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Nodal analysis, choosing reference node confusion

  1. Sep 13, 2015 #1
    I know that when we have to identify the nodes for solving a circuit with nodal analysis we always have to designate a reference node (which will be the ground node) as a reference for measuring the others' voltages. However I was practicing this and I found this example on http://mathonweb.com/help/backgd5b.htm:
    https://scontent-atl3-1.xx.fbcdn.net/hphotos-xlp1/v/t1.0-9/11205488_1641308579487013_7202709367008535386_n.jpg?oh=1008d277d5d6a59d6e351ba41bb3af4e&oe=56601DF3 ​
    It seems that one of them will be the reference or ground node (this means one of them will be at 0 V). The problem chose node N2 as the reference node. After using Kirchhoff's current law for each node they found that:
    V1=-35.88 V
    V2=0 V
    V3= 63.74 V
    V4= 0.19 V
    My question is: what if I chose another node (for example N4) as the reference node for measuring the other voltages? Would it change my final results? And if it did so, how to know which node will be the real reference node? And I'm questioning this because I did this circuit on an online simulator and it turned out that those were the correct results so it seems that they chose the right node as the ground (I know that when we have to choose the reference node it is easier to choose that node the one that has more branches, but in this case both N2 and N4 have the same number of branches connected to them). I'm leaving the URL here for that simulation

    http://lushprojects.com/circuitjs/c...84+0+60 v+192+384+192+296+0+0+40+300+0+0+0.5
     
  2. jcsd
  3. Sep 13, 2015 #2
    You can choose any node as a reference node. Do you understand what reference node is (ground)?
    Try read this: http://www.ittc.ku.edu/~jstiles/312/handouts/312_Introduction_package.pdf (start from page 3)
    To measure the voltage we need two point in the space. One of this point is treat as a reference point. We have a very similarity situation when we try to measure a height of an object. We need a reference point. The most common reference pint is "above mean sea level". But when you measure the height of the table in your house the floor now becomes your reference point.
    https://www.physicsforums.com/attachments/rys0-1-jpg.69517/
    https://www.physicsforums.com/attachment.php?attachmentid=69517&stc=1&d=1399478598


    Yes, the result will change (the voltage result will change but the current will not change).

    There is no such thing as a "real reference node".
     
  4. Sep 13, 2015 #3
    So how did the results actually match the values of the simulator?
     
  5. Sep 13, 2015 #4
    Simply by accident the simulation program chosen N2 as a reference node.

    See this example when we have N2 as GND (the upper one) and N1 as GND (the the lower one)
     

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  6. Sep 13, 2015 #5
    Ah ya, so it doesn't matter at all which node will be chosen as the reference node, so this thing about choosing it arbitrarily can be used as a method to check if currents are solved in the correct manner. Understood. Thanks.
     
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