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Thread: capacitors in series and parallel

  1. #1 capacitors in series and parallel 
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    hello everyone.
    while i understand how to deal with problems with capacitors i can't find an explanation to the following:
    1) why capacitors in series have the same charge.
    2) why capacitors in parallel have the same potential differences.

    i'll be glad for an explanation.
    thank you,
    Rotem.
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  2. #2  
    x0x
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    Quote Originally Posted by rotem300100 View Post
    hello everyone.
    while i understand how to deal with problems with capacitors i can't find an explanation to the following:
    1) why capacitors in series have the same charge.
    Well, the "negative plate of capacitor C1 is connected by a wire to the "positive" plate of the next capacitor in series, C2. If they had different charges the electrons would migrate from one plate to another until the charges equalized.

    2) why capacitors in parallel have the same potential differences.
    Because all "positive" plates are connected to the same node. Same thing for all the "negative" plates. So, Kirchhoff law (theorem, really) mandates that they all have the same potential, if they didn't then electrons would "migrate" again until the potentials equalized.
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  3. #3 thanks 
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    Quote Originally Posted by x0x View Post
    Well, the "negative plate of capacitor C1 is connected by a wire to the "positive" plate of the next capacitor in series, C2. If they had different charges the electrons would migrate from one plate to another until the charges equalized.



    Because all "positive" plates are connected to the same node. Same thing for all the "negative" plates. So, Kirchhoff law (theorem, really) mandates that they all have the same potential, if they didn't then electrons would "migrate" again until the potentials equalized.
    thanks
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  4. #4  
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    Quote Originally Posted by rotem300100 View Post
    thanks
    You are welcome
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  5. #5  
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    Capacitors in some ways in series and parallel circuits act very differently than resistors do. Capacitors absorb and slowy release electric charges, where resistors simply reduce current flow. (In an AC circuit) Capacitors do not allow DC to flow.

    Two equal resitors in a series circuit double the resistance where two equal capacitors in the circuit half the capacitance. The opposite for each is true in a parallel circuit.
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  6. #6  
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    Quote Originally Posted by mayflow View Post
    Capacitors in some ways in series and parallel circuits act very differently than resistors do. Capacitors absorb and slowy release electric charges, where resistors simply reduce current flow. (In an AC circuit) Capacitors do not allow DC to flow.

    Two equal resitors in a series circuit double the resistance where two equal capacitors in the circuit half the capacitance. The opposite for each is true in a parallel circuit.
    Here is yet another example of why knowing too little is a problem.

    Your lack of knowledge makes you treat capacitors as different, and thus unnecessarily complicate what is actually a trivially simple picture.

    If you generalise the notion of resistance to impedance, then there are no separate cases to have to keep track of. Impedances in series always add. Similarly, admittances (the reciprocal of impedance) of paralleled elements also add. Always. These rules apply even if the impedances consist of more than one element, including more than one type of element. One only need to know the formula for the impedance of the various elements:

    1) Impedance of a resistance is the resistance itself (easy). The admittance of a resistance is just 1/R, often called the conductance.

    2) Impedance of a capacitor is 1/(j*2*pi*f*C). The admittance of a capacitor is just j*2*pi*f*C.

    3) Impedance of an inductor is j*2*pi*f*L. The admittance of an inductor is just 1/(j*2*pi*f*L).
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    Quote Originally Posted by tk421 View Post
    Here is yet another example of why knowing too little is a problem.

    Your lack of knowledge makes you treat capacitors as different, and thus unnecessarily complicate what is actually a trivially simple picture.

    If you generalise the notion of resistance to impedance, then there are no separate cases to have to keep track of. Impedances in series always add. Similarly, admittances (the reciprocal of impedance) of paralleled elements also add. Always. These rules apply even if the impedances consist of more than one element, including more than one type of element. One only need to know the formula for the impedance of the various elements:

    1) Impedance of a resistance is the resistance itself (easy). The admittance of a resistance is just 1/R, often called the conductance.

    2) Impedance of a capacitor is 1/(j*2*pi*f*C). The admittance of a capacitor is just j*2*pi*f*C.

    3) Impedance of an inductor is j*2*pi*f*L. The admittance of an inductor is just 1/(j*2*pi*f*L).
    Do you agree with UT, (The University of Texas) on this? _ Capacitors in Series and in Parallel and my friend this is trying to keep it simple because I think the OP was a simple question that was not meant to entail things like complicated circuitries and RC time constants and phase shifts. There are also all kinds of what may be called stray capacitances due to positions of various current carrying paths within the circuitry. What I said was not undereducated, and it was accurate concerning the question in the Original post.
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  8. #8  
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    Quote Originally Posted by mayflow View Post
    Do you agree with UT, (The University of Texas) on this? _ Capacitors in Series and in Parallel and my friend this is trying to keep it simple because I think the OP was a simple question that was not meant to entail things like complicated circuitries and RC time constants and phase shifts.
    In this new round of pigeon chess, you pretend as if your answer was a model of simplicity and relevancy. It was neither. The OP's question had nothing whatever to do with formulas for capacitors in series or parallel. Just look at the actual question of the OP.

