KVL loop: Example wrong, or did I misunderstand?

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    Example Kvl Loop
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Discussion Overview

The discussion revolves around a circuit analysis problem involving Kirchhoff's Voltage Law (KVL) and the interpretation of voltages in a loop. Participants are trying to understand the application of KVL in a specific example from a textbook, particularly focusing on the treatment of a negative voltage source and its implications for the calculated voltage across another component.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion regarding the treatment of a negative voltage (-8V) in the KVL equation, questioning whether their algebraic approach is correct.
  • Another participant suggests that a circuit diagram would clarify the situation, indicating that visual representation may be necessary to resolve the misunderstanding.
  • A third participant notes that the voltages in the loop include a voltage source and voltage drops across resistances, implying a need for clarity on the roles of each component in the KVL application.
  • Several participants emphasize the importance of posting the circuit diagram to avoid assumptions and facilitate accurate discussion.

Areas of Agreement / Disagreement

There is no consensus on the correct interpretation of the negative voltage or the resulting calculations, as participants are still exploring the issue and seeking additional information.

Contextual Notes

The discussion lacks a circuit diagram, which may be critical for understanding the relationships between the voltages and components involved. Participants have not yet resolved the mathematical steps or assumptions regarding the negative voltage's impact on the KVL equation.

EngnrMatt
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In my circuit analysis textbook, there is an example problem using a KVL loop. Though it seems relatively simple, I cannot understand the answer.

the loop consists of 3 voltages: -8, 12, and the unknown, Vr2. The polarities (clockwise) around the loop are respectively +,-,+.

The answer given is 4V for Vr2. I can see this being the case if the 8 wasn't negative, but if I apply algebra, the equation comes out as:

+ (-8) - (12) + Vr2 = 0

where Vr2 = 20

Am I treating the -8 wrong? Because if it were positive 8, I could understand Vr2 being 4, however, I am given to understand negative voltage is different from positive voltage.
 
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A circuit diagram would be helpful.
 
One of those is a voltage source while the other two are the volt drops over resistances huh?
 
Post the circuit diagram otherwise we are guessing what might be wrong.
 

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