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KCL or MESH problem

  1. Nov 9, 2008 #1
    1. The problem statement, all variables and given/known data

    [​IMG]

    Determine I_1 I_2 I_3
    When
    E = 15V
    R1 = 20k ohms
    R2 = 4.7k ohms
    R3 = 10k ohms
    2. Relevant equations

    KCL equation

    3. The attempt at a solution

    i tried to find using KCL.. is my equation right?
    Loop equation 1:
    EB + I3R3 + R1(I3 + I1) = 0

    Loop equation 2:
    R1(I1 + I3) + R2(I1 – I2) = 0

    Loop equation 3:
    EB + I3R3 + I2R2 = 0

    can i use MESH analysis... but there are three unknowns and if i use mesh, i can only have 2 equations with 3 unknowns
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 9, 2008 #2

    Redbelly98

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    Staff Emeritus
    Science Advisor
    Homework Helper

    Welcome to PF.

    There is a problem with your equations.

    The voltage across R1 is is simply R1*I1, not R1(I1 + I3), because the current flowing through R1 is simply I1.

    Likewise, the voltage across R2 is R2*I2.

    Also, these are KVL equations for voltage, not KCL which would apply to currents. KCL says that the sum of all currents entering a node is zero, and will be useful here.

    Note, another way to solve this would be to use the rules for combining resistors in parallel and series.
     
  4. Nov 10, 2008 #3
    I agree with Redbelly98, I think you're confusing KVL for KCL. See if you can combine the resistors to find the current coming out of the supply. From there, I think it's easier if you find the Norton equivalent of the ckt, and then use current divider equations. See link for quick wikipedia articles on them. Mesh analysis will work just fine here as well, but I think it's more obvious using Norton equivalent.
     
  5. Dec 15, 2008 #4
    u can apply kvl at the 2mishes and get 2equa., in 2variables .then solve them
     
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