    I was responding to your answer, not to the OP. It was correct -- and good pedagogy -- to introduce the generalising (and thus simplifying) notion of an impedance. That allows one formula to work always for series-connected elements of any kind. It is an unnecessary confusion to treat capacitors as fundamentally different. It is also irrelevant to the OP. It is, in fact, an example of what you accuse others of doing: You are attempting (unsuccessfully) to appear more schooled.


    There are also all kinds of what may be called stray capacitances due to positions of various current carrying paths within the circuitry.
    True, but again, more irrelevant noise as you poop out more terms in an effort to appear educated.

    What I said was not undereducated, and it was accurate concerning the question in the Original post.
    Your answer does nothing to address the question about charge, nor does it answer the question about why capacitors in parallel all have the same voltage across them. Those were the two questions he actually asked, so your answer was irrelevant to the OP.
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    I am not even sure that all capacitors in series have the same charge. Capacitors are not simple resistances. Different capacitors will pass certain frequencies quite easily, but resist others quite fiercly, however the entire capacitance effect on a series circuit will be a lower capacitance, and will be a higher capacitance in a parallel circuit. This is not the same as the simple resistance of a resistor which will simply resist current fow of any frequency and add in series and divide in parallel.
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  10. #10  
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    Quote Originally Posted by mayflow View Post
    I am not even sure that all capacitors in series have the same charge.
    If you aren't even sure, then you have no business trying to answer (and, by the way, x0x provided the answer to this question, and to the OP's other question, in post #2; the OP thanked him in post #3). In your eagerness to show off, you pollute the thread with noise. Please stop. "Better to remain silent and have people suspect you know nothing, than to open your mouth and remove all doubt."

    Capacitors are not simple resistances... {noise removed}.
    Please stop.

    Learn some physics first. Learn some electronics first.

    When you've figured it out, then you'll be able to contribute.
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  11. #11  
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    Quote Originally Posted by mayflow View Post
    I am not even sure that all capacitors in series have the same charge.
    If you are not sure, were you able to read and comprehend my simple explanation?
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  12. #12  
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    Quote Originally Posted by x0x View Post
    If you are not sure, were you able to read and comprehend my simple explanation?
    I think the capacitive reactance will vary with the frequency dependant on the capacitance value of the capacitor, so one value of capacitor will allow or not different frequencies with different reactances than another. You may have a valid point though as the overall capacitive reactance in a series circuit will be divided between the capacitors in the circuit. This maybe means the same voltage across them each, which is not true of resistors in a series circuit as they will all have the same current flow, but not the same voltage potentials across them each.
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  13. #13  
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    Quote Originally Posted by mayflow View Post
    I think the capacitive reactance will vary with the frequency dependant on the capacitance value of the capacitor, so one value of capacitor will allow or not different frequencies with different reactances than another. You may have a valid point though as the overall capacitive reactance in a series circuit will be divided between the capacitors in the circuit. This maybe means the same voltage across them each, which is not true of resistors in a series circuit as they will all have the same current flow, but not the same voltage potentials across them each.
    The answer has nothing to do with "reactance". You are simply stringing buzzwords.
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    Quote Originally Posted by tk421 View Post
    If you aren't even sure, then you have no business trying to answer (and, by the way, x0x provided the answer to this question, and to the OP's other question, in post #2; the OP thanked him in post #3). In your eagerness to show off, you pollute the thread with noise. Please stop. "Better to remain silent and have people suspect you know nothing, than to open your mouth and remove all doubt."



    Please stop.

    Learn some physics first. Learn some electronics first.

    When you've figured it out, then you'll be able to contribute.

    Dude, it is not that simple. Capacitive reactance is not like a simple resistor. It varies with frequency and creates time delays and maybe phase changes. It has capacities to hold and delay current flow, and I don't know what is meant by equal charges here. Is it equal voltage across the terminals? I don't think it woould mean the same quantity of electron differential would it?
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  15. #15  
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    Quote Originally Posted by mayflow View Post
    Dude, it is not that simple. Capacitive reactance is not like a simple resistor. It varies with frequency and creates time delays. It has capacities to hold and delay current flow where a resistor merely impedes current flow.
    Why do you feel compelled to pollute so many threads?
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  16. #16  
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    Quote Originally Posted by x0x View Post
    The answer has nothing to do with "reactance". You are simply stringing buzzwords.
    If a capacitor had no reactance, it would have no impedence. It is basically two conductive surfaces with some sort of dielectric material between them. It reacts and holds and dissipates AC electrical charges. Resistors are not like that. They may be made of carbon or some other materials that will allow the passage of electrons from atom to atom, but not with as much allowability as copper or silver or gold and such although they all do have some measurable resistance. To say that a capacitor impedes a current the same way a resistor does is nonsense.
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