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Homework Help: Find the total resistance

  1. May 12, 2013 #1
    1. The problem statement, all variables and given/known data

    a) Each box has the restistance 1 Ohm. Find the total resistance between 1) A-B 2) A-D 3) B-C
    b) Find I (electric current(?)) from A-D if you put a battery from B-C
    c) Find I on the Ammeter and the direction (the lower picture)

    a) Use Kircchofs 2nd law on ABED to show that I3 ) 0,5A
    b) Explain how I2 + I5 = 2A. Use Kirchhoffs 2nd law on BEFC to find another equation for uknown I2+I5
    c) Find I1 and I4
    d) Find the resistance R.

    2. Relevant equations

    Kirchoff's circuit law is relevant

    3. The attempt at a solution
    a) 1) Thought it would be just 1 ohm but it's supposed to be 0,63 2) I used 1/Rtotal = 1/R + 1/R + R which means 2+1 = 3 which is wrong while it's supposed to be 0,5.
    This makes the rest pretty complicated to solve.

    I used Kirchoffs law and found following formulas but not sure if it's supposed to be solved as
    5 unknowns while we've only been into 3.
  2. jcsd
  3. May 12, 2013 #2


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    Homework Helper

    Welcome to PF!

    Hi MJay! Welcome to PF! :smile:
    You need to use all the paths from A to B :wink:
    erm :redface:

    calculate it again! :smile:
  4. May 12, 2013 #3
    Hey thanks :)

    I tried a different method now:

    1/R = 1/(AC+CD) + 1/AD
    which gives 2/3 then I insert it in 1/R= 1/(2/3) + 1/(AB+BD) Am I on the right way?
  5. May 12, 2013 #4


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    Hi MJay! :wink:
    (this is part 2)

    Correct :smile:

    (But your notation is terrible, and I suspect is misleading you …

    you can't just call everything R, and expect not to make mistakes :redface:)

    think again :smile:
  6. May 12, 2013 #5
    Yeah understand but it get's so messy while I'm doing this in the way we learned it in school which might get confusing also, I'm not sure what way I should put it. But I have no clue or book to help me out, as this is the toughest exercise (marked with 2**).
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