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Homework Help: RC circuit black box circuit modeling

  1. Feb 7, 2017 #1
    1. The problem statement, all variables and given/known data
    I have a red mystery box which contains a 20, 50, 100, and 200,000 ohm resistor (one of each and ONLY one.)

    There are three terminals on the box you can plug a voltmeter into... testing A-B I got 60k, testing A-C I got 140k, and testing B-C I get 120k ohms. So this is a bit of a puzzle.

    I'm aware of general and relevant equations.

    3. The attempt at a solution
    I really struggle with this for some reason. It's been a while with RC circuits.

    These are pretty large values and it means that no matter what two places you pick, at least some of the current must flow through the 200K ohm resistor I think. Ok I think I can do this... it seems I need to somehow reduce the resistance of the 200,000 ohm resistor by some percentage. But I DO NOT know how to do this without either making the resistance JUST a flat 60 ohms for instance, or I either just make a map so that I get 200,050 ohms...

    I tried thinking of something like... starting with the 200K resistor, then as you move along the circuit splits to a parallel junction with 20 and 50. The junction does not connect back to one line. One end leads off from the 20, and another end leaves from the 50 ohm resistor for instance. This sort of arrangement looked good for a moment but the thing is I think that "current" or whatever is measured won't pass through the other resistor at the parallel junction, right!?? So that just becomes 200,020 or 200,050 ohms and it just adds...
    I was hoping that for instance if the 20-50 junction came back together, I think I could feasibly squeeze out the right values 60k 140k and 120k if you measured immediately after the 20 or 50 ohm resistor, and before it came back together.
  2. jcsd
  3. Feb 7, 2017 #2


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    Staff: Mentor

    How is this an RC circuit? It contains resistors only, no capacitors.

    Are you sure that the 20, 50, and 100 values are Ohms and not k Ohms? I don't see how it would be possible to have all measured results above 170 Ω, let alone in the kΩ range otherwise. Unless it's your measurements that are not correctly scaled?

    Rather than describing circuits in text form it would be much better to post sketches of your ideas. A circuit diagram conveys information more efficiently without the possibility of misinterpretation.
  4. Feb 7, 2017 #3
    I see what you mean. Maybe the resitances are 20kohm 50kohm and so. Anyway I would try first to work with a t or delta model to see wich values of recistance I get. Still I think there is something missing in the problem statement.
  5. Feb 7, 2017 #4
    Ohhhhhhhhhhhh you might be right there....

    This may just be stupidly lazy TA's at work..... I spent an hour trying these diagrams.

    Thanks both.
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