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Capacitor equivalent

  1. May 16, 2016 #1
    1. The problem statement, all variables and given/known data
    https://scontent-kul1-1.xx.fbcdn.net/v/t34.0-12/13228153_10206233880597531_197570509_n.png?oh=32466b5e1ab50b8c8a0f49be9d044411&oe=573C076F
    Find the equivalent capacitance between A and B

    2. Relevant equations
    Capacitor

    3. The attempt at a solution

    https://fbcdn-sphotos-a-a.akamaihd.net/hphotos-ak-xat1/v/t34.0-12/13227986_10206233888797736_1816077667_n.png?oh=b89640bb9bb064cd8dfb82adec3cf894&oe=573BF525&__gda__=1463494744_04b6bbc1efbb4f70cbc46240f62cdcf4
    I assume 1 and 2 is in parallel
    There for 1/C = 1/1+1/1+1/3
    C=8/3
    Seems like this idea is not so convincing, Anyone has better idea on how to solve this question?
     
  2. jcsd
  3. May 16, 2016 #2

    cnh1995

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    Parallel components are connected between same two points. Which capacitors are in parallel then?
    Also, your formula for equivalent capacitance for parallel capacitors is incorrect.
     
  4. May 16, 2016 #3

    ehild

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    @WeiLoong

    Which capacitors are 1 and 2? C1 and C2 are really parallel, but what about the others? Somehow you presented a correct result for the equivalent capacitance, but how? Where is that 1/3 from?
    The red line means the same node A in the circuit.

    upload_2016-5-16_11-55-59.png
     
  5. May 16, 2016 #4
    there is some error in addition.
    moreover your circuit given above does not show the relation.
    check it.
     
  6. May 16, 2016 #5
    c1 and c2 is parallel meanwhile c3 and c4 are series
    c3 and c4 will give me 2/3F
    2/3F is parallel to C1 and C2
    hence C=1+1+2/3=8/3 is that correct?
     
  7. May 16, 2016 #6

    ehild

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    No, C3 and C4 are not in series, as their common point is connected to B. How is the equivalent of C1 and C2 connected to C3?

    upload_2016-5-16_13-2-50.png
     
  8. May 16, 2016 #7
    so that mean C1 C2 C3 are parrallel?
    C123= 3F?
     
  9. May 16, 2016 #8

    cnh1995

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  10. May 16, 2016 #9
    https://scontent-kul1-1.xx.fbcdn.net/v/t34.0-12/13231083_10206234376729934_1801165661_n.png?oh=6c602ace0e3669a2fc756dec529a82d2&oe=573C5142
    Like this? Then 2F and 1 F on top are in series
     
  11. May 16, 2016 #10

    cnh1995

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    Right.
     
  12. May 16, 2016 #11

    gneill

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    Staff: Mentor

    Just because components can be drawn so that their symbols are parallel does not mean that they are connected in parallel. Parallel and series connectivity is a matter of topology, not drawing aesthetics. On a schematic diagram wires and components can be moved or turned or twisted or routed or placed all over the page, but so long as the connections remain the same, the topology of the circuit remains the same.

    You need to get a firm grasp on what parallel and series connections mean and be able to recognize the necessary and sufficient conditions for both.

    Here is a quick test that will let you recognize if two components are in parallel: If you can trace a complete, closed path through the two components following existing wiring and without passing through any other component along the way, then those two components are parallel.

    Your C1 and C2 are such a pair:
    upload_2016-5-16_8-21-57.png
    Note the closed path in green that passes only through C1 and C2 and no other components along the way.
     
  13. May 16, 2016 #12

    ehild

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    Yes.
     
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