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Kirchof's law with Current?

Discussion in 'Electronics Homework Help' started by Bittenfleax, Nov 12, 2014.

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  1. Bittenfleax

    Bittenfleax

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    Nov 12, 2014
    I am having trouble with some Kirchof's law. I am really stuck and have come here for help. I have tried it for about 2 hours and could not get a thing - Thanks

    Photo 12-11-2014 11 32 00 am (Custom).jpg
     
    Last edited by a moderator: Nov 12, 2014
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,389
    2,774
    Jan 21, 2010
    OK, start by showing us the equations you've derived. We can check to see if they're correct or not. Then we can progress from there.

    From the notes on there it looks like you've tried to use KVL, and that's as good a place to start as any.
     
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  3. LvW

    LvW

    604
    145
    Apr 12, 2014
    Did you ever hear about the superposition theorem? That means: How to calculate the total current through a particular part using superposition of different currents (in our case: 2) caused by different sources (in our case: 2) ?
     
  4. Bittenfleax

    Bittenfleax

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    0
    Nov 12, 2014
    Ahhh that is good then. My notes are not available now I am afraid.
     
  5. Bittenfleax

    Bittenfleax

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    0
    Nov 12, 2014
    @LvQ
    I will look at that, however I don't think I can do it now. I will have to do it later. Thanks for the reply
     
    Last edited: Nov 12, 2014
  6. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    First off, this is KCL so starting with loop currents is a mistake.

    Kirchoffs Current Law or KCL, states that the “total current or charge entering a junction or node is exactly equal to the charge leaving the node as it has no other place to go except to leave, as no charge is lost within the node“. In other words the algebraic sum of ALL the currents entering and leaving a node must be equal to zero. (http://www.electronics-tutorials.ws/dccircuits/dcp_4.html)

    So KCL sums the current at a node in terms of the node voltage and the resistance to adjacent nodes. The first task then is to identify the nodes and the voltages at each node. But voltages are measured with respect to a reference, so one of the nodes must be designated the ground node. However, this problem has a complication: multiple voltage sources. The voice of experience says that the equations will be more elegant if the series junction of a single resistor and voltage source is not identified as a node. Of course, it can be a node but then one must write an additional constraint equation. With that said, this problem has only one node; therefore, only one node equation.

    To demonstrate how the node equation is written, I'll use a slightly different circuit. Note that when writing the individual node currents, it was most convenient to just mentally exchange the position of V1 & R4 and V2 & R5.

    Screenshot-38.png
     
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