Node-voltage Method, some misconception?

[M. next]

Node-voltage Method, some misconception??

Please check the attachment:
The question is written upon it

Sorry bad drawing, the circuit is closed from the left hand side but not from the right hand side.

 

[Jony130]

But I don't see any problem. We can choose any node we want as a reference point (GND).

 

[M. next]

But the essential node must connect at least 3 appliances! Here we have only two :/

 

[Jony130]

But you can solve the circuit using nodal analysis without any "essential node".
And I still don't understand why you need "essential nodes"?

 

[M. next]

What's nodal analysis? How does it differ from node-voltage method. Elaborate please.
[I only learned node-voltage method :p]
If we are talking only node-voltage method, how in the world did the Doctor consider the node below a reference when it only connects two appliances[see figure].
That's my question.

 

[M. next]

I just checked, and they mean the same. Yet again, this is against the rule.

 

[Jony130]

Nodal analysis is exactly the same think as node-voltage method.
And we can choose any node we want as a reference point (GND).

See the example

We have a four nodes in our circuit. I pick as a reference point (GND) node 4.
So node 4 by dedefinition has a potential of 0 V.
So we left with three nodes. but we know that voltage at node 1 is equal 9V.
So we only has two unknown nodal voltages 2 and 3.
Now we apply KCL to the nodes where the unknown voltages appears.

For node 2 (I assume that all current flow out from the node)

\frac{V2}{R2} + \frac{V2 - V1}{R1} + \frac{V2 - V3}{R3} = 0

And we notice that V1 = E1 = 10V

Now we write KCL for node 3.

\frac{V3 - V2}{R3} + \frac{V3 - V1}{R4} = 0

And now all we have to do is to solve for V2 and V3

http://www.wolframalpha.com/input/?i=A/3+++(A+-+9)/9+++(A+-+B)/9+=0,+(B-A)/9+++(B+-+9)/9=0

V2 = 3V and V3 = Vth = 6V

 

[M. next]

Lots of thanks Jhony, now I get it..

 

[Jony130]

We always measure all the voltage respect to this common point (reference point), also known as a "ground" (GND). And we assume that GND have zero voltage.
Look ta this examples

 

[M. next]

Thanks, that's very kind of you.
You really were helpful