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Kirchhoff's Rules: Circuit Question

  1. Apr 7, 2014 #1
    1. The problem statement, all variables and given/known data
    If R1 is 3Ω, R2 is 6Ω and R3 is 9Ω, what are the three currents in figure 16-4, in which the batteries are 6.0v and 12.0v?


    2. Relevant equations
    I1+I2+I3


    3. The attempt at a solution

    I've got three equations so far

    I worked out

    +6-3I1-9I2=0

    and

    +12-6+6I2-9I=0

    For the next step I have to find I1 in terms of I2 but I'm not sure how I do that

    http://puu.sh/7ZXv9.png [Broken]
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Apr 7, 2014 #2

    SammyS

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    Your subscripts are inconsistent with you equations and/or the figure.

    Also, I1+I2+I3 is not an equation. Perhaps you meant I1+I2=I3
     
    Last edited by a moderator: May 6, 2017
  4. Apr 7, 2014 #3
    youre right i meant to put I1+I2+I3=0 and

    +12-6+6I2-9I1=0
     
  5. Apr 7, 2014 #4

    SammyS

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    12 - 6 + 6I2 - 9I1 = 0 is correct.

    For I1+I2+I3=0 to be correct, you should draw I3 in the opposite direction.

    You need to correct the other loop equation.
     
  6. Apr 8, 2014 #5
    +6-3I1-9I2=0

    is wrong?

    I think i was getting mixed up over some wording in my workbook. I've got

    2-3I2=I1 now and i'll substitute in the third equation
     
    Last edited: Apr 8, 2014
  7. Apr 8, 2014 #6

    NascentOxygen

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    It looks wrong. Where did it spring from?

    Where did that spring from? If you'd explain the laws you applied to arrive at these, it would make the process of checking your steps easier, ditto for finding mistakes.

    Perhaps you have some currents and/or resistors labelled differently to how you think you have?
     
    Last edited: Apr 8, 2014
  8. Apr 8, 2014 #7
    the question was "If R1 is 3Ω, R2 is 6Ω and R3 is 9Ω, what are the three currents in figure 16-4, in which the batteries are 6.0v and 12.0v?"

    and heres the solution, might make it more clear to you what im missing

    http://puu.sh/80On3.jpg [Broken]

    http://puu.sh/80Oo1.jpg [Broken]
     
    Last edited by a moderator: May 6, 2017
  9. Apr 8, 2014 #8
    The problem we're having is that the signs and indices in your currents and/or resistors in the equations don't seem to match what we see in the picture. Your first job is to fix that. Than we can talk about how to solve the equations.
     
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