Electric Circuits Problem!

Discussion in 'Homework Help' started by sagaradeath, Feb 18, 2011.

  1. sagaradeath

    Thread Starter New Member

    Feb 18, 2011
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    Please look at the attachment!!!

    Please look at the attachment!!!

    The Question is in the attachment!
     
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  2. addousas

    New Member

    Jul 15, 2010
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    I looked at the question over and over. The problem is not right R1 is necessary to solve this problem. Vb=2*(R1*R3/(R1+R2+R3))+4(R1+R2)*R3/(R1+R2+R3)

    To prove my point lets assume that the problem is right
    under this assumption pick two values for R1
    Vb should be the same but that is not the case, thus R1 is needed to solve this proble
     
  3. Georacer

    Moderator

    Nov 25, 2009
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  4. hgmjr

    Moderator

    Jan 28, 2005
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    The reason that R1 is fundamental to the solution of this problem is that it is in parallel with a current source. If it were in parallel with a voltage source then it could be ignored for solving this particular problem.

    hgmjr
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The value of R1 is not required explicitly because of the additional information given. The student is actually given the node voltage Va, and so can solve the problem.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    Ooopss!! You are absolutely Adjuster, I completely missed that important clue to the answer. How silly of me.

    It is still a good idea to recognize the importance of knowing the value of R1 to the solution of such a problem were the voltage across R1 not provided in the problem statement.

    I would not want to lull any newbies into the notion that resistors can be ignored unless a clear explanation of why they can be ignored is provided.

    hgmjr
     
  7. sagaradeath

    Thread Starter New Member

    Feb 18, 2011
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    how do you get R1???
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Well, the problem does not ask for it, but here is how it goes:

    You are given that Va = 5Volts. This should allow you to solve for the current in R2.

    Having found the current in R2, you will be able to find the current in R1, and of course you already know Va.
     
  9. sagaradeath

    Thread Starter New Member

    Feb 18, 2011
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    is the answer of the problem 41.2... i want to check my answer
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    That is not the answer that I obtained.

    How about posting your work so that we can see how you obtained your answer.

    hgmjr
     
  11. sagaradeath

    Thread Starter New Member

    Feb 18, 2011
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    well i was thinking V=IR

    so V=ix*R3

    so V=4*10.3

    41.2
     
  12. sagaradeath

    Thread Starter New Member

    Feb 18, 2011
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    V=IR
    V=ix*R3
    V=4*10.3
    V=41.2
     
  13. hgmjr

    Moderator

    Jan 28, 2005
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    Don't you think that the 5 volt potential and the 5.1 ohm resistor contribute to the votage at the junction the 10.3 and 5.1 ohm resistor?

    hgmjr
     
  14. sagaradeath

    Thread Starter New Member

    Feb 18, 2011
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    so what should i do ....im still new to this
     
  15. hgmjr

    Moderator

    Jan 28, 2005
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    What circuit analysis technique were you instructed to use in solving this problem?

    hgmjr
     
  16. sagaradeath

    Thread Starter New Member

    Feb 18, 2011
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    probably the parallel one
     
  17. hgmjr

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    Jan 28, 2005
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    Actually what I was referring to was which circuit analysis method such as:

    1. Superposition
    2. Kirchhoff's
    3. Thevenin's
    4. Norton's


    hgmjr
     
  18. sagaradeath

    Thread Starter New Member

    Feb 18, 2011
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    oh Kirchhoff's
     
  19. hgmjr

    Moderator

    Jan 28, 2005
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    Have a look at this example for a start. Also you should have a couple of examples in your textbook.

    Then see if you can form the two equations needed to solve for Vb.

    hgmjr
     
  20. sagaradeath

    Thread Starter New Member

    Feb 18, 2011
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    umm well i dont know how to start it...
     
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