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A Pedantic Question

Discussion in 'Members Lounge' started by jpanhalt, Aug 6, 2013.

  1. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    This question may seem a bit pedantic, but I am hoping some thoughtful people will reply. It is often said that AC does not flow through a capacitor or words to that effect. Those writers usually go on to say it just looks like it does.

    For example see here: http://www.electro-tech-online.com/threads/why-doesnt-c1-get-hot.135803/#post1138302

    In contrast, it is generally assumed that AC flows through a copper wire. My question for those feeling pedantic on this lovely evening is, "How do you distinguish AC flow through a wire from what 'looks like' AC flow through a capacitor?" Maybe the wrong question is being asked.

    John
     
    Last edited: Aug 6, 2013
  2. dougy83

    dougy83 Well-Known Member

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    ESR doesn't affect whether current is present or not; that part of the statement is plain falacious.

    If you say that there is no current flowing through a capacitor, then you can only be referring to the fact that the specific electrons entering the capacitor will not exit the other side. There is current in the capacitor leads and plates, just not through the dielectric. Even though a large enough capacitor can behave as if it were a plain piece of wire with AC current applied, no specific electrons pass through the dielectric of the capacitor.

    There is current through a wire, as the electrons are known to pass longitudinally along it; there is no insulator to stop the flow.
     
  3. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    @dough83,

    What experiment can you propose that will show the difference?

    John
     
  4. dave

    Dave New Member

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  5. Ratchit

    Ratchit Well-Known Member

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    jpanhalt,

    Shouldn't this post be in the thread you are referencing, where thoughtful people are residing, and not the members lounge?

    Are you really asking what is the difference between AC current in a branch with no caps as compared with a branch with one or more caps? Answer: There is no difference. I already explained in a previous thread why no current exists through a capacitor, yet exists in the branch containing the capacitor.

    Ratch
     
  6. Ratchit

    Ratchit Well-Known Member

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    doughy,

    Yes, you are right. I was thinking of the energy loss caused by the ESR

    Ratch
     
  7. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    I think the lounge is where it belongs, perhaps in chat, but certainly not as an OT tangent to the original thread. If a moderator sees fit to change it, I am OK with that.

    John
     
  8. dougy83

    dougy83 Well-Known Member

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    Well, you can fill up one side of the capacitor with blue electrons and the other with red electrons. Then apply an AC current. If you have trouble colouring the electrons, you could try the equivalent experiment using dyed water with a rubber diaphragm in place of the dielectric.
     
  9. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Last year there was a long conversation on electrons flowing in a wire. Several people thought electrons don't flow in wires. You put electrons in one end of a wire and different electrons come out the other end. 'no flow'. Either way (wire or cap) electrons in/electrons out. Looks like current flow.

    I am catching electrons and putting a red dot on them. I will see if they come through. lol
     
  10. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    @dougy83
    Can you be a little more thoughtful and serious? As best I know, the particles of water in a perfect wave have negligible forward velocity. We are talking electronics, not urology. :D

    @ronsimpson
    I think we may be in agreement. Maybe entangled electrons would solve the question? I think the capacitor and wire may give the same result.

    John
     
  11. Ratchit

    Ratchit Well-Known Member

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    jpanhalt,

    If you are referencing the thread, especially the "OT" part of the thread, then that is the place to put your posting.

    It says in the purpose statement of the members lounge, "This forum is for talking about non-electronics stuff amongst fellow members."

    Will you have any choice?

    Ratch
     
  12. dougy83

    dougy83 Well-Known Member

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    So red dots are more thoughtful and serious? :confused: I don't know what you are referring to re moving particles of water not moving - do you have a reference for what you're meaning?

    If the drift velocity is high enough, how can you argue that the electrons won't make it all the way along the wire?
     
  13. Ratchit

    Ratchit Well-Known Member

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    ronsimpson,

    You don't remember those discussions very well. An analog example consisting of a water hose filled with marbles was used to explain that if you popped a marble in at one end, a different marble would pop out almost instantaneously at the opposite end. You could insert marbles all day long before the first marble came out. The flow of marbles in the hose was very slow and was called the drift velocity. Same with electrons in a wire. Slow drift flow and almost instantaneous response. That is electron flow and current existence.

    Ratch
     
  14. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Ratchit,

    I am asking about AC, not direct current. Have you considered the effect of "drift velocity" on linear displacement of electrons in a wire?

