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Equivalent Capacitance

  1. Mar 10, 2014 #1

    Maylis

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    1. The problem statement, all variables and given/known data
    Reduce the circuit below into a single equivalent capacitor at terminals a,b. Assume all initial voltages at t = 0 are zero.


    2. Relevant equations



    3. The attempt at a solution
    Okay, this is the circuit I had in mind when I made this thread earlier
    https://www.physicsforums.com/showthread.php?p=4684447#post4684447

    My first thought was to take all the capacitors that to me look to be in parallel and combine them. I said that they are in parallel because they share at least 2 nodes on both sides of the components.

    Then, I said the capacitors on the left side are in series. There is a lot of doubt in my decision here. I say that because they all 3 share the same node on only one side of their component, so I called them in series.

    Then, after that the rest are clearly in series so I combined them.

    Also, I wonder why it matters what terminals I choose to find the equivalent capacitance between, and what would happen if I chose different terminals?
     

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    Last edited: Mar 10, 2014
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  3. Mar 10, 2014 #2

    PhysicoRaj

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    Your understanding of series and parallel combination is still wrong. After you combined the parallels, you say the left 3 are in series.. how is that? If you have gone through the thread you have linked, it contains an answer to the trick.
     
  4. Mar 10, 2014 #3

    Maylis

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    I say they are in series because they all share one node on one side of the component.
     
  5. Mar 10, 2014 #4

    Maylis

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    Alright, I redid it saying the 2C capacitors were in series. However, how do I know the current is the same? I mean, the charges land on the capacitor and don't go to the other side, is the current on a capacitor the same on both sides of the capacitor for one component?
     

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  6. Mar 10, 2014 #5

    PhysicoRaj

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    That's where you have gone wrong. Only two of them can share a point in series.
     
  7. Mar 10, 2014 #6

    PhysicoRaj

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    Yes. That's why it is 'current through a capacitor'
     
  8. Mar 10, 2014 #7

    CWatters

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    Maylis - Your answer in #4 looks correct.

    Yes. Electrons going I one side repel electrons out of the other side.
     
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