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!

Quick Question about KCL

  1. Sep 14, 2014 #1

    Zondrina

    User Avatar
    Homework Helper

    1. The problem statement, all variables and given/known data

    Find ##I_x## and ##I_1## in the following circuit:

    Screen Shot 2014-09-14 at 11.24.24 AM.png

    2. Relevant equations

    ##\sum I = 0##

    3. The attempt at a solution

    I want to know if I'm actually envisioning these currents correctly.

    Applying KCL to the upper left node:

    ##(1) \quad 6 mA - I_1 - I_x = 0##

    Applying KCL to the bottom left node:

    ##(2) \quad I_x + 1 mA + 1.5 I_x - 6 mA = 0##

    Right away now I see that ##(2) \Rightarrow 2.5 I_x - 5 mA = 0 \Rightarrow I_x = 2 mA##.

    Subbing into ##(1)## I see that ##I_1 = 4 mA##.

    Am I applying those properly ^? Why did I not need the other nodes at all? Is this because there are four equations and only two unknowns?

    Would these be the correct equations for the upper right node and bottom right node respectively?

    ##(3) \quad I_1 - 1 mA - 1.5I_x = 0## <- Using this with ##(2)## gives the same answer.

    ##(4) \quad 1 mA + 1.5I_x = 0## <- This does not make any sense and I'm not quite sure why. Is there current flowing out of this node?
     
  2. jcsd
  3. Sep 14, 2014 #2

    berkeman

    User Avatar

    Staff: Mentor

    Your first equation is fine, and the rest may be fine too, but I got vertigo when you chose the bottom right node for your second equation. A more typical approach would be to call the bottom node ground, and write your second KCL equation for the top right node. Do you get the same answer doing it that way?
     
  4. Sep 14, 2014 #3

    Zondrina

    User Avatar
    Homework Helper

    Indeed ##(3)## and ##(2)## in combination give the same answer as ##(1)## and ##(2)##.

    So the bottom right node would be a reference to ground?
     
  5. Sep 14, 2014 #4

    berkeman

    User Avatar

    Staff: Mentor

    The whole bottom line is one node, and yes, normally I would label it as ground.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Quick Question about KCL
  1. KCL Question (Replies: 8)

Loading...