Equivalant resistance between two points

Discussion in 'Homework Help' started by zork, Oct 12, 2013.

  1. zork

    Thread Starter New Member

    Oct 12, 2013
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    Hi everybody,

    In the circuit joined to this post, I'd like to find the value of the current passing through R1 using Thevenin's method.

    I had a problem when I wanted to find the equivalent resistance between A and B.
    I guess that Rth = R3 // R2. But I cannot figure out why R4 isn't considered in calculus.

    Could anyone guide me to find the answer ?
    Thanks in advance.
     
    • rth.png
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  2. WBahn

    Moderator

    Mar 31, 2012
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    Why are you guessing?

    Redraw the circuit with the two voltage supplies set to zero and R1 removed and replaced with a set of terminals.
     
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  3. #12

    Expert

    Nov 30, 2010
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    The important part: re-draw the circuit. Take the, "intentionally confusing" out of it.
     
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  4. zork

    Thread Starter New Member

    Oct 12, 2013
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    I have redrawn the circuit.
    But I still cannot figure out if R4 is taken in consideration in the calculus of Rth.
    Any help ?
    Thanks.
     
  5. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    Redraw with R4 moved down below the "B" node and see how that looks.
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    What does the voltage across R4 have to be? Remember, the voltage between two points is the same no matter which path you take. So pick the simplest path from one end of R4 and the other and see how much voltage can possibly be dropped along that path.

    Or another way of doing it is to start at one terminal and draw a line from that terminal, through the resistor, and to the other end of the terminal without ever touching any particular node more than once. Keep in mind that a node consists of all of the wiring that connects a set of components together and if you touch any part of any of that node's wiring, you have touched that node.

    Or another way of doing it is to take a colored pencil (or something comparable) and color code each of the nodes. Draw a colored line along each node right up to the end of each component that node connects to. What is the color of the left end of R4? What is the color of the right end of R4? What does that tell you?
     
  7. zork

    Thread Starter New Member

    Oct 12, 2013
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    Done. If I use Kirchoff second law I find that the voltage across R4 is zero.

    For the first method : Is the simplest path, the path which doesn't pass across R2 and R3 ? If yes, so the voltage should by zero.
    For the second and third method, I didn't understand very well. I would like more clarification, please.
    Sorry for being to silly, I am a complete beginner in this field.
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    I've taken your circuit and colored the nodes.


    [​IMG]

    Notice that there are just two nodes.

    Notice that both ends of R4 are connected to the same node, hence it is shorted out.

    Notice that if you start at the top terminal and try to find a path through R4 you can't do it. You start on the blue node and go left through R3, at which point you are on the red node. You now go to R4 and after going through it you are on the red node. Since you have been on this node before, this is not a valid path. So try another path since you only need to find one valid path. But in this case you can't.
     
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  9. zork

    Thread Starter New Member

    Oct 12, 2013
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    Thanks for your quick answers.
    So, according to this. In the attached circuit the resistance between A and B should be zero, right ?
     
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  10. WBahn

    Moderator

    Mar 31, 2012
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    Yep.

    One thing to keep in mind is that you can move A to any point as long as it is on the same node. So move it to the top left corner of the diagram. Now move B to the lower left corner of the diagram. Now it's clear that they are shorted to each other. Or go one further and move B to the top left corner as well and now it's clear that they are, in fact, the exact same point! What's the resistance between a point and itself? Pretty small.
     
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  11. zapta

    New Member

    Sep 30, 2013
    3
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    Zork, your diagram is exactly like this

    [​IMG]
     
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