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Equivalent resistance between two nodes help

  1. Mar 12, 2014 #1
    1. The problem statement, all variables and given/known data

    r2wmNF7.jpg

    2. Relevant equations

    Parallel-Series resistors combinations.

    3. 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=200


    Is my approach correct? is the resistance between a and b the same as the equivalent resistance?

    Edit:
    Forgot to add the diagram+the statement
     
    Last edited: Mar 12, 2014
  2. jcsd
  3. Mar 12, 2014 #2

    gneill

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    Staff: Mentor

    Was there supposed to be a problem statement or at least a circuit diagram?
     
  4. Mar 12, 2014 #3

    gneill

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    Staff: Mentor

    Okay, the circuit diagram helps but...
    What is the problem statement? What are you supposed to determine about the given circuit?
     
  5. Mar 12, 2014 #4
    Sorry again, the question is: determine the resistance between A and B
     
  6. Mar 12, 2014 #5

    gneill

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    Staff: Mentor

    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:

    attachment.php?attachmentid=67575&stc=1&d=1394666069.gif

    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...
     

    Attached Files:

  7. Mar 12, 2014 #6
    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"?
     

    Attached Files:

  8. Mar 12, 2014 #7

    gneill

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    Staff: Mentor

    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.

    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.
     
  9. Mar 12, 2014 #8
    Thanks :)
     
  10. Mar 15, 2014 #9
    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?
     
  11. Mar 15, 2014 #10

    gneill

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    Staff: Mentor

    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.
     
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