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AC flowing through a cap. What actually happens?

Discussion in 'General Electronics Chat' started by qtommer, Feb 25, 2011.

  1. MrAl

    MrAl Well-Known Member Most Helpful Member

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    I can see by this remark you just want to argue, and frankly i have better things to do. Goodbye to you.
     
  2. BrownOut

    BrownOut Banned

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    Good riddens to you, and your unimformed personal remarks.
     
  3. The Electrician

    The Electrician Active Member

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    Are you acknowledging that you mistakenly interpreted my use of "conduction current" to be synonymous with "real current", even though I wrote as some length to avoid just that conjunction of terminology? Others may have made that connection, but I have been at some pain to avoid it. You can see from my posts that I think most of the disputation in this thread is due to semantic confusion brought on by failure to carefully define terms. Please don't attribute to me generalizations made by others.

    A non sequitur is an error of logic, not of choosing names:

    http://en.wikipedia.org/wiki/Non_sequitur_(logic)

    Your point 4. above is most emphatically NOT what I suggest. I have in my posts consistently suggested that the unqualified word "current" is not well defined by most who have used it.

    I think I have explained my position quite clearly in my posts.

    I have also said that anybody may choose to use the unqualified word "current" to mean a flow of material charge carrying particles, but if that is what they mean by the word, they should say so before they begin using it in that sense. Most have not been careful in that respect, and I think it's better to be a little redundant and use the phrase "conduction current" for a flow of charge carrying particles.

    You have done just the thing I complain about. In red above, you have used the phrase "real current" without defining the term. I don't recall having read in one of your posts that you wish your readers to understand that whenever you use the term "real current", you mean something specific which you then describe. So how are readers of post #476 to know for sure just what you mean by that phrase?

    At any rate, you see how this thread has become a discussion of semantics rather than a discussion about just how does AC current in one lead of a capacitor also appear in the other lead?
     
  4. dave

    Dave New Member

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  5. BrownOut

    BrownOut Banned

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    It is generally accepted that the bar magnet has small, circulating currents that produce the magnetic fields, and so I would call the phenomenon current. What I would call the bar magnet is irrelevant, becuase I'm only interested in the mechanism that produced the fields.
     
  6. Mr RB

    Mr RB Well-Known Member

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    For me to have "mistakenly interpreted" it I would have to be wrong. Do you have a better synonym for "real current" than the term "conduction current"? If so please state it.

    Or we could put it to a poll, to see who thinks "conduction current" is synonomous to "real current"? Who thinks that was a good interpretation or a mistaken interpretation?

    I'm well aware of what it means.

    I clearly outlined where I believed the non sequitur existed, and you chose to avoid answering the question and chose instead to nitpick.

    I will make the question of your LOGIC really simple for you this time; Was your premise that because the 2 phenomenons BOTH produced magnetic fields that it is ok to call them BOTH "current"? Surely that is simple enough for you to understand and now answer with a yes or no instead of nitpicking.
     
    Last edited: Apr 5, 2011
  7. Mr RB

    Mr RB Well-Known Member

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    Normally I agree with most of what you say Brownout, but I'm finding that to be a bit of a stretch. Are you saying "current" is the best term to describe what is happening with a permanent magnet? What is wrong with "magnetism" as a description?
     
  8. BrownOut

    BrownOut Banned

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    I couldn't care less what the 'best' term is. I'm saying that at the core of the magnetic field is still a current, and so calling the mechanism a current would be true and correct.
     
  9. The Electrician

    The Electrician Active Member

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    The issue is not whether others may have interpreted "conduction current" as synonymous with "real current". The issue is whether I interpret "conduction current" to be synonymous with "real current". Where you were wrong was in your assumption that I did so interpret. I didn't, I don't, and I pointed out to you that I have never said so. That is where you were wrong.



    Then why do you use it incorrectly?

    These nits are of the essence. Again, you use the unqualified word "current".

    I didn't avoid anything. I didn't respond to your assertion that it is a non sequitur other to say that it isn't a non sequitur, because I don't believe that it is. I responded to what I believe the issue is, namely a matter of semantics.

    You ask whether it is ok to '...call them both "current"'.

    The issue you have raised is about what we call things, not about what they are. It's a matter of semantics, not logic.

