Help identifying mistake in source transformation problem

Discussion in 'Homework Help' started by WilkinsMicawber, Jun 5, 2017.

  1. WilkinsMicawber

    Thread Starter New Member

    Jun 5, 2017
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    The solution I get for the following problem is not the book's, yet I cannot figure out where I erred. Any help would be appreciated. I apologize for the poor-quality image. It was the best I could do on with my phone.

    Circuits 5th edition, chapter 4, problem 30)
    Use source transformation to find ix.
    hwimage.jpg
    The circuit at the bottom does not give a proper value for ix, which is supposed to be .255A.
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    Have you checked either your solution or the book's solution against the original problem to determine which on (if either) is actually correct? Books do have wrong answers occasionally, for a host of reasons ranging from having grad students who really don't care preparing the solutions, to a change in the problem from one edition of the text to another, to a publisher's mistake, to the author misinterpreting their own problem or just making a mistake in solving it.

    You should get in the habit of ALWAYS verifying that your answer really is a solution to the problem -- and that goes double once you are in the "real world". One of the nice things about engineering problems is that the correctness of a solution to a problem can usually be confirmed from the solution itself.

    Leaving that aside, how are you solving that bottom circuit? You don't have any current labeled ix in that circuit, so how can you know what your dependent voltage source is putting out?

    The control current, ix, is a specific current at a specific location in the original circuit. If you perform a transformation that involves that location, you have to be very careful to transform the parameter as well. In this case, ix is the current either in the 12 V voltage source of the 24 Ω resistor. But when you did your first source transformation turning the voltage source and the series resistor into a current source and a parallel resistor, you lost all information related to ix -- neither the current in the current source nor the current in the resistor is ix! The current ix is the current in the portion of the top wire connecting the 24 Ω resistor and the 30 Ω resistor.
     
  3. WilkinsMicawber

    Thread Starter New Member

    Jun 5, 2017
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    Ah!

    I do believe you're right, that the information about Ix was lost when I did the source transformation.

    Thank you.
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    Transforming the circuit from the left while maintaining the integrity of the control signal may or may not be possible and, even if it is, it might be difficult. Try it -- it might also be real easy. But keep in mind that you also have the option of transforming the circuit from the right, in which you only need to properly transform the controlled signals, which is generally pretty straightforward.
     
  5. MrAl

    AAC Fanatic!

    Jun 17, 2014
    5,273
    1,136
    Hello there,

    I had a little trouble reading the original circuit schematic so here is a somewhat improved image for that.

    SourceTransformation-2.jpg
     
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