Hi,
my friend need to solve this circuit and I'm not sure how to do it on paper with simple mesh current theorem, I still didn't study them.
they ask you to find the resistance of Rx, from LTspice putting a current source I find the resistance to be 8.3333ohms.
what's the easiest way to solve such thing? maybe model Rx as a current source?
what's the easiest way to solve such thing?
what's the easiest way to solve such thing?
You calculate the replacement source which is (V1 + V2) / 2 (because R1 = R3) and R1 || R3
Giving a 35V source with 15 Ohms.
For 1.5 A you will need a total resistance of 35V/1.5A = 23.33 Ohms
leaving 8.33 Ohms for R2.
With best regards
Andreas
Kirchoff's current law says that the sum of the currents through the two resistors is 1.5A
So
20/(30 + Rx) + 50/(30 + Rx) = 1.5
Re-arranging gives Rx = 16.66 ohms
Edit: I think
Andreas and Tim are correct.Edit: I think
Wrong, the current through the resistors is not 20/(30 + Rx) or 50/(30 + Rx) respectively. You can't just totally disregard the current flowing through the common resistor from the other branch.
If you want to apply superposition, then you have to set the other voltage source to zero, i.e., a short circuit, and you have to contend with the part of the current going through the other resistor, which is a whole can of worms.
Andreas' approach is much simpler, and also correct.
what's the easiest way to solve such thing?
You calculate the replacement source which is (V1 + V2) / 2 (because R1 = R3) and R1 || R3
Giving a 35V source with 15 Ohms.
For 1.5 A you will need a total resistance of 35V/1.5A = 23.33 Ohms
leaving 8.33 Ohms for R2.
With best regards
Andreas
eq1 : 0 = (V1-VA)/R1 +(V2-VA)/R3 - (VA/Rx);
eq2 : 1.5 = (VA/Rx);
eq1 : 0 = V1 - (I1*R1) - RX*(I1+I2);
eq2 : 0 = V2 - (I2*R2) - RX*(I1+I2);
eq3 : 1.5 = I1+I2;
I so much prefer Nodal Analysis for these kinds of problems...
Define a node VA at the junction of the 3 resistors
Assume current is flowing into the node from V1 and that current is (V1 - VA) / R1
Assume current is flowing into the node from V2 and that current is (V2 - VA) / R3
Assume current is flowing out of the node VA and that current is VA / Rx and is given as 1.5A
So, you wind up with 2 equations and 2 unknowns (VA and RX)Code: [Select]eq1 : 0 = (V1-VA)/R1 +(V2-VA)/R3 - (VA/Rx);
eq2 : 1.5 = (VA/Rx);
And, yes, RX = 8.333 Ohms
To solve the problem using Mesh Analysis, you wind up with 3 equations and 3 unknowns (I1,I2,RX)Code: [Select]eq1 : 0 = V1 - (I1*R1) - RX*(I1+I2);
eq2 : 0 = V2 - (I2*R2) - RX*(I1+I2);
eq3 : 1.5 = I1+I2;
For the mesh, I assume the current flow is from ground up through each source. The left loop is clockwise, the right loop is counterclockwise.
Attached is wxMaxima solution for both Nodal and Mesh. I am trying to maximize my use of wxMaxima, hence the reply.
Thanks! quite a simple problem I see, don't know why I struggled.
Thanks! quite a simple problem I see, don't know why I struggled.
It's a process, a simple yet methodical writing of the equations. It may be worthwhile to take the equations I wrote for each of the approaches along with the schematic and see where I got them.
Nodal: One node VA with two currents coming in and one current leaving and that current is given. This is absolutely simple since the total current is given.
Mesh: Two loops where the only complication is the center resistor and the polarity of the voltage drop across the resistor from each loop current. In this case (most cases if you do it right), the currents add so the same voltage term [RX(I1+I2)] winds up in both equations and the factor (I1+I2) is given.
There are many videos on the Internet but there is an entire semester long video course at Digilent Inc
https://learn.digilentinc.com/classroom/realanalog/
Thanks, I completely understand how you did that, it makes sense, but I wasn't thinking hard enough because this question was a little bit outside the box for mesh analysis, usually the resistance is given and they want you to find the current, but now that I know how you did that I know how to solve such questions.
digilent looks interesting, thanks!
Is this how you would solve the problem of having an infinitely large grid or array of resistors and trying to find the current/volts of one in the middle?
what's the easiest way to solve such thing?
You calculate the replacement source which is (V1 + V2) / 2 (because R1 = R3) and R1 || R3
Giving a 35V source with 15 Ohms.
For 1.5 A you will need a total resistance of 35V/1.5A = 23.33 Ohms
leaving 8.33 Ohms for R2.
With best regards
AndreasYes, that's how I'd do it.
Attached is the .asc file for completeness.