Soving for resistor values and source current-image included

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Discussion Overview

The discussion revolves around solving for resistor values (R2, R3) and source current (Is) in a circuit, as presented in an accompanying image. Participants explore methods for calculating these values using Kirchhoff's Current Law (KCL) and discuss the implications of their approaches.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant calculates R3 using the formula R3 = V/I and finds R3 = 12 ohms, but struggles with R2, calculating R2 = 12/4 = 3 ohms instead of the expected 4 ohms.
  • Another participant suggests that the method is correct but questions the current value used for R2, indicating it is not 4A.
  • A different participant proposes that Is can be determined without first calculating R2 or R3, hinting at the application of KCL.
  • One participant confirms understanding of KCL, stating that the sum of currents entering a node equals the sum of currents leaving it, and applies this to the circuit.
  • Another participant explains that if 4A leaves the node at the top of R2, then 3A must come from the R2 branch, suggesting a method to calculate Is using KCL.
  • A participant expresses appreciation for the KCL approach, noting it clarifies the presence of different current values in the circuit.
  • One participant cautions against the simplification of KCL to "the sum going in = the sum going out," advocating for a method that defines current directions to avoid sign errors in more complex circuits.

Areas of Agreement / Disagreement

Participants generally agree on the application of KCL but express differing preferences for how to approach the calculations and define current directions. The discussion remains unresolved regarding the correct method for calculating R2 and the implications of the current values used.

Contextual Notes

There are limitations in the assumptions made about current values and the definitions of current directions, which may affect the calculations. The discussion does not resolve these uncertainties.

Josh225
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Question is in image included

So, I need to find R2, R3, and Is. I was able to get the correct answer for R3 by doing R3= V/I (R3= 12/1 = 12) . however when I apply that equation to R2 ; R2= V/I. (R2=12/4 =3), I do not get the correct answer. The answer is supposed to equal 4 ohms. Am I using an incorrect method and just getting lucky finding R3?

Once I find the value of the 2 resistors, I can figure out Rt and from there find Is.
 

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Josh225 said:
So, I need to find R2, R3, and Is. I was able to get the correct answer for R3 by doing R3= V/I (R3= 12/1 = 12) . however when I apply that equation to R2 ; R2= V/I. (R2=12/4 =3), I do not get the correct answer. The answer is supposed to equal 4 ohms. Am I using an incorrect method and just getting lucky finding R3?

Once I find the value of the 2 resistors, I can figure out Rt and from there find Is.

Method is correct but the current in R2 is not 4A.
 
Josh225 said:
Once I find the value of the 2 resistors, I can figure out Rt and from there find Is.

Actually there is a way to find Is without working out R2 or R3 first.

Have you studied KCL yet?
 
Do you subtract 4 and 1 to get you 3? 12/3 would equal 4.. But i don't see why you would do that. I've just been working from a textbook and haven't came across that.

And yes, KCL states that the sum of the current going in equals the sum of the current going out. I haven't learned beyond that though if there's anything else.
 
Last edited:
Yes. The current going into a node equals the current coming out. Google Kirchoff's Current Law.

4A leaves the node at the top of R2 so 4A must enter. 1A comes from the R3 branch so 3A must come from the R2 branch.

You can also use KCL to calculate Is.
 
Wow... That makes a lot of sense. I wouldn't have thought of that. I don't know why my textbook didn't show me to think like that. I won't forget it now though!

It had me wondering why there were 2 different current values... Because according to kirchhoff's current law, the sum going in = the sum going out. Now it makes complete sense! Thank you!
 
Josh225 said:
according to kirchhoff's current law, the sum going in = the sum going out.

Despite what I said above I find it's a bit dangerous to do "the sum going in = the sum sum out". It's better to define current going into a node as +ve and then make a sum to zero..

In other words I prefer to write..
+IR2 +1 +(-4) = 0
then solve for IR2

rather than write
4 = 1 + IR2

The reason is that later on when you get more complicated circuits it's very easy to make sign errors. In many cases you don't know the direction of one or more currents. In those cases you have to define a direction as positive, then later when you solve the simultaneous equations you might discover that one of them is negative. At that point you can refer back to your definition to work out what that means.

Edit: I should add that this is my personal preference. Either way works but I find this way I make fewer sign errors.
 

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