# the voltage of resistor parallel with wire >>>/!\ Help please/!\

#### !!Miss.EE!!

Joined Oct 17, 2007
38
Hi all , I'm new member here and I'm really happy I found this amazing site.
I don't know how to start since it is my first time writing here, However. I hop I'll do something to this forum in return. ^___^
I have a problem in standing why the voltage must be zero for R=2K in the figure below (I mean Vo)... (Note: I simulate the circuit using Orcad that why I knew it is 0 V ) but why I really don't know . Can you please help me ??!!)
Further I know when resistor is parallel with wire(short circuit ) the current through that resistor is equal zero , is it by somehow that is the reason the voltage through that resistor is equal to zero too!!) ....

#### Dave

Joined Nov 17, 2003
6,969
The image is too small to read.

Dave

#### !!Miss.EE!!

Joined Oct 17, 2007
38

#### recca02

Joined Apr 2, 2007
1,212
the image is too large
havent solved yet but it seems it should not be zero.
however a short circuit in parallel isnt the only reason for voltage difference acrosss a element to be zero.
sometimes the voltage across to element is same(like in wheatstone circuit) and hence the voltage difference is zero.

#### !!Miss.EE!!

Joined Oct 17, 2007
38
thanks but we didn't take wheatstone circuit yet T___T

#### recca02

Joined Apr 2, 2007
1,212
where did the original circuit go?(i didnt mean to complain b4)
by the way i dont think answer shud be zero for that circuit.
by wheatstone circuit example i meant there are some times conditions that voltage drop across elements is such that there are points in circuit whose potentials are same and hence no potential difference exist between them.

#### !!Miss.EE!!

Joined Oct 17, 2007
38
Ok sorry I didn't say it before
But I have to use superposition theorem

#### chulangj

Joined Oct 11, 2007
3
you have shorted out the resistors to the output and form a new path for the current to pass. The voltage across this wire is very very very small and would read zero. A pic of the new circuit is shown here

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#### !!Miss.EE!!

Joined Oct 17, 2007
38

this is the Q...I solved the other two cases ...just the one above Couldn't understand it.

#### !!Miss.EE!!

Joined Oct 17, 2007
38
you have shorted out the resistors to the output and form a new path for the current to pass. The voltage across this wire is very very very small and would read zero. A pic of the new circuit is shown here

Isn't like what I said in my first post????

#### !!Miss.EE!!

Joined Oct 17, 2007
38
you have shorted out the resistors to the output and form a new path for the current to pass. The voltage across this wire is very very very small and would read zero. A pic of the new circuit is shown here
Thank you Very much >>>> I got it >>>>

#### recca02

Joined Apr 2, 2007
1,212
i thought u meant o v in the actual problem
anyways for the above case like u said the resistances are in parallel to a short hence the current thru 2 +2 ohms loop is zero so no votage drop across the 2 ohm resistor.

#### !!Miss.EE!!

Joined Oct 17, 2007
38
Vo is across one resistor with 2K which is also parallel with the another 2K, so V across them is equal , hence it is zero 'cause they are parallel with wire(short) ... right !!!

#### recca02

Joined Apr 2, 2007
1,212
that will be a bit confusing way to look at the circuit .
actually that way if u consider the circuit then the two resistors of 2K have a terminal floating.
so the answer in any case is that all current flows thru the parallel short circuit and hence no current flows thru the 2k resistances and so no P.D.
or u can also say that since voltage diff across a short is zero hence voltage drop across the 2k resistors in series(in this case) is zero.
there are a lot of ways to look at this more than i can even post right now.

#### !!Miss.EE!!

Joined Oct 17, 2007
38
this is the original question::

and I approached this case and now I'm confused again:

first is the node I circled is the same???
then some one can help me find Vo in detail please; 'cause each time something come up on my mind

#### recca02

Joined Apr 2, 2007
1,212
they are all same
if u really want to solve it the old fashioned way use kvl
just take those three squares as separate loops.

#### !!Miss.EE!!

Joined Oct 17, 2007
38
some thing new .... ( sorry but I don't give up till all my idea is finished)
I redraw the circuit (don't LAUGH )

now it is clear both of 2K resistors are with open node >>>> Thats why they have Zero volt. ( it is a question) -____-!!

#### recca02

Joined Apr 2, 2007
1,212
now it is clear both of 2K resistors are with open node >>>> Thats why they have Zero volt. ( it is a question) -____-!!
that is what i meant.

that will be a bit confusing way to look at the circuit .
actually that way if u consider the circuit then the two resistors of 2K have a terminal floating.
now.
the more thought u give the better u will arrive at same conclusion but with new methods and will have much more clearer idea.

#### !!Miss.EE!!

Joined Oct 17, 2007
38
thank you very much llllllllllllllllllllllllllllllllllllllllllllllllll 100%

#### JoeJester

Joined Apr 26, 2005
4,390
Miss EE,

Complete all the steps of the superposition. Just because the first step you've done has 0 as Vo doesn't preclude you from having an output when you add the remaining steps.