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Homework Help: Circuits Homework: Find the Power Absorbed in a Current Source using Node-Voltage

  1. Sep 30, 2012 #1
    1. The problem statement, all variables and given/known data

    Here's a link to a picture of the problem:

    V(delta) is above the 7 Ohm resistor, and V(x) is described by the positive Node between the 7 Ohm resistor and the 5 Amp Source, and the negative Node between the 9 ohm resistor and 150 Volt Source.

    i(omega) runs down the 4 Ohm source on the left.

    The top dependent voltage source is 3*V(x)
    The left dependent voltage source is 5*i(omega)
    The right dependent voltage source is 13*v(delta)

    2. Relevant equations

    I advise to use the node-voltage method to solve this. Plus, it will help me understand it since that and the Current-mesh are the tools our professor expects us to know for the coming exam.

    3. The attempt at a solution

    I have spent far too many hours and countless papers and methods trying to solve this problem in a way that gave me the answer that the book has. I've begun to think that the book may be wrong in regards to their answer?

    Anyway, I'll detail my usual procedure:

    Using the node-voltage method:

    I chose to ground the node in the top between the 1 Ohm and 2 Ohm resistors. Since a wire connect that node to the one under it, I concluded that both of those nodes are grounded. I pretty much erased that wire and connected both nodes into one.

    I then created node-voltage equations, and use a program called MATLAB to solve matrices that I created out of the linear equations (As my professor has told us to do for every problem).

    The answer in the answer manual says that the Power absorbed is 23.68 W.
    They show a procedure to doing it, but they always skip the 150 Volt source in their equations. It's as if it doesn't exist. Why is this?

    If someone could either do it and get the right answer, or tell me why they chose to omit the 150 Voltage source in their calculations, that'd help me greatly. Also, you could just tell me a different method to doing the problem and I'll give it a go.

    What really has me confused is the usage of the Voltage and Current source on the bottom wire. I don't know how to represent those in a node-voltage equation correctly.

    Thank you if you can help me in anyway.
  2. jcsd
  3. Sep 30, 2012 #2
    We have voltage source and current source in series. This configuration gives constant current at any voltage so it is actually current source.
  4. Sep 30, 2012 #3
    So what you're saying is that they are right in that they ignore the 150 Voltage source?
    Cause the way I'm thinking is that the voltage source would also create a current through the bottom resistor, such that the current in the bottom can be represented as a sum of the Current source and Voltage source through the resistor.
  5. Sep 30, 2012 #4
    The current source produces 100 mA current regardless of what is connected to it.
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