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Circuit problem

  1. Apr 11, 2012 #1
    Hi I am little confused by star resistors configuration. The problem is asking for equivalent resistor for the circuit. I have attached the image below (Image is little sloppy because I can't draw with a mouse :D).

    My logic is as follows:

    First I calculated R1 and R2 as parallel resistors.

    Req1 = (1/R2 + 1/R1)^-1
    Req1 = 11.25;

    Then I calculated that value in series with 4.5 resistor.

    Req2 = 11.25 + 4.5 = 15.75;

    Since, Req2 is parrallel with R4 then they should also add reciprocally.

    Rt = (1/R4 + 1/Req2)^-1 = 6.81;

    This value is wrong though I don't know what I did wrong.

    Thanks for help in advance.
     

    Attached Files:

  2. jcsd
  3. Apr 11, 2012 #2

    ehild

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    Gold Member

    Which resistors are R1 and R2?

    ehild
     
  4. Apr 11, 2012 #3
    Sorry for the confusion I accidently uploaded the image that doesn't contains the name of the each resistor.

    Anyway they are as follows:

    R1 = 18 OHM
    R2 = 30 OHM
    R3 = 4.5 OHM
    R4 = 12 OHM


    Thanks.
     
  5. Apr 11, 2012 #4

    ehild

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    The 18Ω and 30Ω resistors are not parallel. Calculate the parallel resultant of the 12Ω and 30Ω ones.

    ehild
     
  6. Apr 11, 2012 #5
    I see but how can I know if they are parallel or not ? If they this star shape ? The way I analyzed this shape is first taking following charge from Positive terminal then when it goes to junction that has R2 and R1 I thought then one charge would go one way while other goes the other way.

    Thanks.
     
  7. Apr 11, 2012 #6
  8. Apr 11, 2012 #7
    I see that makes sense thanks :).
     
  9. Apr 11, 2012 #8

    gneill

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    Staff: Mentor

    Don't let slanted components and wires fool you :smile: If there's an unbroken wired connection between points they all comprise the same node. Feel free to rearrange the drawing so long as the same connections are maintained:

    attachment.php?attachmentid=46110&stc=1&d=1334149935.gif
     

    Attached Files:

  10. Apr 11, 2012 #9
    I see thanks alot gneil its easier to draw it in this configuration better than delta configuration. That statement about continuous made me understand the logic alot because when my book solves questions like this it assumes u already know how to change from configuration to configuration without understanding the process.
     
  11. Apr 11, 2012 #10
    I think it also helps to imagine a real battery connected to it with a real wires and see the situation that comes from these connections. Helped me to think about that way.
     
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