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Homework Help: Kirchoff's Rule(How do I find equivalent resistance?)

  1. Apr 21, 2013 #1
    1. The problem statement, all variables and given/known data
    Please take a look at the attachment to see the circuit. My question is how do I find the equivalent resistance of this circuit? I have done the other two parts but this part has got me stumped. I also drew my currents on the picture.

    2. Relevant equations

    3. The attempt at a solution
    I attempted to categorize which resistors were in parallel and in series to each other but that just got me really confused... If anyone can walk me through this problem it would be greatly appreciated.

    Here are my loop equations:

    -9I1+3I2+9I3+8I4 = -12
    -3I3 = -12
    -6I1-4I2+6I3+6I4 = -12
    -I1+I2+I3-5I4 = -12

    I1 = 7.324841
    I2 = 0.878981
    I3 = 4
    I4 = 1.910828

    current across the 5 ohm resistor = 1.911 amps
    Vab = -8.484 volts

    Attached Files:

    Last edited: Apr 21, 2013
  2. jcsd
  3. Apr 21, 2013 #2


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    Gold Member

    Do you know how to write loop equations? You need to show some effort on your own so we know where you are stumped. Just giving the answer is against forum rules.

    EDIT: "walking you through it" is equivalent to giving the answer. We're more into hints.
  4. Apr 21, 2013 #3
    yes, I know how to write loop equations. However, I thought the equivalent resistance was just adding up the resistances so I didn't think I would need to show my loop equations. I have edited my first post with the loop equations and edited picture.
    Last edited: Apr 21, 2013
  5. Apr 21, 2013 #4


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    Gold Member

    Since you have the current being drawn out of the 12V supply, you have the equivalent resistance, yes?

    The problem is that "adding up the resistances" just doesn't work unless everything is simple series/parallel combinations and you've already figured out that that is not the case with this circuit else you would have been able to do it.
  6. Apr 21, 2013 #5
    oh my gosh, I cannot believe I didn't see it. Thank you for the help!
    Last edited: Apr 21, 2013
  7. Apr 22, 2013 #6


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    Staff: Mentor

    Actually, if you note that there are "missing" resistors on the diagonals you might see that direct simplification is quite possible. Easy in fact :smile:


    Attached Files:

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