammar w said:
Is the wrong with the negative voltage (-8) ??

Yes, it looks like whoever created the circuit diagram created confusion when they labeled the leftmost resistor's potential drop. As it is marked, strictly speaking there should be a -8V potential drop across that resistor proceeding from bottom to top, which translates into a +8V potential rise as you've properly interpreted it. However, it would appear that they (confusingly) intended the "+ -" indicators to show the actual direction of the potential change of 8V.

Bottom line: assume that the "-8V" is really "8V" oriented according to the "+ -".
 
thank you
but I see that there is no difference if it was "+8" instead of "-8" ?
 
Ammar w said:
thank you
but I see that there is no difference if it was "+8" instead of "-8" ?

Um, how can it make no difference if you change the sign of a potential change?

For part (a) your equation summing the potential drops would become: +8-12+VR2 = 0
 
ok, how the solution of (a) will be if :
https://www.diigo.com/item/image/2sb3i/y0us
 
Last edited by a moderator:
Ammar w said:
ok, how the solution of (a) will be if :
https://www.diigo.com/item/image/2sb3i/y0us

Just solve your equation that I presented in my previous post...
 
Last edited by a moderator:
I'm sorry
but can you tell me the equation of (a) if the potential change was "-8"?
I thought that the equation +8-12+VR2 = 0 is used when the potential change = -8.
 
Ammar w said:
I'm sorry
but can you tell me the equation of (a) if the potential change was "-8"?
I thought that the equation +8-12+VR2 = 0 is used when the potential change = -8.

Yes, that's right. The problem is not with your understanding, it's with the problem itself; the resistor is not labeled correctly for the given answers. The resistor should have its potential labeled as "8V", not "-8V", or alternatively, the + and - tags should be reversed.
 
aha
thank you Mr. gneill
 

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