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Homework Help: Equivalent capacitor in the circuit

  1. Jul 21, 2012 #1
    Find the equivalent capacitance of the combinations shown in the fig.
    (refer to the file attached)

    Is there any easier method to solve instead of this method by assuming a constant potiential difference is applied across the circuit and a total charge Q flown in it.

    Attached Files:

  2. jcsd
  3. Jul 21, 2012 #2
    this can be dne by charge distribution . and applying kirchoff's loop law.
  4. Jul 21, 2012 #3


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  5. Jul 21, 2012 #4
  6. Jul 22, 2012 #5


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    Staff: Mentor

    No easier. Let current in top 2F cap be I1, and in top 4F cap be I2.
    And current down through vertical 4F cap is I1 - I2.

    Solve for I´s in terms of applied voltage V and ω.

    Does the textbook give the answer?
  7. Jul 22, 2012 #6
    the ans is 20/7 F i think...is tht correct?
  8. Jul 22, 2012 #7
    So, the star delta transformation isn't applicable here?
    I had a quick search on google about this. The first link directed me to this thread. Check this post by gneill in that thread, he mentioned a formula which could be of use here. Can you explain that formula NascentOxygen?
  9. Jul 22, 2012 #8
    tht is nt a wheatstone bridge hw cn u apply the star delta thn?
  10. Jul 22, 2012 #9
    Yes the ans. is 20/7 F and I also get it by using krichoff's law
  11. Jul 22, 2012 #10
    I also want to know that can we use star to delta formation here as in this case applying of
    krichoff's law is easy , but what if all the value of capacitance is different ?
  12. Jul 22, 2012 #11


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    The formulae given in the wikipedia article are general formulae, so their transform is applicable where the three branches are all different. On this page http://en.wikipedia.org/wiki/Y-Δ_transform under the heading Equations for the transformation from Y-load to Δ-load 3-phase circuit you are shown how to relate the impedance of each arm of Δ to that of the Y.

    https://www.physicsforums.com/images/icons/icon2.gif [Broken] In their formula, instead of resistances, you will use impedances, remembering that the impedance of a capacitor C = (ωC)⁻¹

    I tried it on your capacitor network, transforming the upside-down Y shape of the vertical capacitor and the two lower ones into a delta. This changes the network to an uncomplicated arrangement of capacitors in parallel, and in series. I got the same answer, 20/7 F :smile: :wink:
    Last edited by a moderator: May 6, 2017
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