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: Kirchhoff and Potentials question.

  1. Aug 24, 2008 #1
    Excuse me but, this is a rough translaton from a Portuguese homework.
    Also, excuse me for my lack of knowledge in some physics, I do a programming course and one of the teachers said we had to know about this.
    1. The problem statement, all variables and given/known data
    http://img181.imageshack.us/img181/7084/87202322av0.jpg [Broken]
    http://g.imageshack.us/g.php?h=181&i=87202322av0.jpg [Broken]

    Taking into account the circuit above, answer these questions:
    a) What is the value of i1, i2, i3
    b) Calculate the potentials Va, Vb, Vc, Vd, Ve and Vo taking in account the ground (sorry I don't know the correct term, it's the value on the right.

    2. Relevant equations
    Kirchhoff law i1 = i2 + i3
    V = RI
    U = V - RI

    3. The attempt at a solution
    This teacher is known for being nasty with the students and he gave us this homework without any explanation only saying that we'd need to know about the Kirchhoff laws.
    I only found about you guys today, and this homework is due tomorrow (it's worth the grades for the bimester)
    So if possible, help! I'm reading a lot of books ATM and will keep updating here 'til the time to deliver the work.

    Thanks in advance.
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Aug 24, 2008 #2


    User Avatar
    Homework Helper

    Hmm, I'd wager that fish don't get thirsty at all.

    Anyway, as you said you have to use KCL and KVL. Write a loop KVL equation for I1, I2, I3 around the left circular mesh and one for the right mesh, plus KCL. Then solve for the unknowns.

    As for the potentials, note that you didn't specify which point in the circuit it taken to be ground reference; your picture shows the ground node in the middle of the wires and not connected to them.
  4. Aug 24, 2008 #3
    That'About the ground reference, that's how it is on paper.
    There is a third question that asks what would happen if it changed to Vo.

    Btw Co is should've been Vo. Sorry for the mistake.
  5. Aug 24, 2008 #4


    User Avatar
    Homework Helper

    Well, assuming Co is the ground potential, then you should use some nodal analysis to get the currents. Forget about KVL for now and the mesh analysis earlier, write a node current equation for every node Va, Vb, Vc, Ve. Note that by KCL, current flowing into a node = current flowing out of it.
  6. Aug 24, 2008 #5
    Sorry, but what does KVL and KCL mean? I think I'm lost in translation here.
    I got the part that in = out. I'm having problems finding out which equation I should use for the nodes. Should I do something like U = V ± R . I ?
    If so, how should I put it from Va to Vb and from Vb to Vc ? U = V ± R . I1 + I2 + I3 ?
    Thanks for the help!
    I know very little physics though and since school ended (quite long ago) I haven't looked into it much.
  7. Aug 25, 2008 #6


    User Avatar
    Homework Helper

    KVL = Kirchoff's voltage law
    KCL = Kirchoff's current law

    You need to write an equation for the current flowing out and into each and every node. For example, the current flowing from Va to Vb across the 6k resistor is given by [tex]\frac{V_a - V_b}{6k}[/tex]; current from Vb to Va in the opposite direction along the same path is given by [tex]-\frac{V_a - V_b}{6k}[/tex].
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook