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Kirchoff's Rules- Problem

  1. Mar 26, 2009 #1
    1. The problem statement, all variables and given/known data
    In the circuit of Figure P28.28, determine the current in each resistor and the potential difference across the 200-ohm resistor.
    FigP28.28.jpg


    2. Relevant equations

    Kirchoff's Rules
    V=IR

    3. The attempt at a solution

    The way I did it was like this:
    FigP28.28d.jpg
    I set up a system of equations-
    I1 + I2 + I3 + I4 = 0
    - 40 V = I2 (80Ω) - I1 (200Ω)
    40 V - 360 V = I3 (20Ω) - I2 (80Ω)
    - 360 V - 80 V = I4 (70Ω) - I3 (20Ω)

    And I get
    I1=1.83 (for 200Ω)
    I2=4.07 (for 80Ω)
    I3=.30 (for 20Ω)
    I4=-6.2 (for 70Ω)
    Then I do V=IR to get the potential difference, which I got was 366 volts.

    But in the back of the book, it says the answers are:
    1.00 A up in 200Ω
    4.00 A up in 70 Ω
    3.00 A up in 80 Ω
    8.00 A down in 20 Ω
    200 V

    What did I do wrong
     
  2. jcsd
  3. Mar 26, 2009 #2

    Delphi51

    User Avatar
    Homework Helper

    The signs on the 40V and the 360V should be the same because they are in series - the potentials add.
     
  4. Mar 26, 2009 #3
    thanks
     
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