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Hello everyone, having troulbes applying Kirchoff's Rule~DC circuits

  1. Oct 21, 2005 #1
    Hello everyone, this is my first problem dealing with Kirchoffs Rule and I thought I did it right...here is the circuit...
    http://www.webassign.net/hrw/hrw7_27-40.gif
    the info is the following:
    In Figure 27-40, the resistances are R1 = 1 , R2 = 2.2 , and the ideal batteries have emfs E1 = 2.0 V, and E2 = 3 = 3 V.


    Fig. 27-40

    (a) What is the current through each battery? (Take upward to be positive.)
    battery 1 A
    battery 2 A
    battery 3 A

    (b) What is the potential difference Va - Vb?
    V

    I came out with the following 3 equations:
    E1 + (-R1)(I_1) + R2*I_2 - E2 -R1*I_1;
    -R2*I_2 + R1*I_3 - E3 + R1*I_3 + E2;
    I_1 + I_2 + I_3 = 0;

    When i solve for I_1 = -I_2 -I_3
    Where do I plug I_1 into? i tried the first equation and i couldn't solve for any I, and i can't for the 2nd, because there is no I_1, did i do this right? thanks!
     
  2. jcsd
  3. Oct 21, 2005 #2

    Fermat

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    Substitute for I_2 = -I_1 - I_3 into both the first two eqns, since it is common to both.
    Then you will have two eqns in two unknowns.
     
  4. Oct 22, 2005 #3
    Thanks! that worked out great, but I messed somewhere writing out the 2nd circuit on the right....This is the equation i got and the work:
    http://img477.imageshack.us/img477/956/lastscan6lq.jpg
    its wrong....
    The first one, i did, on the left, was right though. Any help would be great.
     
  5. Oct 23, 2005 #4

    Fermat

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    On your last line - in the attachment - the -R1.I2 should be +R1.I3.

    It looks like this mistake was copied through from the line before it.
     
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