Nodal and Mesh mismatch...where am I going wrong

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Discussion Overview

The discussion revolves around discrepancies in current calculations for a circuit using nodal and mesh analysis methods. Participants are exploring the approaches to determine the current ia, with a focus on the application of nodal analysis and its justification.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant calculated the current ia using mesh analysis as 1.71mA and nodal analysis as 2.25mA, indicating a potential issue in the nodal analysis approach.
  • Another participant suggested checking the substitution for v1 in the node equation, noting the connection of the 6 V source through a resistor to the reference potential.
  • A participant mentioned adjusting the voltage v1 as 3-v, which they believe provides the correct current value, but expressed uncertainty about the validity of this approach.
  • Further responses encouraged justifying the substitution for v1 in the node equation and asked how to express v1 in terms of the current passing through the associated resistor.
  • One participant acknowledged that their approach looks good, indicating some validation of their reasoning.

Areas of Agreement / Disagreement

Participants are engaged in a debate regarding the correct approach to nodal analysis, with no consensus reached on the validity of the current calculations or the justification of substitutions made in the equations.

Contextual Notes

There are unresolved aspects regarding the assumptions made in the node equations and the specific definitions of voltage and current in the context of the circuit analysis.

DFrentzen
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upload_2016-2-8_15-48-6.png


Given the circuit diagram above, I am interested in finding the current ia. Using mesh analysis, I determined it to be 1.71mA, but nodal analysis results in a value of 2.25mA. Solution attempts provided below. I think the issue is in my nodal analysis approach.

Mesh Analysis..I1 is the left mesh, I2 the right mesh.
upload_2016-2-8_15-52-53.png


Nodal analysis attempt...current calculation at the bottom.
upload_2016-2-8_15-53-45.png
 

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For the node equation, check your substitution for v1: Note that the 6 V source is connected through a resistor to the reference potential.
 
I tried to take that into account on one of my attempts, the voltage v1 as 3-v provides the correct value for current. I was unsure if I'm on solid ground to do that.

upload_2016-2-8_16-13-31.png
 
You're on the right track. But how to justify your substitution?

Consider that you have a term in your node equation for the current that passes through the "v1" resistor. How would you express v1 in terms of that current?
 
gneill said:
You're on the right track. But how to justify your substitution?

Consider that you have a term in your node equation for the current that passes through the "v1" resistor. How would you express v1 in terms of that current?

I see v1 given as

upload_2016-2-8_16-33-6.png

Which allows me to plug in and

upload_2016-2-8_16-33-45.png
 
Looks good.
 
Thank you.
 

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