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Series or parallel circuit?

  1. Jun 9, 2012 #1
    We are shown a circuit diagram as in attachement
    It shows an open switch and two capacitors (both of which have been charged fully in previous experiments

    Question: The capacitors are joined as shown in the circuit. When switch S is closed the voltage across both capacitors is the same. Why?

    The answer says: they are in parallel

    Surely this is wrong? They are in series, no?
    As the charge can only follow one path it must be series, right?

    Thanks
     

    Attached Files:

  2. jcsd
  3. Jun 9, 2012 #2

    Dale

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    When two components are in parallel they have the same voltage across them, and when they are in series they have the same current through them. So actually, they are both in series and in parallel. I.e. for a circuit this simple there is no difference between series and parallel.
     
  4. Jun 9, 2012 #3

    surely as the capacitance of the two is different the p.d cannot be the same

    would you agree though they are in series....
     
  5. Jun 9, 2012 #4

    phinds

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    Yes, but that is a temporary artifact of the initial conditions and is irrelevant to whether they are in series are parallel.

    I agree w/ DaleSpam
     
  6. Jun 9, 2012 #5

    I don't understand why they would aquire the same p.d
    If we have a series circuit with resistors...the p.d is NOT the same across all of them if they are different...please could you explain how it is possible for p.d to be the same?

    note: they both store the same charge as they are in series....Q=CV therefore V cannot be the same for the two components
     
    Last edited: Jun 9, 2012
  7. Jun 9, 2012 #6


    Here is the direct question
    I realise I may not have been so clear
     

    Attached Files:

  8. Jun 9, 2012 #7

    Dale

    Staff: Mentor

    They must acquire the same p.d. A wire has the same potential everywhere, that is the basic purpose of a wire. So the top wire is one potential and the bottom wire is another potential. So there is the same potential between every pair of points between the top and bottom wires, including across each capacitor. That is why parallel components always have the same voltage across them, the wires constrain it to be that way.

    They do not store the same charge. Being in series means that they have the same current go through them, which means that the change in the charge is the same. That is not at all the same as having the same charge unless they both start uncharged or with the same charge.
     
    Last edited: Jun 9, 2012
  9. Jun 10, 2012 #8
    I think this makes sense...however, if they were on the SAME side of the wire (i.e looked like this)

    ______
    |
    |
    C
    |
    |
    C
    |
    ______
    (i cannot do the last side....C = capacitor)

    Would the potential across each capacitor be different now because they have different capacitance valeus or would they still be the saem. If so, why?


    Ok, thanks
     
  10. Jun 10, 2012 #9

    sophiecentaur

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    This is yet another thread about 'Definitions'. There is absolutely no need to get aerated about which things fit into which category as long as you can grasp the underlying relationships. Kirchoffs laws do not explicitly use the words 'series' or 'parallel' because they are not really relevant. As long as you remember the basic idea of a node or a loop, everything sorts itself out.

    The distinction between series and paralell in situations like this is actually about as relevant as whether Pluto is or isn't a planet. Pluto doesn't care what you call it - it still has the same orbit!
     
  11. Jun 10, 2012 #10
    ok...thanks
    could you take a look at my post in #8
    thanks

    also I am not sure I do know the difference between a node or loop (or even what you mean by them)
    edit: yes I do...just looked it up :)

    and therefore this explains the answer to post #8...it makes no difference
    this was very useful...thanks for your helkp :)
     
    Last edited: Jun 10, 2012
  12. Jun 10, 2012 #11

    sophiecentaur

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    A node is a part of a circuit where two or more components are joined together. A loop is a path that you can trace on a circuit with your finger, through any number of components that will bring you back to the same point.
    In this simple case, points where the two capacitors are joined are nodes but you can get many components coming together at a node in some circuits (allowing many 'parallel paths'). Likewise, loops can involve dozens of components ('in series').
    The actual layout of a circuit - how it's drawn on paper, makes no difference to how it will behave.
     
  13. Jun 10, 2012 #12
    yes I just realised this....thanks so much :)
    problem solved :)
     
  14. Jun 10, 2012 #13

    sophiecentaur

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    Once you get onto Kirchoff, things will probably make more sense. :smile:
    'Early Science' is littered with definitions and classifications that confuse people who actually try to THINK about it. This is obviously your problem and it used to be mine too. I just don't care, these days haha.
     
  15. Jun 10, 2012 #14

    Dale

    Staff: Mentor

    They would still be the same. Why would they be different? You haven't changed the circuit any, just drawn it differently. They still have a wire connecting one side and another wire connecting the other sides and so all of my description above still applies exactly the same.

    Btw, sophiecentaur mentioned loops and nodes. If you haven't gotten there yet then once you are exposed to those it should clear things up quite a bit. My prefered method is the node voltage analysis.
     
  16. Jun 10, 2012 #15
    yes when this was pointed out to me I realised how foolish I was being...thanks for your support :)
     
  17. Jun 10, 2012 #16

    Dale

    Staff: Mentor

    No worries. Honestly, if they haven't given you Kirchoff's laws then I don't think that they should be having you solve circuits, but that is just my viewpoint (apparently not shared by your professor).
     
  18. Jun 10, 2012 #17

    sophiecentaur

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    I totally agree, DS. Too many teachers seem to rely on rote definitions and stock answers rather than encouraging some deeper understanding. I think that, in many cases, it's because they are not too sure of some topics. To be fair, Electricity seems to give more trouble than other topics. Something to do with the abstract logic, lack of opportunity for 'purple passages' in descriptions and the need for a Maths approach from the start, probably.
     
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