1. Not finding help here? Sign up for a free 30min 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!

Using Kirchhoff's laws to find current in a circuit

  1. Sep 4, 2012 #1
    1. The problem statement, all variables and given/known data
    Use KCL and KVL to determine the currents I1 to I3 in the circuit below:

    aodth.png
    2. Relevant equations

    Ʃ[itex]v_{n}[/itex] = 0
    Ʃ[itex]i_{n}[/itex] = 0
    v=ir


    3. The attempt at a solution
    I got two different answers for [itex]I_{1}[/itex] using the two different laws. I'm not sure if I'm doing it right.

    KVL @ loop 1: -18 + 6 + 8[itex]I_{1}[/itex] = 0
    8 [itex]I_{1}[/itex] = 12
    [itex]I_{1}[/itex] = 1.5A


    KCL @ node a: 3 = [itex]I_{1}[/itex] + [itex]I_{2}[/itex] + 1
    KCL @ node b: [itex]I_{3}[/itex] = 1 + [itex]I_{2}[/itex]



    0 = [itex]I_{1}[/itex] + [itex]I_{2}[/itex] -2
    0 = -1 - [itex]I_{2}[/itex] + [itex]I_{3}[/itex]
    1 = [itex]I_{3}[/itex]
    0 = [itex]I_{2}[/itex]
    2 = [itex]I_{1}[/itex]
     
  2. jcsd
  3. Sep 4, 2012 #2

    berkeman

    User Avatar

    Staff: Mentor

    Your KVL solution for I1 is correct. For the KCL solution, you have 3 unknowns but you have only written 2 equations. That isn't sufficient to solve for I1 yet...
     
  4. Sep 4, 2012 #3
    Could I substitute the value that I got for I1 from the KVL equation into KCL?
    I wasn't sure if I could do that because I thought the question was asking me to find the values separately using the two different laws.
     
  5. Sep 4, 2012 #4

    berkeman

    User Avatar

    Staff: Mentor

    It does sound like they want you to solve the circuit with each technique separately. It's a pretty easy circuit to solve by inspection anyway, so they must want you to go through all the steps of each technique separately for practice.

    So can you see another node where you could write a 3rd KCL equation?
     
  6. Sep 4, 2012 #5
    The one between the 8 ohm and 4 ohm resistor?
     
  7. Sep 4, 2012 #6

    berkeman

    User Avatar

    Staff: Mentor

    That might work...
     
  8. Sep 4, 2012 #7
    nvm I figured it out, thanks for the help.
     
    Last edited: Sep 4, 2012
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook