Basic circuit problem -- Resistor network and a voltage source

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Discussion Overview

The discussion revolves around a circuit problem involving a resistor network and a voltage source, specifically focusing on the application of Kirchhoff's Voltage Law (KVL) in analyzing loop equations. Participants are examining the validity of different equations derived from the circuit and the impact of current direction on the signs of voltage across resistors.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the validity of an equation for a specific loop in the circuit, noting a discrepancy in the sign of the voltage for a 180-ohm resistor compared to another source.
  • Another participant suggests that the direction of current affects the signs used in KVL equations, raising a question about the implications of current direction on voltage signs when applying KVL.
  • Some participants agree that assuming a direction for current is standard practice, and if the assumption is incorrect, the result will yield a negative value, indicating the actual current direction is opposite to the assumption.
  • There is a discussion about the implications of maintaining a negative answer and how it affects the overall results, with one participant asserting that current is a vector quantity, while another challenges this characterization.
  • One participant refers to examples from their textbook to illustrate differing approaches to current direction and sign conventions in KVL applications.

Areas of Agreement / Disagreement

Participants generally agree that the direction of current affects the signs in KVL equations, but there is disagreement regarding the characterization of current as a vector quantity and how different assumptions about current direction can lead to different results.

Contextual Notes

Participants express uncertainty about the implications of their assumptions regarding current direction and voltage signs, and there are unresolved questions about the consistency of applying KVL in different loop directions.

edgarpokemon
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Homework Statement


http://www.chegg.com/homework-help/questions-and-answers/problem-2-variableh-resistor-r-circuit-fig-p224-adjusted-va-equals-60-v-find-value-r-q8083343

Homework Equations

The Attempt at a Solution


Hi, I attached a pdf file with the problem. My question is, for the lower right loop, why is the equation that they have valid? I worked the problem, but the equation that I got for that loop is...

voltage of 180 ohm + voltage of 30 ohm + voltage of 12 ohm + voltage of 18 ohm =0. so the voltage for the 180ohm, is -180V, but they have a positive 180V. How did they got that?
 

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edgarpokemon said:

Homework Statement


http://www.chegg.com/homework-help/questions-and-answers/problem-2-variableh-resistor-r-circuit-fig-p224-adjusted-va-equals-60-v-find-value-r-q8083343

Homework Equations

The Attempt at a Solution


Hi, I attached a pdf file with the problem. My question is, for the lower right loop, why is the equation that they have valid? I worked the problem, but the equation that I got for that loop is...

voltage of 180 ohm + voltage of 30 ohm + voltage of 12 ohm + voltage of 18 ohm =0. so the voltage for the 180ohm, is -180V, but they have a positive 180V. How did they got that?

I just noted something. Does the direction of the current affects the signs (+ or -)? For example, if i have a KVL with a loop going clockwise, and when it reaches a resistor, that has a current going in the opposite direction (to the left), does that matter? If a kvl with a loop going clockwise hits the + sign of a resistor, will it always have a + sign regardless of the direction of the current on that resistor?
 
Yes the direction of the current affects the sign. Typically you assume a direction and if you're wrong you'll get a negative answer. It's as simple as that.
 
analogdesign said:
Yes the direction of the current affects the sign. Typically you assume a direction and if you're wrong you'll get a negative answer. It's as simple as that.

but if a keep the negative answer, I get a different result?
 
No, the negative answer just means the current goes in the opposite direction from what you initially assumed. Remember current is a vector quantity with a magnitude and a direction.
 
analogdesign said:
Yes the direction of the current affects the sign. Typically you assume a direction and if you're wrong you'll get a negative answer. It's as simple as that.

look at this picture from my book. In this example, what you said is correct, the assumed direction of current affects the sign. But on the other picture that i posted, they are not doing this. I think they are using a counterclockwise loop for the lower left loop in the other picture, but that should not matter. I used a clockwise direction and i got a negative answer, they used a counterclockwise direction, and they got a positive.
 

Attachments

  • 20180213_194110 (1).jpg
    20180213_194110 (1).jpg
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analogdesign said:
Remember current is a vector quantity with a magnitude and a direction.
I agree with your explanation of the negative current in OP's answer, but current is NOT a vector.
 

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