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Parallel Vs. Series - Capacitance?

  1. Mar 22, 2012 #1
    What exactly is the reasoning behind deciding to use parallel vs series? What is the value behind capacitance, really, as opposed to a general electric circuit? Thanks.
     
  2. jcsd
  3. Mar 22, 2012 #2

    davenn

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    Normally you wouldn't see series capacitors in a circuit, unless they were part of say a filter circuit that had inductors going from the junction of the two capacitors to ground.
    But you wouldn't normally just see two series caps on their own, instead you would just use a smaller value single capacitor.
    Remember working out the total values of series and parallel capacitors is opposite to working out the values of series and parallel resistors.....
    Series resistors add their values.... Parallel capacitors add their values.

    Now it's very common to see multiple capacitors in parallel specially on RF gear like I work on.
    In a RF circuit power supply rails must often be decoupled by multiple capacitors of different values and types so as to keep wide frequency range RF off the power rails.
    Eg. You mite have a 10 uF electrolytic or tantalum and a 0.1uF disc ceramic and a 0.001uF disc ceramic ..... Just an example of cap use in my world

    Cheers
    Dave
     
  4. Mar 23, 2012 #3
    Vs(DC) ---------||-------- ground

    In direct current, electricity cannot flow through a capacitor (it is actually a discontinuity in the circuit).

    Vs(~) ---------||-------- ground

    In alternating current, the capacitor acts as a kind of resistor (it has an associated impedance).
     
  5. Mar 23, 2012 #4

    phinds

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    Which, of course would mean that the caps are NOT in series.
     
  6. Mar 23, 2012 #5

    phinds

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    And what does that have to do with the question?
     
  7. Mar 23, 2012 #6

    phinds

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    In addition to what davenn pointed out, another reason to use parallel caps is simply to increase the capacitance in the circuit if you don't have a single cap that is big enough.

    I can't think of any good reason to put caps in series other than (as a REAL stretch) to end up with a total capacitance that you can't achieve with the capacitors you have on hand by putting them in parallel.
     
  8. Mar 23, 2012 #7

    davenn

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    Wellllll.... ;) Would almost be tempted to dispute that. Haha.
    My thoughts are always in the RF field. I don't do much with filters etc in audio work.
    If I have a "T" filter with 2 caps in series and an inductor in parallel from the junction of the 2 caps, for all intents those 2 caps are still in series. How else can we describe it ?

    Always willing to learn. :)

    Dave
     
  9. Mar 23, 2012 #8

    phinds

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    Calling it a T seems quite reasonable. Calling it series caps is just flat wrong and misleading.

    If all three of the elements you describe were resistors, would you describe the arms of the T as series resistors? Just flat wrong.
     
  10. Mar 23, 2012 #9

    Redbelly98

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    It would be described by a circuit schematic. phinds is correct, the capacitors would not be in series; series elements have the same current because the wire that connects them is not connected to anything else.

    EDIT: Another answer to your question: it can be described as a T filter, just as you have done.
     
    Last edited: Mar 24, 2012
  11. Mar 24, 2012 #10

    NascentOxygen

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    A common use of series capacitors is to allow polarised capacitors to work with AC. (Polarised means they have a positive end and a negative end, and these polarities must be respected when connecting the capacitor to any circuit. Electrolytic capacitors are inherently polarised.)
     
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