- #1
arestes
- 80
- 3
Hello!
First of all, this is not a HW problem and I already solved it. Actually, it most likely was a HW problem for my friend who gave it to me. I would take the opportunity to ask exactly how to handle this: I'm not a student but I like going back to school exercises in Physics and gain deeper understanding now. Should I always mention this? Should I take this to the homework forum to avoid raising (incorrect) suspicion (in which case i don't think I would get the appropriate response, which is to discuss rather than solve)?
Anyway: This system:
At first I tried to do it with the mesh currents circulating as shown and I found out the system is undetermined.
These are the mesh current equations for the four inner loops:
$$ 9=300 I_1 +100 I_2+200I_3+0I_4 \\
9=100 I_1 +400 I_2+ 0I_3+300I_4 \\
9=0 I_1 +300 I_2+400I_3+700I_4 \\
9=200 I_1 + 0 I_2+600I_3+400I_4 $$
There are infinitely many solutions!
This is not the first time I came across this but it usually meant that I had forgotten to eliminate an extra nonexistent current.
However, I solved this by assigning actual (branch) currents going through each resistor and assigning an unknown voltage Vc to the central node and then applying Ohm's law. (I assigned a reference voltage of zero at the node between A and B).
$$ V_c=100i_1 \\
V_c=400 i_4\\
9-V_c=200 i_2\\
9-V_c=300 i_3\\
i_1+i_4=i_2+i_3$$
Solving this system I got that the currents are [itex]i_1=36mA \quad i_2= 27 mA \quad i_3= 18 mA[/itex] and [itex]i_4=9mA[/itex] with the central voltage ##V_c=3.6 V##. This is the solution.
I am puzzled as to two things:
(A) What is the underlying reason the system formed by considering MESH CURRENTS (as drawn in the image) is undetermined: I recall this has happened to me when I could clearly see that there was a fictitious mesh current but here there are four, which is the same number of real currents.
(B) Moreover, these mesh currents are the ones the are supposed to get through the sources, they should MEAN something.
(C) I can see that this network has the four sources aligned such that they don't contradict each other with regards voltage. Part b. of the problem asks what happens if one of them is reversed. This would be a contradiction for voltages. I honestly don't know what would happen and my best guess is that it would break the batteries.
Any thoughts?
cheers
First of all, this is not a HW problem and I already solved it. Actually, it most likely was a HW problem for my friend who gave it to me. I would take the opportunity to ask exactly how to handle this: I'm not a student but I like going back to school exercises in Physics and gain deeper understanding now. Should I always mention this? Should I take this to the homework forum to avoid raising (incorrect) suspicion (in which case i don't think I would get the appropriate response, which is to discuss rather than solve)?
Anyway: This system:
At first I tried to do it with the mesh currents circulating as shown and I found out the system is undetermined.
These are the mesh current equations for the four inner loops:
$$ 9=300 I_1 +100 I_2+200I_3+0I_4 \\
9=100 I_1 +400 I_2+ 0I_3+300I_4 \\
9=0 I_1 +300 I_2+400I_3+700I_4 \\
9=200 I_1 + 0 I_2+600I_3+400I_4 $$
There are infinitely many solutions!
This is not the first time I came across this but it usually meant that I had forgotten to eliminate an extra nonexistent current.
However, I solved this by assigning actual (branch) currents going through each resistor and assigning an unknown voltage Vc to the central node and then applying Ohm's law. (I assigned a reference voltage of zero at the node between A and B).
$$ V_c=100i_1 \\
V_c=400 i_4\\
9-V_c=200 i_2\\
9-V_c=300 i_3\\
i_1+i_4=i_2+i_3$$
Solving this system I got that the currents are [itex]i_1=36mA \quad i_2= 27 mA \quad i_3= 18 mA[/itex] and [itex]i_4=9mA[/itex] with the central voltage ##V_c=3.6 V##. This is the solution.
I am puzzled as to two things:
(A) What is the underlying reason the system formed by considering MESH CURRENTS (as drawn in the image) is undetermined: I recall this has happened to me when I could clearly see that there was a fictitious mesh current but here there are four, which is the same number of real currents.
(B) Moreover, these mesh currents are the ones the are supposed to get through the sources, they should MEAN something.
(C) I can see that this network has the four sources aligned such that they don't contradict each other with regards voltage. Part b. of the problem asks what happens if one of them is reversed. This would be a contradiction for voltages. I honestly don't know what would happen and my best guess is that it would break the batteries.
Any thoughts?
cheers
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