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Equivalent Capacitance and Voltage Across

  1. Feb 8, 2016 #1
    1. The problem statement, all variables and given/known data
    Screen Shot 2016-02-08 at 6.32.36 PM.png

    2. Relevant equations
    Series: 1/Ceq=1/C1+1/C2
    Parallel: Ceq =C1+C2

    3. The attempt at a solution
    So to find the equivalent resistance do I short (put a wire through the open terminals) the circuit and start adding up the capacitors? Once I do that do I just create a new circuit with the equivalent capacitance and inductor and find the voltage?
     
  2. jcsd
  3. Feb 8, 2016 #2
    Here is a bigger picture of the circuit
    Screen Shot 2016-02-08 at 6.42.26 PM.png
     
  4. Feb 8, 2016 #3

    NascentOxygen

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    Staff: Mentor

    You treat it as an arrangement where some capacitors are in series, and some are in parallel, and you combine what you can, then combine that result with others, and continue in this fashion. You are determining the equivalent capacitance. Resistance is absent from this picture.

    Note: you have a capacitor mistakenly labelled in μH and another in μV.
     
  5. Feb 8, 2016 #4
    Sorry that was a typo in my post. Meant to say capacitance.

    Also, that is how the problem was given. I am not sure if that is a mistake or meant to be there. But then again, inductors have a different symbol, so I guess that needs to be changed. Also, what is the point of telling me all the voltages across those capacitors? Will I need that to find the voltage later? Or do I find the voltage using only the equivalent capacitance?
     
  6. Feb 8, 2016 #5

    NascentOxygen

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    The voltage seen at the terminals is a separate problem; you could do it first, or leave it till later.
     
  7. Feb 8, 2016 #6
    Ok, I'll try both and see if I can get it.
     
  8. Feb 8, 2016 #7
    I got 16.3 uF for my equivalent capacitance. I'm not sure how I can find the voltage using that though. I'm not sure how to do it separately either
     
  9. Feb 8, 2016 #8

    NascentOxygen

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    To find the total voltage, work your way around from one terminal to the other, adding up voltages as you go.
     
  10. Feb 8, 2016 #9
    Is that how you add voltages? What if you reach a junction with voltages? How would you treat that? Also, it looks like some of the capacitors were not labeled with a voltage, like 5uF and 24uF. Should I just ignore those or is there a way to find the voltages across those capacitors?
     
  11. Feb 8, 2016 #10

    cnh1995

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    Homework Helper

    Yes. In any loop, the net addition of voltages should be 0. This is according to KVL.
    Think harder:wink:! There's a way and I'm pretty sure you'll see it if you looked at the arrangement carefully..
     
    Last edited: Feb 8, 2016
  12. Feb 8, 2016 #11
    I don't think there is an equation for this. I was thinking that voltage is the same in parallel, so would the 24uF be at 2V too? But I don't know about that because the 5uF is in a weird position. It's in parallel with 2 different voltages. I don't think that's how you'd find it either.
     
  13. Feb 8, 2016 #12

    cnh1995

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    Parallel connection means having common end points. 5μC is in parallel only with 25μC and no other capacitor.
    If you are still confused, use this.
     
  14. Feb 8, 2016 #13
    Ok so if I wanted to find the voltage for the 5uF I would use KVL for the top box and it would be +10V -V5 =0. But how do I find the voltage across that capacitor? Is there an equation for that? One that relates voltage and capacitance? I don't know the energy stored or the charge, so I'm not sure how I can find the voltage.
     
  15. Feb 8, 2016 #14

    cnh1995

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    Doesn't that give V5=10V? This is the voltage across the 5μF capacitor.
     
  16. Feb 8, 2016 #15
    Oh... Wow. Yes it does. I don't know what I was thinking. -_-
    facepalm_400x400.jpeg
     
  17. Feb 8, 2016 #16
    So once I get those missing voltages (10V for V5 and 5V for V24) how would I get the voltage across the terminals?

    This is what NQ said, but I'm not sure how. What do I do when I get to a junction or if I have voltages in parallel, like the box on top? Is there a way to add voltages in parallel? I think if they are in series you just add them together.
     
  18. Feb 8, 2016 #17

    cnh1995

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    :smile::smile:
    Edit: This is for the above image..My login expired and that got this post here, after your queries..:wink:
     
  19. Feb 8, 2016 #18

    cnh1995

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    Choose any loop containing the terminals and have a "KVL walk" along it.
     
  20. Feb 8, 2016 #19
    Ohhhhhhhhh. :confused::confused::confused: I don't know why I am overthinking this so much. So much facepalm. 25V across the terminal.
     
  21. Feb 8, 2016 #20

    cnh1995

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    Right.
     
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