Hey there, right when I thought I was making progresses with (basic) electronics, here's one example that unfortunately brought back confusion.
Given:
E
1 = 12V
R
1 = 1 Ω
R
2 = 3 Ω
R
3 = 4 Ω
R
5 = 2 Ω
i
2 = 2 A
I need to find the value of R
4This is what I did (no subscripts anymore for the sake of making the post easier to write)
1. Identified three meshes: left one with R5, R1, E1 / center one with E1, R1, R2 / right one with R2, R3, R4
2. at first, I ignored the right mesh and just went on with calculations
Assumed clockwise current flow, wrote the following equations, with I
L / I
C / I
R being the currents of each mesh (Left / Center / Right respectively)
-E1 - R5 * IL - R1 * IL = 0
E1 - R1 * IC - R2 * IC = 0
3. I chose to temporarily ignore the righmost mesh and went on with calculations, ended up with IL = -4 A and IC = 3 A; since the first number is negative I understand it just means the actual current flow is the opposite of the assumed one, but the number is supposed to be right... right?
And here's the issue: by reproducing the circuit with the assumed solution (R4 = 2 Ω)
in this circuit simulator I get different results, namely 3A instead of 4 at R5, and 2A instead of 3 at R2. Now, I understand R2 might be wrong due to having to take into account the other resistors, but shouldn't R5 be right anyway? Or perhaps the whole approach is flawed and hence led to wrong results in the first place?
===============================
To cut a long story short: I think I don't get this law as well as I thought.
And that is especially true since I've seen the dedicated, excellent EEVBlog video (#820) about nesh/nodal analysis.
Problem is, I don't think I've ever seen what he did with the middle resistor (which is shared between the left and right mesh), i.e. take into account the current from the right mesh when writing the left equation and viceversa. And that, albeit extremely clear in the video, definitely confused me as a beginner.
So the question is: do I have to apply that kind of calculation in this example, too? And what would be the equation of the rightmost mesh, i.e. the one with R2, R3, R4 ?
Would it be something like - R2 * IR - R3*IR - R4 * IR = 0 or does this not even make sense?