Equivalent Resistance and Branch Currents in Non-simple Circuits

AI Thread Summary
To find equivalent resistance and branch currents in complex circuits, applying Kirchhoff's Voltage Law (KVL) and Kirchhoff's Current Law (KCL) is essential, though it may lead to circular calculations. The assumption of treating the input current as if it comes from a battery with ε = i*R_equivalent can be useful. When traditional series or parallel combinations are not apparent, Δ-Y transformations and symmetry can simplify the analysis. Recognizing symmetrical properties allows for the identification of equivalent nodes, enabling the replacement of certain components with half-value resistors. Utilizing these techniques can effectively lead to a clearer understanding of the circuit's behavior.
BeRiemann
Messages
14
Reaction score
0
I'm asked to find an equivalent resistance and the current through each branch of the following circuits. (r is a fixed arbitrary value for all resistors in the circuits)

IMAG0063.jpg

IMAG0064.jpg

Homework Equations


V=IR
Sum of I at a node = 0
Sum of V about a closed loop = 0

The Attempt at a Solution


I've been playing with both KVL and KCL equations trying get a working answer, but none of my attempts seem to be anything but cyclically finding me zero. My problem is how I consider the input current. Is it right to make the assumption that I essentially have a battery there which has the value of ε = i*R_equivalent. The circuits cannot be broken down in terms of parallel or series from what I've seen.
 
Physics news on Phys.org
Are you familiar with \Delta -Y conversions? I would start by implementing these conversions to determine equivalent resistances.
 
Sometimes symmetry offers opportunities for circuit simplification, particularly in 'contrived' circuits like these; the problems are designed to get you to think about and recognize these opportunities. Take the first circuit for example.

Yes, when you don't find obvious serial or parallel resistor combinations to exploit you could bring out bigger and better weapons like Δ-Y transformations or even mesh or nodal analysis. But you might also recognize the fact that there is a line of symmetry running through the circuit, where the top half is a mirror image of the bottom half. What does that do for you? It means that the currents in each of the "reflected" paths and components will be the same. It also means that image-pairs of nodes will have the same potential.

When two nodes always have the same potential they are essentially equivalent nodes. You could run a wire between those nodes and it would not affect circuit operation in any way. In effect then you can treat them as being the same node! When you do that, a number of new effective parallel connections appear.

Imagine folding the circuit along the line of symmetry. Any components that get overlaid with their mirror partner are now parallel pairs -- so replace them with half value resistors. Observe the new simplified circuit!
 
I'm not familiar with delta-Y conversions, but I'll have to take a look at them if they're useful.
I was thinking along those same paths for symmetry, my biggest obstacle was just knowing if it was actually a valid technique. Thanks!
 
Thread 'Voltmeter readings for this circuit with switches'
TL;DR Summary: I would like to know the voltmeter readings on the two resistors separately in the picture in the following cases , When one of the keys is closed When both of them are opened (Knowing that the battery has negligible internal resistance) My thoughts for the first case , one of them must be 12 volt while the other is 0 The second case we'll I think both voltmeter readings should be 12 volt since they are both parallel to the battery and they involve the key within what the...
Thread 'Trying to understand the logic behind adding vectors with an angle between them'
My initial calculation was to subtract V1 from V2 to show that from the perspective of the second aircraft the first one is -300km/h. So i checked with ChatGPT and it said I cant just subtract them because I have an angle between them. So I dont understand the reasoning of it. Like why should a velocity be dependent on an angle? I was thinking about how it would look like if the planes where parallel to each other, and then how it look like if one is turning away and I dont see it. Since...
Back
Top