Is decreasing resistance always a valid proof of no resistance increase?

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Discussion Overview

The discussion centers around the question of whether decreasing the resistance of one resistor in a system of resistors will always result in a decrease in the total resistance measured between two points. Participants explore this concept through various configurations of resistors and seek proofs or counterexamples.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant questions if decreasing the resistance of a single resistor in any system will always lead to a decrease in total resistance.
  • Another participant notes that in the case of two resistors in parallel, reducing one resistor's value results in a lower equivalent resistance.
  • Concerns are raised about whether this principle holds for more complex systems, with a request for counterexamples or proofs.
  • A participant points out that reducing resistance in a series configuration also affects the total resistance, but the implications differ from parallel configurations.
  • There is speculation about the possibility of a counterexample in complex systems where decreasing one resistance might lead to an increase in total resistance due to current rearrangement.
  • One participant asserts that it is always true that equivalent resistance will decrease if current flows through the resistor being reduced, suggesting a proof involving star-mesh transforms and derivatives.
  • Another participant agrees with the idea of using star-mesh transforms and mentions using computational methods to derive a proof, indicating that the derivative of total resistance with respect to resistance values yields a non-negative result.

Areas of Agreement / Disagreement

Participants express differing views on whether the principle holds universally across all resistor configurations, with some asserting it is always true while others remain skeptical and seek further clarification or proof.

Contextual Notes

The discussion does not resolve the question of whether there are exceptions to the principle in complex systems, and assumptions about current flow and circuit configurations remain unexamined.

chingel
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Is it always true that if I have any system of resistors and I calculate the resistance between two points, when I decrease the resistance of one resistor, then the resistance measured between the same two points as previously will not increase?

I.E if i have lots of resistors between two points and I decrease the resistance of one of them, will the total resistance always not increase? If so, is there a proof?
 
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It depends on your 'system'.
In the simplest case of two resistors in parallel, if you reduce the resistance value of one resistor, then the resistance of the combination also reduces.
 
I'm wondering if it is true in general, no matter what the system is like, as long as it only has resistors? Perhaps someone has a counterexample or knows if this is true.
 
Hmm, actually what I posted above is also true for two resistors in series, but the net effect is different.
In general if you have a circuit and reduce the resistance in some part of it, more current will end up flowing through the circuit.
Which is the same thing as saying that the resistance of the circuit as a whole is reduced.
 
It does intuitively seem like that, but is there a proof?

I could have a very long and complex system, and maybe if i decrease a particular resistance by a specific amount the currents will rearrange in such a way that the total resistance will increase. Is there a way to show that it cannot happen?
 
You might be able to get the sort of result you're talking about if the circuit also included one or more transistors.
 
It is always true. Equivalent resistance will go down if and only if there is current flow through the resistor that reduces its value, and it will never increase.

You can probably show that via star-mesh transforms using some derivatives with respect to the changing resistance.
The special case of no current is trivial.
 
That is a good idea, using the star-mesh transform we can always transform it to just four nodes and then by letting the computer do the algebra, we can solve for the total resistance of the general case with four nodes. Then I let the computer calculate the derivative (it was some horrible long expression) of the total resistance with respect to each resistance and it came out to be the square of something, which is never negative and therefore proves the statement.
 
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