    John
     
  15. Ratchit

    Ratchit Well-Known Member

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    jpanhalt,

    What difference does AC make? It only means that the charge carriers reverse the direction of their drift velocity periodically. By the way, drift velocity is a net effect, not the behavior of a single electron. There could be uncountable electrons going every which way, but as long as the net flow is moving a one direction, that defines current.

    Ratch
     
  16. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Ratchit,

    Please, let's try to stay on topic. You and several others have said that there is no AC flow through a capacitor. Unsaid, but clearly implied is that there is AC flow though a piece of copper wire.

    A variety of vague theories have been put forth to support the the first proposition. One cannot show that conduction through both is the same, but those who claim the capacitor doesn't conduct should be able to show there is a difference.

    So, boiled down, what experiment can you do to show that AC conduction does not occur through a capacitor, but does occur through a wire?

    Comments about where to post this question, colored balls in a pipe, water waves, rubber bladders, and so forth really do not address that question.

    Can you answer it?

    John
     
  17. Mr RB

    Mr RB Well-Known Member

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    * AC current through a wire will not cause the two ends of the wire to be at different voltages.
    * The same AC current through a capacitor will cause a voltage on the two legs of the capacitor.

    It's about the simplest test you can do?
     
  18. Ratchit

    Ratchit Well-Known Member

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    jpanhalt,

    I, for one, have been staying on topic. I cannot speak for others. You asked about AC in a wire in your previous post. I answered that question. Was there something you did not understand about the answer?

    You were not given theories by me, you were given explanations for both of those phenomena.

    Sure, I already have done so. Experimentation is not needed. Just a knowledge of material properties and a basic knowledge of physics.

    Current exists in a wire because it is conductive. That means that a current will exist in the wire if a voltage is applied across its ends. The alternating polarity of the voltage makes no difference, conduction will still occur.

    A capacitor is a dielectric sandwiched between two plates. Because the dielectric is nonconductive, no current will exist through the capacitor. See http://www.thefreedictionary.com/dielectric
    Didn't you see post#8 of this thread? http://www.electro-tech-online.com/threads/understanding-transistors-capacitor-and-diode.135727/. It explains why current is present in the capacitor branch, but not through the capacitor itself. There is no difference between the currents in a plain wire and a wire containing a capacitor, unless you vary the excitation enough to limit the component response.

    Ratch
     
    Last edited: Aug 6, 2013
  19. ronv

    ronv Well-Known Member Most Helpful Member

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    It is the difference between the practical and the physics. I for one am to old to worry about the physics but when I look below it makes perfect sense. Current must flow thru the capacitor (where else could it be going?). Now we (you) can argue about what kind of current and what color electron. They all look brown to me. :rolleyes:
     

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  20. Ratchit

    Ratchit Well-Known Member

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    ronv,

    I can understand what physics means in the above sentence, but what does practical mean?

    If not worry, at least have knowledge of the physics. Let's see if it makes sense.

    Tsk, tsk, where indeed? The current is accumulating charge on one plate and depleting charge on the other plate, but no charge is flowing through the capacitor because the dielectric is an insulator. So the current in shown in the Spice analysis is the current of the branch, not the current through the capacitor.

    You don't have to worry about kind or color. You have to discern where the current is and is not.

    Ratch
     
  21. rumpfy

    rumpfy Active Member

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    Can I weigh in?
    A capacitor is a 'CHARGE' device. within the dielectric, charge is stored. The amount of charge stored is given as the quantity of coulomb stored.
    Since coulomb is given as AMPS x TIME, then we can say that Q= I*T.
    Also, we know that the value of capacitance is given as 'so many COULOMB per volt'.
    So, if we apply a 1 volt EMF to a capacitor, the voltage across it will be 1 volt and if it is a 1 farad capacitor, it will store 1 Coulomb of charge. So, Q= C*V
    If we substitute I*T for Q, we get C*V=I*T
    Now if we differentiate this expression with respect to time, we get
    C* dV/dT = I (C being a constant)
    What this says is that the current through a capacitor is equal to the time rate of change of voltage. If the voltage across the capacitor is DC then the time rate of change of voltage is zero; ie the capacitor has no current through it if the applied voltage is DC.
    A capacitor is then just another circuit component whose Time rate of change characteristics affect the circuit and the other circuit components. An AC signal current has a Time rate of change which is greater than zero ie dI/dT > 0.0
    Since current is an electron flow, then I'm not sure what the crap about electrons in wires being somehow different to electrons in dielectrics is all meant to mean.
    I think we all should sit down now and have a nice glass of red (Aussie Shiraz is good!!!).
     

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