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: Nodal analysis with dependent voltage source

  1. Dec 12, 2015 #1
    1. The problem statement, all variables and given/known data

    In attached image. Just curious as to how their nodal analysis is correct.

    2. Relevant equations

    ##I_{in} = I_{out}##

    3. The attempt at a solution

    Solution in image.

    I am just not sure how they applied nodal analysis here to find that ##I_s = \frac{12}{12} + \frac{12}{6} + \frac{12-8}{4}##. If I'm not mistaken, they are just finding V/R, correct? But isn't the voltage in ALL the resistors dependent in some way on both the 12 V source attached and also the dependent voltage source? Why does it seem like they only included the dependent voltage only on the 4-ohm resistor? Is their solution correct?

    Attached Files:

  2. jcsd
  3. Dec 12, 2015 #2


    User Avatar

    Staff: Mentor

    Their solution is correct. If the applied independent source were changed to another value all the currents would be affected by the change, yet the resulting ratio of V/Is would turn out the same, yielding the same Rth.

    Perhaps you have some doubt about how the node equation itself was written?
  4. Dec 13, 2015 #3
    I guess I'm just not entirely understanding why the dependent voltage source is not considered in these equations for the other 2 resistors. Doesn't this voltage source affect the true voltage across the 6- and 12-ohm resistor, thus requiring this to be included when analyzing the current passing through these resistors? It seems to me that they are considering the voltage through the 6- and 12-ohm resistor to be simply 12 V, but that doesn't appear obvious to me since there is more than one voltage source in this entire circuit.
  5. Dec 13, 2015 #4


    User Avatar

    Staff: Mentor

    The dependent source is not connected to the node where the 6 and 12 Ohm resistors connect; The external source is. So the external source must set their currents. Nothing can alter the potential difference imposed by a fixed voltage source and in this case the 12 V source is wired directly across those resistors.
  6. Dec 13, 2015 #5
    So referencing node b as 0, that makes perfect sense! I was blind, and now I see! :P Thanks!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted