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: Draw a Circuit based on the info and find the voltage values

  1. Jan 23, 2014 #1
    1. The problem statement, all variables and given/known data
    Consider a circuit containing four nodes
    labeled a, b, c, and d. Also, we know that
    vab= 12V,vcb=−4V,and vda= 8V. Deter-
    mine the values of vac and vcd. (Hint: Draw
    a picture showing the nodes and the known

    2. Relevant equations
    ∑ voltages in a loop = 0

    3. The attempt at a solution

    I had no idea how to approach the problem, just a couple ideas. I tried sketching this out:


    This one didn't make much sense to me since applying the loop rule, I get differing answers for ##v_{ac}##

    So I tried a few other ways to draw it and this was the closest I could get:


    This seems like it makes more sense, but I'm not entirely sure. Am I on the right track?
  2. jcsd
  3. Jan 23, 2014 #2


    User Avatar
    Homework Helper
    Gold Member

    I recommend the first thing to change is you need to change your voltage symbol to indicate some sort of polarity. They blue squares alone are not sufficient. Either use a symbol with obvious polarity such as a battery symbol, or (and this is just as good too) label the '+' and '-' signs.

    What I mean by that is if you look at the 8 V element in the upper left. Imagine, hypothetically, flipping the polarity of that element, making it VAD = 8 V. If you did so, it wouldn't look any different than the way it is now with VDA = 8 V. You need some way of indicating the polarity. You need to know by looking at your picture that if VDA = 8 V, then VAD = -8 V

    Secondly, don't draw that stuff in the middle. It's not necessary. Just keep it as a square if you wish (or even three sides of a square), with nothing in the center. Note that VAC = VAB + VBC
  4. Jan 24, 2014 #3


    User Avatar
    Homework Helper

    It is easier to work with potentials. If Vab means the potential of b with respect to a : Vab=Vb-Va=12 V. Write 0 at point a and 12 V at point b. Draw a battery between the nodes to ensure the given potential difference.
    Draw the batteries and determine the potential of each node. Calculate the potential differences Vac=Vc-Va and Vcd=Vd-Vc.

    You can connect a resistor between two nodes and an other one between the other two nodes, the potential differences would not change.


    Attached Files:

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted