Find caраcitоr voltages and сurrеnts аnd rеsistоr сurrеnts fоr thе сirсuit

Discussion in 'Homework Help' started by Feride, Feb 11, 2015.

  1. Feride

    Thread Starter New Member

    Feb 11, 2015
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    Help me please with HOMEWORK
    deadline is in two hours... And I have no ideas

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  2. bertus

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    Apr 5, 2008
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  3. Feride

    Thread Starter New Member

    Feb 11, 2015
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    Hello, I wrote a KCL equation for a top node so it looks like on a photo. You know, I've never met such problems before - do you have any ideas how to solve it?

    Usually in second order problems we write KVL for a loop, then take derivative, etc. But here, I dont know at all. Deadline is coming so far :(
    Thanks in advance
     
  4. WBahn

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    Mar 31, 2012
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    Your KCL equation mixes up TWO nodes. You have one node at the top of C1 and a different node at the top of C2. So you need TWO node equations.
     
  5. Feride

    Thread Starter New Member

    Feb 11, 2015
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    OK, and what do I do after writiong the second equation? :)
     
  6. Feride

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    Feb 11, 2015
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    Here are BOTH node equations. What then?)
     
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  7. WBahn

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    First, you need to get the node equations right. The first step in doing that is learning how to apply KCL at a node.

    Also, you have undefined variables in your equations. You have Vs, Vc1, and Vc2, which are defined well enough. But then you have Vc. What voltage is that?

    Take a step back and write the KCL equations for each node (let's call the node above C1 Node A and the node above C2 Node B) in terms of the currents given (hint: each node equation should involve three of the five labeled currents).
     
  8. Feride

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    Feb 11, 2015
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    like this?
     
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  9. Feride

    Thread Starter New Member

    Feb 11, 2015
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    In terms of currents.
     
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  10. Feride

    Thread Starter New Member

    Feb 11, 2015
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    Is there an algorithm of solving such problems? Could you write please the general steps like a further explanation, cause I still do not see what am I doing
     
  11. WBahn

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    The algorithm is to apply KCL to each node. Node A has exactly three currents flowing into or out of it: I1, I2, and Ic1. Set the sum of the currents flowing into the node equal to the sum of the currents flowing out of the node. Do the same for Node B, which also has exactly three currents flowing into or out of it. Can you see which three those are?
     
  12. Feride

    Thread Starter New Member

    Feb 11, 2015
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    yeap, I2, I3, Ic2. What do I do next? :)
     
  13. Feride

    Thread Starter New Member

    Feb 11, 2015
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    Maybe I can combine C1 and C2 so it becames C12=6mF and then i can solve it as first-order Rc circuit?
     
  14. WBahn

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    You write the two KCL equations, one for each node, in terms of the three currents that enter/exit that node.

    You haven't gotten that part right yet, so there is no point doing anything with component values or Ohm's Law or the constitutive equation for a capacitor until you have those two equations correct.
     
  15. WBahn

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    Mar 31, 2012
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    Before just throwing two capacitances at a formula that takes two capacitances, you might ask if that formula applies to those two capacitances.
     
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