Equivalent resistance between two nodes help

[JasonHathaway]

Homework Statement

Homework Equations

Parallel-Series resistors combinations.

The Attempt at a Solution

Using an approach much like the one used to find the Thevinin resistance:

1- Open circuit between A and B (R1, R2, R3), and short circuit the voltage source.
2- (R4//R5)+R6=170
3- Returning R1, R2 and R3 and find their equivalent (R1//R2//R3=30).
4- Rab=170+30=200Is my approach correct? is the resistance between a and b the same as the equivalent resistance?

Edit:
Forgot to add the diagram+the statement

 

[gneill]

Was there supposed to be a problem statement or at least a circuit diagram?

 

[gneill]

Okay, the circuit diagram helps but...
What is the problem statement? What are you supposed to determine about the given circuit?

 

[JasonHathaway]

Sorry again, the question is: determine the resistance between A and B

 

[gneill]

Sorry again, the question is: determine the resistance between A and B

Okay, presumably that means they want you to find the equivalent resistance that represents the combination of resistors R1, R2, and R3. In other words, the equivalent resistance of this circuit segment:

Unless there's more to the problem such as the resistors between A and B are to be taken as a load and you're to find the Thevenin equivalent for the rest of the circuit that's driving that load...

 

[JasonHathaway]

But aren't R4 and R5 also share the node B and should be considered?

P.S. Does the same approach you did applies on this circuit if the problem states: "Find the equivalent resistance between a and b as seen by the source"?

 

[gneill]

But aren't R4 and R5 also share the node B and should be considered?

Really it depends on what they mean by "between A and B". Unless they are asking for a Thevenin equivalent , I'd interpret it to mean the resistors as I've shown. It's the simplest interpretation without other information. Perhaps it's a slightly vaguely posed question, but that would be my interpretation.

P.S. Does the same approach you did applies on this circuit if the problem states: "Find the equivalent resistance between a and b as seen by the source"?

No, in that case the nodes a and b are where the source itself connects to the resistor network, so the logical interpretation would be that they want you to reduce the entire network down to a single equivalent resistance that represents the net load on the source.

 

[JasonHathaway]

Thanks :)

 

[JasonHathaway]

Just to be sure, If A and B were:
http://i.imgur.com/Kilpty3.jpg?1

Then, the equivalent resistance between A and B would be (90//90//90)+(200//2000)=130, isn't?

 

[gneill]

Just to be sure, If A and B were:
http://i.imgur.com/Kilpty3.jpg?1

Then, the equivalent resistance between A and B would be (90//90//90)+(200//200[STRIKE]0[/STRIKE])=130, isn't?

Sure. In general it's better to remove any ambiguity by indicating the intended portion of the circuit more specifically, say by drawing a circle around it. When circuits are more complex and other paths between the nodes in question are available, it's important to be able specify the intended path(s) and the excluded ones.