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Homework Help: Question relating to Kirchhoff's law

  1. May 31, 2016 #1
    1. The problem statement, all variables and given/known data
    Determine the magnitudes of the currents through R1 and R2 in (Figure 1) , assuming that each battery has an internal resistance r = 1.2 Ω .
    Express your answers using two significant figures separated by commas.

    2. Relevant equations
    Junction rule: I3 = I1 + I2 (any current going into one junction must come out with the same amount)
    loop rule: The sum of all the potential differences around a complete loop is equal to zero.

    3. The attempt at a solution
    Please refer to the image I attached.
    First I set up the directions for current flow. I got the equations:

    I3 = I1 + I2.
    For the top loop, I got -18*I2 - 22*I1 +9V = 0.

    For the bottom loop, I got 6V + 18*I2 = 0. I isolated I2 to get -6/18 = -0.33333.

    Then I plugged in I2(-0.33333) into the top loop's equation to isolate I1. I got I1 = 0.6818.

    However, the masteringphysics keeps telling me it's wrong. Can anyone help me with this?

    *I've randomly tried changing up the signs of the two numbers, which didn't work so I don't know where I went wrong.

    Attached Files:

  2. jcsd
  3. May 31, 2016 #2


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    Gold Member

    So where are I1, I2, and I3, and in what direction? I'd suggest dropping I3 as you only need 2 (and you don't use it anyway)
  4. May 31, 2016 #3
    Yes haha I dropped the first equation basically since I3 wasnt needed. I1 i drew to point towards the left from the top loop. As for I2, I drew it pointing towards the right on the middle segment. I3 I drew it point towards the right from the battery on the bottom loop.

    Attached Files:

  5. May 31, 2016 #4


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    Gold Member

    So how did you work the magic of making I1 not flow through the middle resistor?
  6. May 31, 2016 #5
    Sorry, not too sure what you mean. I included 18*I2 for the first loop. Is that not the current going through resistor in the middle?
  7. May 31, 2016 #6


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    Gold Member

    Draw a full loop current in each loop. Where do you think I1 goes when it hits the node? Does it just evaporate?
  8. Jul 10, 2016 #7


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    Staff: Mentor

    The problem asks for the current magnitudes. Magnitudes are never negative.
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