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Finding the currents in this system

  1. May 15, 2009 #1
    given the following:
    http://lh6.ggpht.com/_H4Iz7SmBrbk/Sg04JIIg9vI/AAAAAAAAA-o/xDycbG90Gmc/s720/C.jpg [Broken]

    and given that

    ε1=3V
    ε2=2V
    R1=1Ω
    R2=2Ω
    R3=3Ω

    i am asked to find the current flowing throiught R1,R2, R3

    what i did was

    I3=I2+I1
    I1R1-I2R21

    but i am short of one equation to solve this, what i would like to do is somehow find R12, as an the sum of R1 and R2, but my problem is that i have ε1 between them and i have no idea how to tackle this,

    had ε1 noot been there i would have said R12=(1/R1+1/R2)-1 and then i could make a third equation using this.

    how can i solve this?
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 15, 2009 #2

    tiny-tim

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    Hi Dell! :smile:

    Stop trying to simplify it … it's simple enough already! :rolleyes:

    There are three loops, of which two are independent …

    you've already applied Kirchhoff to the R1R2-loop, so now just apply it to the R2R3-loop. :wink:
     
  4. May 15, 2009 #3
    thanks,.. for the second part of the question they ask what the difference in voltage is from pioint A which lies between R3 and ε2, and point B which is between R1 and ε1,

    can i use kirchoff here too, and igonre the part with R2 and R3, so I1R1=ε1-v
    v=ε1-I1R1=2.8

    ?
     
  5. May 15, 2009 #4

    tiny-tim

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    sorry, I don't understand that :confused: … and what happend to E2?

    You need the potential difference across E2 and across R1 (or across E1 and across R3):smile:
     
  6. May 15, 2009 #5
    sorry meant to be v=ε2-I1R1=2-0.2=1.8,

    but is it okay to just ignore the whole part that includes R2 and I2??
     
  7. May 15, 2009 #6

    tiny-tim

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    Yup … Kirchhoff applies to only one loop at a time! :smile:
     
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