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!

[Circuits] Solving a KCL Problem

  1. Jan 20, 2014 #1
    1. The problem statement, all variables and given/known data

    FEoNQ4I.png

    2. Relevant equations



    3. The attempt at a solution

    sqMF18w.png

    Here is what I don't understand: how can this circuit exist? If you look at the V2 node, there are 3 currents going in but none going out. Isn't this a contradiction? From what I've heard, you can arbitrarily draw current arrows and if the direction of the arrows is wrong, you'll just get a negative current. But don't you have to at least be consistent in the rotation? If you look at the bottom left mesh, the currents are going clockwise and then there is a current that is going counter-clockwise.
     
  2. jcsd
  3. Jan 20, 2014 #2

    gneill

    User Avatar

    Staff: Mentor

    So long as you're consistent in writing the math according to your assumed currents, the math will always take care of itself and present you with the right values (positive or negative) for each.

    In fact, when writing node equations it's actually easier to maintain consistency by always assuming that all currents are flowing either into or out of a given node. That way you never trip up by forgetting which one or ones you had going in and which ones going out. Later, if you need the value of a particular current, you use solved-for node voltages and Ohm's law for a given branch.
     
  4. Jan 20, 2014 #3
    No...there isn't any contradiction .The confusion clears as soon as you calculate V2 ,which you haven't .V2 = 62.89 V which means it is higher than both 60V and V1 .So,in reality a current of magnitude 0.29A flows from V2 to 60V across the 10Ω resistor and 4.90A from V2 to V1 across 2Ω resistor .

    In other words at node V2 ,current of magnitude 5.19A enters whereas 0.29A and 4.90A leave .

    It is your assumption that all currents are flowing into node V2.But in reality that is not the case .

    Hope that helps
     
    Last edited: Jan 20, 2014
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted