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!

Circuit source current question

  1. Sep 13, 2014 #1
    1. The problem statement, all variables and given/known data
    Find is of the circuit below

    2. Relevant equations
    i=V/R
    v1+v2+..=0
    is=vs/Req


    3. The attempt at a solution
    Resistance of circuit as seen by source = 7kΩ
    6V(source)/7kΩ(Req)=.857i

    My question is, how come i3 is also 0.857i? Shouldn't it be different since it just went through two resistors? Can someone explain to me why the currents are the same? Is it because the resistances are in series?
     

    Attached Files:

  2. jcsd
  3. Sep 13, 2014 #2

    gneill

    User Avatar

    Staff: Mentor

    is alone leaves the voltage source, while i3 alone re-enters the voltage source. You can't have one different than the other through the same component...
     
  4. Sep 13, 2014 #3
    If it had another current source or voltage source below one of the resistors, would this still be true?
     
  5. Sep 13, 2014 #4

    gneill

    User Avatar

    Staff: Mentor

    There is no way that the current entering a component can be different from the current leaving it. Otherwise charge would be building up on the component or be created or destroyed. Can't happen! Components in series all carry exactly the same current.

    If you change the circuit so that i3 is no longer solely feeding the one voltage source but has some other path available (so it feeds into a node shared with the voltage source and something else, perhaps leading to another voltage source), then you no longer have the same setup and that other "something" may contribute to the total current making its way to the voltage source.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted