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Capacitors Complicated Circuit

  1. Mar 4, 2013 #1
    1. The problem statement, all variables and given/known data
    In Fig. 26-32, the battery has a potential difference of 20 V.
    http://www.webassign.net/hrw/26_32.gif

    2. Relevant equations
    If Capacitors are in series, then (1/C1) + (1/C2) = (1/Cnew)
    If Capacitors are in parallel, then you just add them, C1 + C2 = Cnew

    3. The attempt at a solution
    I'm not sure how to get the equivalent capacitance from here.
    Would I just add the 12/7 to the 2uF parallel from it to make one capacitor? What happens then to the 2uF originally in series with the 12/7 uF?
    This is what I did: Combine the 3 and 4 uF capacs, then the 12/7 and 2 uF(parallel), then that and 3 uF (series) and then that with the remaining 2 uF and 3 uF, in series.
    I ended up with 0.696, which is apparently wrong. Is there a correct way how to resolve all these capacitors? PEMDAS for circuits? Any help will be appreciated
     
  2. jcsd
  3. Mar 4, 2013 #2
    Here's a picture showing where I'm stuck at:
     

    Attached Files:

  4. Mar 4, 2013 #3

    phinds

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    Uh ... how did you get 4 in series with 4 = 12/7 ???
     
  5. Mar 4, 2013 #4
    ....Thaaaat appears to be my problem. Thanks! I'll have a go at this again
     
  6. Mar 4, 2013 #5
    So, I did the work again, fixing that 3 uF into a 4 uF, and I ended up with getting 12/17 uF, but that's not right either.
    Could somebody advise me as to which branches/capacitors I should deal with first, and so on?
     
  7. Mar 5, 2013 #6

    NascentOxygen

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    Are you using a calculator?

    It is pretty obvious when capacitors are in series, so you might as well start there. (But it doesn't matter where you start.) Re-draw the circuit after each step.

    So you have 2x 4µF in series, what equivalent capacitance is that?

    I can see a pair of 3µF in parallel.
     
  8. Mar 5, 2013 #7

    phinds

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    You need to show your work so we can see how it is that you are getting these nonsensical answers

    Show step by step how you reduce two 4-uf's in series to a single equivalent cap.
     
  9. Mar 5, 2013 #8
    I hope this picture helps! It goes from left to right, just in case you didn't know.
    EDIT: Sorry, wrong picture
     

    Attached Files:

  10. Mar 5, 2013 #9
    Here we go! My REVISED work.
     

    Attached Files:

  11. Mar 5, 2013 #10

    phinds

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    You KEEP not showing your work, just the end results of each step. Let me repeat myself:

    You need to show your work so we can see how it is that you are getting these nonsensical answers
     
  12. Mar 6, 2013 #11

    NascentOxygen

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    You overlooked my hint? ("I can see a pair of 3µF in parallel")
     
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