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Homework Help: Multi-loop circuit (kirchoffs laws)

  1. Jan 29, 2008 #1
    1. The problem statement, all variables and given/known data

    The circuit is attached, i want to know what I4 is ( Answer 15mA ). Been working on this problem for 3h and i always end up when i try to multiply the invert matrix "A" with the matrix "B" then my calculator says "It´s not invertable"..

    2. Relevant equations

    Kirchoffs Laws

    3. The attempt at a solution

    junction equations = 2
    loop equations = 4

    1: I1+I2-I3=0
    2: I1+I6-I5=0
    a: R*I3-R*I4=6
    b: -R*I1-R*I3=-12
    c: R*I4+R*I6=6
    d: R*I3+R*I6=12


    Matrix A:

    6x6
    1 1 -1 0 0 0
    1 0 0 0 -1 1
    0 0 100 -100 0 0
    -100 0 -100 0 0 0
    0 0 0 -100 0 -100
    0 0 100 0 0 100

    Matrix B:

    6x1

    0
    0
    6
    -12
    6
    12
     

    Attached Files:

  2. jcsd
  3. Jan 29, 2008 #2

    pam

    User Avatar

    It's easier to do with three loops. This gives 3 eqs in 3 unks.
    Call the loop currents A,B,C. Then the loop equations are
    12-100A-100(A-B)=0
    -6-100(B-C)-100(B-A)=0
    -6-100(C-B)-100C=0
     
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