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: Kirchoff's Rules help

  1. Oct 5, 2009 #1
    1. The problem statement, all variables and given/known data

    I am trying to calculate the currents on this 2 power source circuit using Kirchoff's Rules.

    http://img101.imageshack.us/img101/4151/kirchoffc1.jpg [Broken]

    Ignore the numbers on the lines connecting the components and their length

    2. Relevant equations



    3. The attempt at a solution

    I've got loop 1 on the leaf and loop 2 on the right.

    Loop1: 6+(-8*I#2)+(6*I#1)=0

    Loop2: 12+(-1*I#3)+(-8*I#2)=0

    This does not seem right though. Could someone take a look at the circuit and help me with the solution. For some reason I am having much difficulty using Kirchoff's Rules.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Oct 5, 2009 #2


    User Avatar
    Homework Helper

    Show with arrows the estimated current direction in each branch of the network. Go round each loop and add the potential differences along the path.

  4. Oct 5, 2009 #3
    I do not use the loop rule much directly. you usually get the direction
    of some of the currents wrong (as you did here).

    It's usually better assign a potential of 0 to some point of the circuit,
    usually on the negative side of the voltage source, and then find any
    other potentials you can, and assign unknowns for the one you can't

    Then work out all of the currents, and use Kirchhof's current rule in all
    the points with an unknown potential.

    In this case I assign a potential of 0 to point 4. Point 1 must then be at
    6 V, and point 2 at 18 V.
    Assign an unknown potential of V to point 3.
    The current in R2 is I1 = (18-V)/2
    The current in R1 is I2 = ........
    THe current in R3 is I3 = ........
    you can take all currents in the direction away from point 3.
    Now you can find V with I1+I2+I3 = 0.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook