missing resistor

Discussion in 'Homework Help' started by phantomvs, Jan 21, 2016.

  1. phantomvs

    Thread Starter Member

    Jan 19, 2016
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    I am stuck on another question that asks me to find the missing resistor of AB in the attached diagram.
    I know the following :
    R1= 30 ohms
    R2= 40 ohms
    R3 = 10 ohms
    R4= 10 ohms
    R5=60 ohms

    Now i know the formulas for calculating the R value for parallel and series circuits but can't figure it ou.
    I tryed seperating the circuits in smaller circuits .
    I did R2 & R5 as parallel and do i do series as R1 , R3 and R4 ?

    Thanks !
     
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  2. WBahn

    Moderator

    Mar 31, 2012
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    This makes no sense. Think about it. If I connect up those five resistors according to the diagram provided, how could you possibly tell that there even IS a resistor that is missing between A and B, let alone what value it is?

    You are missing some piece of information that gives the resistance between some pair of points in the circuit that involves the missing resistor.
     
    ISB123 likes this.
  3. phantomvs

    Thread Starter Member

    Jan 19, 2016
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    Your right, but that is the only info i see. I found it wierd that there is no voltage or amps indicated anywhere in the circuit.
     
  4. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    WBahn is right.
     
    Last edited: Jan 21, 2016
  5. WBahn

    Moderator

    Mar 31, 2012
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    o_O Methinks thou dost haveth wrong thread.
     
  6. phantomvs

    Thread Starter Member

    Jan 19, 2016
    39
    1
    Anyway, according to my study sheet the answer is 33.33 ohms

    Not sure how they get it, but thats all the info i have.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    They are not asking for the value of some missing resistor between A and B. They are asking for the equivalent resistance of THOSE resistors as seen between A and B.
     
  8. absf

    Senior Member

    Dec 29, 2010
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    Now can you solve it?

    A8.PNG

    Allen
     
  9. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    That looks like Thevenin equivalent resistance problem.

    Just to clarify. Is English your native tongue?
     
  10. dannyf

    Well-Known Member

    Sep 13, 2015
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    It is probably asking what you is the (equivalent) resistance between A and B. Fairly easy once you redraw the picture.
     
  11. phantomvs

    Thread Starter Member

    Jan 19, 2016
    39
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    I think i figured this one out thanks to ABSF new diagram.

    I found the total resitance of R2, R5 and R1 which is 13.33 ohms

    Then added in R3 and R4 for 33.33 Ohms

    Just re-drawing the schematic, i don't get why R1 goes in parallel when its in series in original diagram.
    Cause R2 and R5 are in parallel which in turn is in R1.
     
    absf likes this.
  12. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    R1 is not in series, its joined at both ends, so its in parallel...
     
    phantomvs likes this.
  13. WBahn

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    Mar 31, 2012
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    Two resistors are in series if they are the only two resistors on a given node.

    Two resistors are in parallel if both ends are connected to the same nodes.

    Identify the nodes.

    A simply way to do that is to color them.

    colored.png
    R1 is connected to the red node and the greed node, thus it is in parallel with any other resistor connected to the red node and the green node.

    The resistors on the green node are R1, R2, R3, and R5. Of these, R1, R2, and R5 are also connected to the red node. Thus R1, R2, and R5 are all in parallel.
     
    phantomvs likes this.
  14. phantomvs

    Thread Starter Member

    Jan 19, 2016
    39
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    You see thats my problem its hard for me to tell if a resistor is in series or parallel.

    The original diagram , it looks like R1 is in series.
     
  15. phantomvs

    Thread Starter Member

    Jan 19, 2016
    39
    1
    Thanks Wbahn

    I appreciate you taking your time to point that out for me .

    You learn something new everyday !
     
  16. phantomvs

    Thread Starter Member

    Jan 19, 2016
    39
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    So like in this diagram i saw posted .

    R3 & R4 are in parallel R1 & R2 are in series
     
  17. WBahn

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    Mar 31, 2012
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    Remember the definition of series. To be in series, whatever current flows in one MUST flow in the other. That means that whatever current flows out of one has to have no choice but to enter the other. Look at that green node and imagine a current flowing up out of R1. Does it HAVE to flow into R2? No, there is another path it could take. Similarly, consider current coming up out of the supply. Does the current then goes through R1 have to also go through R2? No.

    In order for the current flowing out of one to have to flow into the other, there must be no other paths available. If the two resistors are connected directly together at a certain node, this means that there must be no other devices connected to that same node.
     
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  18. phantomvs

    Thread Starter Member

    Jan 19, 2016
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    ok i get it so basically all those resistors in that diagram are in parallel
    R1 R2 R3 R4 are all parallel
     
  19. WBahn

    Moderator

    Mar 31, 2012
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    What are the color of the nodes connected to R3?

    What are the color of the nodes connected to R1?

    Can they be in parallel?
     
    phantomvs likes this.
  20. phantomvs

    Thread Starter Member

    Jan 19, 2016
    39
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    R3 nodes blue / red
    R1 nodes red / green

    no not in parallel due to R2
     
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