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Equivalent capacitance, circular arrangement

  1. Nov 29, 2015 #1
    Mentor's note: Post moved to new thread since it's a new problem.

    One more problem
    1. The problem statement, all variables and given/known data
    CAJ.png

    Find the equivalent capacitance between A and B.
    2. Relevant equations

    3. The attempt at a solution
    I think all the Capacitors are in series but according to my book only three capacitors are in series.How?
     
    Last edited by a moderator: Nov 29, 2015
  2. jcsd
  3. Nov 29, 2015 #2

    gneill

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    When the problem named and labelled the two nodes A and B between which you are to determine the capacitance, it effectively implied connection terminals for the circuit. These terminals provide a favored point of view for you to interpret the circuit.

    Fig1.png
     
    Last edited: Nov 29, 2015
  4. Nov 29, 2015 #3
    How did you switch from one figure to the other,wire A and wire B seems flipped and how you decided that 2 μF location?
     
  5. Nov 29, 2015 #4

    gneill

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    Your original figure is just rotated to bring the terminal leads horizontal. The two circuits are mirrored about the center where the arrow is. Follow outwards from the center and confirm that you find the corresponding components in the same (mirrored) locations.
     
  6. Nov 29, 2015 #5
    When it is mirrored, left would come in right one's place and right would go in left one's place.Similarly, the vertical flip would occur, right?
     
  7. Nov 29, 2015 #6
    Sorry, I did not understand.
     
  8. Nov 29, 2015 #7

    davenn

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    That's not important ... the point is the circular and square arrangements are the same circuit layouts
    and it is now easier to visualise and to solve the problem

    so now have a go at solving the total capacitance value between points A and B


    Dave
     
  9. Nov 29, 2015 #8

    gneill

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    Mirrors reverse front to back, not top to bottom. Otherwise you'd be looking at your feet in the mirror rather than your eyes :smile:

    Surely you can see the symmetry between the two circuits? Confirm that all the components of both circuits make the same connections to each other.
     
  10. Nov 29, 2015 #9

    epenguin

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    Gracy if you don't mind, from yr many questions I have got the impression you are working hard and putting pressure on yourself but it feels like you have your eyes frowning four inches (10 cm) from the book. And that you need to relax, take somehow a cool distant look at these problems (and no doubt the texts of the corresponding books) and ask yourself what is this about? What does the question mean? What principles/equations do I have to use? (They are probably in the text preceding the excercise).... etc.

    Questions are made for verification of understanding but not usually intended to be fiendish.

    You're asked the capacitance between A and B. What is there between A and B? One way there is one capacitor, the other way there are three capacitors in series. No big deal to alternatively draw them as in #2.
     
  11. Nov 29, 2015 #10
    If the question would have asked just equivalent capacitance of the circuit rather than the equivalent capacitance between A and B then all the capacitors would be in parallel, Right?
     
  12. Nov 29, 2015 #11
    Which center?
     
  13. Nov 30, 2015 #12

    SammyS

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    For the most part such a question does not make sense if it's not specified how the group of capacitors are to connected to some external circuit,

    They definitely can;t be considered to all be in parallel. That would require having only two nodes, which you should know. In some sense they are all in series, if taken in isolation from any other circuit, but then equivalent capacitance makes no sense.

    gneill meant: from the center of the figure, outward, i.e. from a point between the two circuit diagrams.
     
  14. Nov 30, 2015 #13
    I knew it, I don't know what happened to me as I wrote in parallel.
     
  15. Nov 30, 2015 #14
    What does equivalent capacitance mean?
     
  16. Nov 30, 2015 #15

    SammyS

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    What do you suppose it (equivalent capacitance) means? After all, this whole thread has to do with finding the equivalent capacitance.

    Does your textbook or do your notes show how the formulas for equivalent capacitance are derived for capacitors in parallel and those in series?
     
    Last edited: Nov 30, 2015
  17. Nov 30, 2015 #16

    ehild

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    The original network of capacitors are connected into a circuit, or to a battery.
    Equivalent capacitance between point A and B is the capacitance of the single capacitor you replace the whole network between A and B with, so as nothing changes in the voltages and charges in the external network.
     
  18. Nov 30, 2015 #17

    ehild

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    @gracy, have you seen real capacitiors and resistors? Such things like in the picture? I guess, you have seen batteries.

    http://getmfg.com/wp-content/uploads/2014/10/rsz_eck-10pct20leanpct20electronicspct20componentspct20-960x650.jpg [Broken]

    You can make a network/circuit from these components by joining the terminals and connect it to a battery,
    It is as making a necklace from beads. You need a thread to connect the beads and enclosures at the ends.
    In A, the beads are connected in series. In B, the beads are parallel. How are the beads of different color connected in C?
    Here you can see real necklaces. Can you see, where are only series and where are both series and parallel connections? http://www.fortheloveof.net/how-to-make-an-earthy-beaded-necklace/
     

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    Last edited by a moderator: May 7, 2017
  19. Nov 30, 2015 #18
    Both series and parallel connections.
     
  20. Nov 30, 2015 #19

    ehild

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    Yes!
     
  21. Nov 30, 2015 #20
    But why gneill rotated my diagram in OP 90 degrees to the right?
     
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