1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Kirchhoff's law problem

  1. Jan 7, 2010 #1
    1. The problem statement, all variables and given/known data

    Find I1,I2,I3

    2. Relevant equations

    Attached the circuit diagram

    2. The attempt at a solution

    I1 - I2 + I3 = 0

    -120I1 - 60I2 + 0I3 = -1.8

    0I1 -60I2 -20I3 = -1.2

    I1 = 1/150 A

    I2 = 1/60 A

    I3 = 1/100 A

    Is that correct ? The answer is my textbook is different :(
     

    Attached Files:

    • k1.jpg
      k1.jpg
      File size:
      6.5 KB
      Views:
      100
  2. jcsd
  3. Jan 7, 2010 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Masterx00! Welcome to PF! :smile:
    Nooo … :redface:
     
  4. Jan 7, 2010 #3
    Thanks :)
    I took the 1st loop clockwise and the second loop anticlockwise, so why the 1st equation is wrong then ?
    Can you please explain further ? :)
     
  5. Jan 7, 2010 #4

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Your 1st equation (I1 - I2 + I3 = 0) doesn't involve loops, it's for a vertex, so it only involves "in" or "out".

    According to the diagram, I1 is in, and both I2 and I3 are out. :smile:
     
  6. Jan 7, 2010 #5
    But, If I made it I1 - I2 - I3 = 0, I1 = 0, I2 = 0.03, I3=-0.03, that sounds correct ?
     
  7. Jan 7, 2010 #6

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    No … how did you get that? :confused:
     
  8. Jan 7, 2010 #7
    Kirchhoff's first law says that the current into a juction is equal to the juntion out. So use that on the top middle junction and have another think about that first question (remembering the directions of currents!)

    Could I also ask if the current directions were given or if you chose them?
     
  9. Jan 7, 2010 #8
    By substituting it with the other 2 equations (using the calculator eqn solver):
    I1 - I2 - I3 = 0

    -120I1 - 60I2 + 0I3 = -1.8

    0I1 -60I2 -20I3 = -1.2
     
  10. Jan 7, 2010 #9

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    No, I2 and I3 are in opposite directions.
     
  11. Jan 7, 2010 #10
    aha
    Now it gives correct answer (0.02,-0.01,0.03), I though direction of I3 was same like I1, because of the direction of the drawn battery poles :shy:
    Thanks for help tiny-tim :smile:
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook