Nodal analysis physical problem !

  • #1
in the nodal analysis -one of ways of analysis DC circuit - i couldn't understand the physical roots of this way .

i mean how could i make the voltage of a node equal to zero by make it as a reference point without having any effects on the voltage(electric potential) of the rest nodes .

from algebra view i'm sure 100% that this way is correct but physically i find difficult to say that the potential difference of the branch "ab" is:
[tex]U_{ab}=v_{a}-v_{b}[/tex]
and put for example 'a' as reference point and the [tex]_{ab}[/tex] become [tex]-v_{b}[/tex] .

could you explain the answer of this physical problem ! please.

thank you
 

Answers and Replies

  • #2
berkeman
Mentor
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in the nodal analysis -one of ways of analysis DC circuit - i couldn't understand the physical roots of this way .

i mean how could i make the voltage of a node equal to zero by make it as a reference point without having any effects on the voltage(electric potential) of the rest nodes .
You can't. When you choose a node as the reference node and call its voltage zero, all the other node voltages will be referenced to that node.

from algebra view i'm sure 100% that this way is correct but physically i find difficult to say that the potential difference of the branch "ab" is:
[tex]U_{ab}=v_{a}-v_{b}[/tex]
and put for example 'a' as reference point and the [tex]_{ab}[/tex] become [tex]-v_{b}[/tex] .

could you explain the answer of this physical problem ! please.

thank you


At each node, you use the KCL to write an equation that involves the voltage at that node, with respec to the voltages of surrounding nodes. The sum of the currents leaving the node has to equal zero, so that's how you write the equation for each node. The voltage differences are used to express the currents leaving the node in the different directions.

Does that help?
 
  • #3
1,482
3
from algebra view i'm sure 100% that this way is correct but physically i find difficult to say that the potential difference of the branch "ab" is:
The potential difference is defined as a path-integral of a charge moved in an electric field from point A to point B which produces a simple potential difference like V2 - V1 if Electric field is constant as is in a voltage or current source.
 
  • #4
thank you berkeman for this useful informations
The voltage differences are used to express the currents leaving the node in the different directions
as what 'waht' said:
The potential difference is defined as a path-integral of a charge moved in an electric field from point A to point B which produces a simple potential difference
attachment.php?attachmentid=26743&stc=1&d=1277794792.jpg


and from this to definitions i can tell you exactly what i mean:
when i attach the node to ground i had zero voltage in this node . physically
the electrons must go from the smaller voltage point[tex](v_{ground}=0)[/tex] to the biger voltage(upper positive point) > how this works , or is this way of thinking is wrong?.

thank you again:smile:
 

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  • #5
dlgoff
Science Advisor
Gold Member
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You don't need to connect a ground to the circuit. And current is defined as the direction of positive charge flow. i.e. the opposite direction of the electron flow.

Take a look at this example of http://docs.google.com/viewer?a=v&q...q8nrJ&sig=AHIEtbSm6Plp9OVqQqU3rTv2tnAQgOEhFg"

The first step in the analysis is to label all the nodes except for the common node.....

Secondly, label the currents entering or leaving each node.....

The next step is to write the KCL equation for each node except the common node....
 
Last edited by a moderator:
  • #6
thank you dlgoff but i know how to do analyzing using this method but i can't understander
the case of connecting the circuit to ground without eletrons going up to circuits and have influence to other voltages
 
  • #7
dlgoff
Science Advisor
Gold Member
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You can call any point the circuit ground and it won't change the analysis. If you are talking putting a wire connection to the earth, electrons can't just enter the circuit.
 
  • #8
yes! thank you very much dlgoff this is what i wanted to hear
electrons can't just enter the circuit.
 

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