EE people! i need your help!!

[Omegachi]

i am stuck on this circuit analysis problem, can anyone help me?

problem

 

[SuperTool]

Originally posted by: Omegachi
i am stuck on this circuit analysis problem, can anyone help me?

problem

Please post problem 2.41

 

[Omegachi]

2.41 is practically the same problem, the circuit just looks a little different. its no help.

oh yea, sorry about the junky resolution on the camera, took the pic too close.

 

[SuperTool]

My guess is Req2 =Req1
If R=infinity, Req1=Req2
If R=0, then Req1=2+(2||4) Req2=2+(1||2)+(1||2), so again Req2=Req1.

 

[Omegachi]

can you explain where you get your values from? plzz

 

[Ranger]

I think you are right, there is no current flow between a and b
so the value of R doesn't matter

 

[SuperTool]

well, you have those 2 resistors next to a and b for both cases.
Req1 is 2+(2||4) (the path trhough a has 2 resistors and path through b has 4 resistors)
If R=inf. then Req1 is identical to Req2, because that's as if there was empty space there instead of R.
If R=0, then a=b is same point. then above a, you have 2||1, and below a you have 2||1, and you also have the 2 on the left
so Req2= 2+(2||1)+(2||1)=2+2*(2||1)=2+(4||2)=Req1. That part is simple. your task is to show that between 0-inf, it's equal too.

 

[Omegachi]

there isn't?! why is that?

 

[SuperTool]

Look at it in terms of symmetry.
The voltage between A and B is going to be halfway between the endpoint voltages, due to symmetry, because whatever the actual resistance is, it's pretty obvious that resistance to the top terminal from A is same as resistance from A to the bottom terminal. Same thing for B. So Va=Vb=(Vtop+Vbot)/2, so there isn't going to be any current flow through the resistor I=(Vab)/R =0