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Node analysis help

  1. Mar 12, 2013 #1
    1. The problem statement, all variables and given/known data
    2zibyc9.png

    2. Relevant equations
    3. The attempt at a solution

    At node 1: (V1-10)/5 -2 + (V1-V2)/4 = 0 --> 9/20V1 - V2/4 = 2
    At node 2: (V2-V1)/4 + (V2 - 4*I)/6 + V2/2 = 0
    and i use also I= V2/2

    i get V1=5.83V and V2=2.5V but its not the rite answer. what am i diong wrong??
     
    Last edited: Mar 12, 2013
  2. jcsd
  3. Mar 12, 2013 #2

    CWatters

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    I've only looked at your equation for node 1 but you don't appear to have been consistent with the sign.

    If I define current into the node as +ve I get..

    (10-V1)/4 + 2A + (V2-V1)/4 = 0
     
  4. Mar 12, 2013 #3

    berkeman

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    Also you should be more careful with your parens -- they are misplaced in at least 2 places...
     
  5. Mar 12, 2013 #4
    its the same eqtn as mine...you just have eq current going into node but i have current going out. so, i use -2A because current going into node. if it was gonig out it would be +2
     
    Last edited by a moderator: Mar 12, 2013
  6. Mar 12, 2013 #5

    berkeman

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    Thanks for fixing the parens in your original post.

    Could you show your work on the simultaneous equations? I agree with your fixed equations, but I get different (simpler) answers...
     
  7. Mar 12, 2013 #6
    i always meant it like that but i got lazy with parenths.

    equation 1 i reduced to:
    (9/20)V1 - V2/4 = 2
    9V1-5V2=40

    equation 2, substitute I=(V2/2):
    (V2-V1)/4 + (V2-4(V2/2))/6 + V2/2 = 0
    V2/4 - V2/6 + V2/2 - V1/4 = 0
    (7/12)V2 = V1/4
    V1 = (7/3)V2

    substitute V1 into eq 1:
    9((7/3)V2)-5V2=40
    21V2-5V2=40
    16V2=40

    V2=2.5V
    V1= (7/3)(2.5)= 5.833V
     
    Last edited: Mar 12, 2013
  8. Mar 12, 2013 #7

    berkeman

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    Can you show how you got that reduction for equation 1? There is an error in your result. You will get V1 = X*V2 for the result...
     
  9. Mar 12, 2013 #8
    (V1-10)/5 -2 + (v1-v2)/4 = 0

    V1/5 -2 -2 + v1/4 -v2/4 = 0
    (9/20)V1 - V2/4 = 4
    9V1 - 5V2=80

    hence
    16V2=80
    V2=5V
    V1=11.6V

    that is the rite answer. and sorry i made an error, forgot to add that 2 to my 1st eq >.>
     
  10. Mar 12, 2013 #9

    berkeman

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    Great! :smile:
     
  11. Mar 13, 2013 #10

    CWatters

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    Perhaps I made a mistake but last night I thought you had written +2A not -2A.
     
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