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Homework Help: Kirchhoff's Rules and Systems of Equations

  1. Feb 10, 2007 #1
    1. The problem statement, all variables and given/known data
    To find the current traveling through a circuit with parallel and series connections. The circuit looks something like this.

    *Right now my webserver is giving me fits and won't let me login. I will try and get a picture to those who cannot understand what I am about to describe below, and with my poor description skills, will probably be everyone.

    It's a series/parallel circuit. First there is R1(97.8), and then the circuit splits into parallel with R3(553) on the outside and R2(325) on the inside, and then continues around to a battery that has a measured voltage of 2.62 volts.

    We measured the current in the lab, so we have the answers. The problem is, were supposed to use systems of equations to discover the answer as if we didn't measure it. IO have tried my best to do something about, and these are the equations I have come up with (we were only told to use 3, not the 4th for the inner current) and tried solving for each individual variable to no avail. This is what I've done.

    2. Relevant equations

    Outer current
    (1) V-R1I1 - R3I3 = 0
    Parallel Loop Current (Inside parallel circuit from R3 to R2)
    (2) -R3I3 + R2I2 = 0
    Junction Rule
    (3) I1 = I2 +I3

    I'm pretty sure my equations are correct.

    I substituted I1 in equation 3 into equation 1 to get

    V-R1(I2 + I3) - R3I3 = 0
    Then I solved for I2
    -R1I2 - R1I3 - R3I3 = -V
    -R1I2 - I3(R1 + R3) = -V
    -V + I3(R1 + R3) = -R1I2
    (Divide by R1)

    -V + I3(R3) = - I2

    I2 = V - R3I3

    Then I substituted what I got for I2 into equation 2

    -R3I3 + R2(V-R3I3) = 0

    -R3I3 - R2V - R2R3I3 = 0

    -R3I3 - R2R3I3 = R2V

    -I3(R3 + R2R3) = R2V
    Divide by R3 + R2R3 yields

    -I3 = V/2R3

    which gives me something similar to the current I measured for I3, but 1 decimal off.

    Measured currents are below

    I1 = 7.43 mA
    I2 = 24.3 mA
    I3 = 24 mA

    Where did I go wrong? When I plug in the answers they don't come anywhere close to what I measured across the circuit. I'm pretty sure we measured the current correctly. :/
    Last edited: Feb 10, 2007
  2. jcsd
  3. Feb 10, 2007 #2


    User Avatar

    You went wrong here:
    R1 + R3 divided by R1 is not R3.
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