Current source in parallel with resistors

Thread Starter

antiantianti

Joined Aug 4, 2016
45
Hi
Can someone please explain tome why the current Ix2 is the same current that flows through the resistor R2 and why the current that flows through R3 is not relevant . I remember someine telling me that a resistor to be relevant must always be in series with the voltage source and resistors must be parallel to current source can someone please give me examples
I appreciate your answers
 

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wayneh

Joined Sep 9, 2010
16,102
As drawn, Ix2 is not independent of current in R3. The "Ix2" label appears twice and the upper right one is incorrect for any value of R3 less than infinity.

If that upper right label is removed, then it is true that the R2 current and Ix2 are independent of R3 as long as the source is not affected by the R3 current.
 

AlbertHall

Joined Jun 4, 2014
8,187
If that upper right label is removed, then it is true that the R2 current and Ix2 are independent of R3 as long as the source is not affected by the R3 current.
AIUI It is a current source and the current will be shared between the two resistors for non-zero and non-infinite values of resistor.

If the two Ix2 labels imply that that those two currents are equal, then R3 can be ignored as the current through it is zero. That could be true only if either R3 is infinite or R2 is zero (or both).
 

Thread Starter

antiantianti

Joined Aug 4, 2016
45
well i will post the original problem maybe I did something wrong
its a problem and i already have the solution but cant understand it
In my first post I didnt only wrote R3 instead of R34
 

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ErnieM

Joined Apr 24, 2011
7,991
As redrawn R1 is completely irrelevant as both ends are connected to the same point.

Ix2 is indeterminant As it measures the current in one leg of two shorted wires.

Is is correct to say R2 R3 and R4 are in parallel.
 

WBahn

Joined Mar 31, 2012
24,576
That makes even less sense than the first diagram. Ix2 = IR2. Sack the tutor!
How can you say that Ix2 = IR2?

While you can readily determine how much current flows in each resistor, you have no way of determining whether the current leaving the other side of the resistor returns through the upper path or the lower path.
 

AnalogKid

Joined Aug 1, 2013
8,081
well i will post the original problem maybe I did something wrong
its a problem and i already have the solution but cant understand it
In my first post I didnt only wrote R3 instead of R34
That is a very poorly drawn schematic, with a serious error. There is no indication that the crossed lines to the right of R3 are connected or not. Without knowing this, all is for nought, failure is assured, the crew and their luggage are lost at sea, and I have to paint the front porch. Thank god the Buckeyes won!!!

ak
 

AlbertHall

Joined Jun 4, 2014
8,187
How can you say that Ix2 = IR2?

While you can readily determine how much current flows in each resistor, you have no way of determining whether the current leaving the other side of the resistor returns through the upper path or the lower path.
Consider resistor R1. It is short-circuited therefore there is no voltage across it. If it has any value other than zero the current through it must be zero and therefore the only path for IR2 is through the Ix2 route. QED.
 

WBahn

Joined Mar 31, 2012
24,576
Consider resistor R1. It is short-circuited therefore there is no voltage across it. If it has any value other than zero the current through it must be zero and therefore the only path for IR2 is through the Ix2 route. QED.
Huh? Since it is short-circuited, it can have ANY current flowing through it (or what ever is shorting it) without having ANY voltage across it -- just like any wire that is treated as a short circuit path.
 

AlbertHall

Joined Jun 4, 2014
8,187
Huh? Since it is short-circuited, it can have ANY current flowing through it (or what ever is shorting it) without having ANY voltage across it -- just like any wire that is treated as a short circuit path.
On that basis, Ix2 can be any current you like from zero to I from the current source so we are back to sack the tutor.
For the question to be answerable requires that we take the diagram as the physical representation. Then the situation is as I described..
 

WBahn

Joined Mar 31, 2012
24,576
On that basis, Ix2 can be any current you like from zero to I from the current source so we are back to sack the tutor.
For the question to be answerable requires that we take the diagram as the physical representation. Then the situation is as I described..
No, it's not. R1 is shorted, which means that it is effectively replaced with a wire. That gives two zero-resistance paths for the current to split between and how it splits is indeterminate. The best guess argument that you could make is that, by symmetry, it will split equally.
 

ErnieM

Joined Apr 24, 2011
7,991
No, it's not. R1 is shorted, which means that it is effectively replaced with a wire. That gives two zero-resistance paths for the current to split between and how it splits is indeterminate. The best guess argument that you could make is that, by symmetry, it will split equally.
The best argument does not entail guessing which leaves us with an indeterminant situation.
 

WBahn

Joined Mar 31, 2012
24,576
The best argument does not entail guessing which leaves us with an indeterminant situation.
I absolutely agree -- that was actually the none-to-well-stated point of that statement. While it may be the "best guess" argument, or even the best "guess argument", the word "guess" in either case eliminates it from consideration as the "best argument" unless you are in a situation in which "guess arguments" are all that are available and you MUST choose from among them (and, unfortunately, that situation is actually pretty common in the real world, either because your customer won't/can't provide you with enough information to avoid making a guess. And there are many problems for which that is the nature of the beast -- there IS not way to solve the problem (at least that we know of) in an acceptable amount of time without making a guess of some kind.
 
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