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Two methods -- neither works, thevenin

  1. May 27, 2015 #1
    1. The problem statement, all variables and given/known data
    we are asked to find the tension between the R6 = 20 ohms resistor, using thevenin

    2015-05-27 19.52.02.jpg


    2. Relevant equations
    the given solution for Ethev is 27.78

    3. The attempt at a solution
    is seen in the pictures below

    thank i really appreciate your help as it gets frustrating not to know what is wrong . 2015-05-27 19.52.41.jpg
    i tried a different approach and got this but still wrong
    thank you again. 2015-05-27 20.19.10.jpg
     

    Attached Files:

  2. jcsd
  3. May 27, 2015 #2

    gneill

    User Avatar

    Staff: Mentor

    Hi potatowhisperer, Welcome to Physics Forums.

    One of the nice things about Thevenenin's and Norton's theorems is that you can apply them step-wise. You can "break" a circuit at successive locations, incorporating components into an evolving Thevenin or Norton model as you go. Often this can be easier than performing a mesh or nodal analysis of the entire circuit. Consider the suggested circuit "breaks" here:

    Fig1.gif

    At break (A) you have a simple voltage divider to the left. So a Thevenin equivalent is trivial to find. At (B) an "open" resistance R3 is tacked on, modifying the Thevenin resistance. Simple! Then at (C) the then-current Thevenin model is part of a voltage divider with R5, so again a trivial reduction...

    Give it a try!
     
  4. May 27, 2015 #3
    Bon jour, potatowhisperer,

    Your first solution, the nodal analysis, is ok until the very end. You did something wrong when you used your matrix solver.

    ?temp_hash=852ab311d37be5847cf92a2503f323ba.png

    You must have made some such error.
     

    Attached Files:

  5. May 27, 2015 #4

    NascentOxygen

    User Avatar

    Staff: Mentor

    Your answer is right, but that's for VA. You need to go further and determine V2.
     
  6. May 28, 2015 #5
    I love y'all
    i m glad that the first method was just a calculation mistake , though i am embarrassed to have taken your time just because of a sign, i really appreciate your help again.
    about the second method , i can not thank you enough for telling that it was unfinished calculations , as i was confused cause now i can think more logically about what i m doing , i have had many instances when i got so frustrated with my mistakes that i would just stop ........ so thank you very much.
     
  7. May 28, 2015 #6
    GNEILL,I am definitely going to try that method, it does simplify things , A LOT .
     
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