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I don't understand why the node voltage here is zero(this is for circuits class)

  1. Sep 17, 2011 #1
    My teacher was going over a thevenin and norton circuit and i understand the thevenin and norton part of it, but the part that confused me was when he did the node voltage at the 2 circled nodes to find Ix. I understand why the bottom node would be zero because it is the reference node but why is the top one zero. I have uploaded a picture of the notes and circled the node where i need help with. Since its connected to Vt wouldn't it have some voltage?
     

    Attached Files:

  2. jcsd
  3. Sep 17, 2011 #2
    Haven't you shown both nodes connected directly together?
     
  4. Sep 17, 2011 #3
    the circuit can be redrawn like this. Can you explain why the voltage at the node is zero and why the current running through the resistor is zero?
     

    Attached Files:

  5. Sep 17, 2011 #4
    can anybody help me?
     
  6. Sep 18, 2011 #5
    Are you sure that bottom one is reference node?

    Can you please post a full problem. I have little to work with here, and I cannot see everything clearly. Post the original schematics with variables etc.
     
  7. Sep 18, 2011 #6

    uart

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    If the "bottom node" is the reference and if you remove the apparent short circuit that you had in you first diagram then yes you are correct, the voltage at that node is not zero (unless VT=0). It is in fact [itex]4 V_T /19[/itex].
     
  8. Sep 18, 2011 #7

    jim hardy

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    how sure are you about your sketch?

    i observe Vt's polarity is + up.
    I observe the current source's polarity is + right so they oppose.
    I cant read what is value of current source - is it 2ix ?

    If so, write KVL around circuit

    start at bottom of vvoltage source go ccw

    -Vt + 5i5 +4ix = 0
    ;
    ;
    and KCL at upper circled node

    i5 -ix -2ix = 0
    which yields: i5=3ix
    ;
    ;
    plug into KVL equation

    -Vt +5(3ix) +4ix = 0

    Vt = 19ix
    ix = Vt/19

    what uart said

    i used to make lots of mistakes from less than meticulous handwriting so i reverted to printing.

    I suggest something is not transcribed correctly.
    You need to work KVL and KCL until they become as natural to you as a Crescent wrench and you have confidence in them.
    They will never lie, but one must be meticulous.

    Believe me, i have earned the right to preach that message. "Sinners make good saints for they've learned the hard way"

    what if your current source were 2i5 instead of 2ix ?

    What if the question is to find value of current source that makes your node in question be zero?

    check with some other students to verify you are answering right question, then show instructor what KVL and KCL have to say..be prepared, solve for the value of current source that WOULD make ix zero and show him.

    old jim
     
    Last edited: Sep 18, 2011
  9. Sep 18, 2011 #8
    This question is well above high school level since it appears to involve a controlled current source.

    We normally expect some sign of effort on the part of the student, is that evident here?
     
  10. Sep 18, 2011 #9
    had the same feeling.
     
  11. Sep 18, 2011 #10

    jim hardy

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    that's a big ten-oops there, good buddies.
     
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