Cannot find V and I in this circuit

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Homework Help Overview

The discussion revolves around finding the current (i) and voltage (v) in a circuit involving resistances and conductances. The problem is situated within the context of circuit analysis, specifically applying Ohm's Law and Kirchhoff's Laws.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss various methods for combining resistances and conductances, with some attempting to simplify the circuit into series configurations. Questions arise regarding the correct interpretation of units (Siemens vs. resistance) and the implications for calculations.

Discussion Status

The discussion is active with multiple participants sharing their approaches and reasoning. Some guidance has been provided regarding the distinction between conductance and resistance, which has clarified a misunderstanding for one participant. However, there is no explicit consensus on the correct method to solve the problem.

Contextual Notes

There is confusion regarding the use of Siemens as a unit of conductance, which some participants initially misinterpreted. This has led to varied approaches in calculating total resistance and current.

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Homework Statement



Obtain the values of i and v in the circuit below:
http://img33.imageshack.us/img33/7538/newpicturenl.png
Answers: i = 6 Amps, v = 3 Volts

Homework Equations



Ohm's Law, Kirchhoff's Current Law & Kirchhoff's Voltage Law

The Attempt at a Solution



I have spent the last 90 minutes on this question!
I can see that it is simple, but I must be approaching it wrong as I keep on getting the same wrong answers.
I just need to know where to start...
Thanks
 
Last edited by a moderator:
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id try combining the furthest resistances to the right because it is a series circuit, to get 9S then finding the total resistance of that and 2S which is 1/9S+1/2S=1/Rt ->2/18+9/18->11/18->18S/11= Rt. then find the total resistance of that and4s because its a series circuit. so its 18S/11 + 4S which is 62S/11... not sure if this is correct/and if it is idk where to go from there
 
You approached it the same way I did :)
I thought that way would work, but I kept ending up with i = 1.35 A
 
hmm, couldn't you do it that way then "simplify" it to a series circuit so I is constant. then can't you do V=IR and plug in 9 for Itotal and whatever the total resistance was for R and find V total? only problem here is S though.
 
The S unit is Siemens, which is conductance, not resistance. For an individual resistor, S = 1/R. This means that conductances in parallel add, while those in series add like resistances in parallel.

Sometimes it's easier just to convert all of the individual conductances to resistances...
 
Last edited:
I had no idea they were different units!
I thought the S was a variable...

Thank you for clearing that up for me - I've solved the question now, thanks. :smile:
 

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