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Electrical circuit porblem using ohms law

  1. Jan 18, 2013 #1
    1. The problem statement, all variables and given/known data
    Using all the following resistors : 2 x 12 ohms , 2 x 24 ohms , and 2 x 36 ohms. create a circuit where the total resistance will be 4 ohms.

    The problem states that I have to use ALL of the resistors..making it so complicated


    2. Relevant equations
    1/Req = 1/R1 + 1/R2 + 1/R3... (in parallel)
    Req + R1+R2+R3... (in series)


    3. The attempt at a solution
    The closest I got was (1/12) + (1/(24+36)) + (1/36) + (1/12) + (1/24). Is there a trick or something..

    I could have 4 parallel resistors.. 12 parallel to 12 parallel to 24 parallel to 24.
    (1/24) + (1/24) + (1/12) + (1/12) = 1/4 ... but I did not use the 2 x 36 ohms.. help will be appreciated.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jan 18, 2013 #2

    Simon Bridge

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    Welcome to PF;
    I don't think there's a trick exactly - beyond understanding the problem.
    i.e. since all the resistors are greater than 4ohms, you will need to combine a lot in parallel to reduce the overall resistance.

    Also 12, 24, and 36, are all multiples of 4... your target resistance.

    I guess you could design the circuit with a short circuit across unwanted resistors... but that is usually considered the same as not using them.
     
  4. Jan 18, 2013 #3

    phinds

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    Hm ... I tried a few combinations and I can't get it either. My attempts were hardly exhaustive, but I sure don't see it.
     
  5. Jan 19, 2013 #4

    Simon Bridge

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    3.9560Ω (the closest so far) is 4.0Ω rounded :)

    Then there is the 12s and 24s in parallel, in series with the 36's in parallel - but short-curcuit the 36s :)

    Since all values divided by 3 as well as 4 I figured looking at triples.
    12 = 4x3x1
    24 = 4x3x2
    36 = 4x3x3

    Then there is just trying every possible combination of series and parallel.
    I don't think there would be more than two series sections... there won't be that many combinations.
     
  6. Jan 19, 2013 #5
    Thank you guys, I'll just stick with the 3.9560ohms answer.
     
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