    If you, or I, or anyone else, wishes to call the dD/dt term of Maxwell's equation "displacement current", that's ok, but they should clearly say that they are doing so beforehand.

    If you, or anyone else, wishes to limit the term "real current" to a flow of material charge carrying particles, they may do so, and I would hope that they would make that definition beforehand. But, if someone does make this limited definition, I may disagree with it and choose not to use it.

    You want to know if it's ok to '...call them both "current"' without telling me for whom it's going to be ok. It's ok for me, provided the appropriate qualifier precedes "current". I have made clear my reasons for calling the dD/dt term of Maxwell's equation "displacement current". And, it's not for me to decide what others may find "ok"; I can only recommend, and I recommend against using "current" in this thread without a qualifier. It's a matter of definition, not logic.
     
  10. Mr RB

    Mr RB Well-Known Member

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    Another long-winded avoidance of answering the question huh? ;)
     
  11. BrownOut

    BrownOut Banned

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    I can understand The Electrician's reluctance to answer the question, as there is no 'ye's or 'no' answer. There is no "the reason" we call both current, rather a framework of reasoning that he, I and others have shown. Dumbing down the question to a singular reason makes it not possible to answer in any reasonable way.
     
  12. Mr RB

    Mr RB Well-Known Member

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    Thank you Brownout that is quite resonable and much more productive than nitpicking a heap of details of questionable relevance. I would like to state that narrowing things down to a Yes or No is one of the most powerful tools in logic and we should at least try to refine areas of the argument in this binary way as it may lead to an eventual solution.

    Back to the original capacitor topic, I never had a disagreement with it being CALLED current. My disagreement is with the idea that it IS current.

    I think it's drifting into dangerous waters expanding the definition of "current" to inlcude all sorts of magnetic or charge phenomenon between 2 objects.

    If you can answer some questions raised by your previous statement about a permanent magnet it would be appreciated;
    1. If the source mechanism of the magnetic field is a current, would you call the field itself "current", say where it extends external to the magnet body?
    2. If that external magnetic field passes through another object, would you say there is "current" flowing between the 2 objects?
     
  13. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again MrRB,

    You'll note that there are a number of things that just dont sum up when trying to call something a 'current' which is not really a current. A current flow has three dimensions, length, width, and height. The length is sometimes omitted and we just go with width and height as the area through which the current flows. Every current has an area, and in a round wire that means it flows through a circular cross section which can be described by a simple diameter or a radius. Thus we might say that a current of 1 amp flows through a wire of cross sectional area of 1 square cm, which we could then say that the current density is 1 amp per square cm, or 100 amps per square meter. With any other real current we can do the same thing. We might use other units, but it's always a flow through some cross sectional area of reasonable size. For example, we will have a hard time pumping 100 amps through a number 32 gauge wire without burning it up. Other things too through, such as a river. We could describe the flow through a cross section of the river.
    This is the way all currents work otherwise there would not be a current.

    Point is, try doing that with the so called 'displacement current' which has been the subject of most of this thread. Try figuring out where that cross section is. All along we have been asking the question, "what is displacement current and how should we call it", when really we should have been asking, "where is that displacement current and where is its cross sectional area located, and what is that area (in square meters for example) in which it flows".
    If we have a capacitor with two plates of 1 meter square each separated by say 1cm, where does that displacement current flow, and what is the cross sectional area of that flow with a current of 10 amps. See what kind of crazy answers you'll get when you ask someone that believes that displacement current is real.

    If it is some sort of 'flow' we can call it current, but it's not a flow of anything because it's not there. The field lines are not a flow of anything either, so we can not accurately say that the field lines flow either. Dare someone to show you what the cross sectional area of the presumed 'flow' is and *where* it is located inside the capacitor. If it is real we can measure it. It's not real so we can't measure its cross sectional area or say where that cross sectional area is located, like we can with real current. If this is not clear, simply try to point out where it is and how wide and tall it is, and optionally, how long it is.
     
    Last edited: Apr 8, 2011
  14. The Electrician

    The Electrician Active Member

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    What would you like to call the dD/dt term in the relevant Maxwell's equation?
     
  15. killivolt

    killivolt Well-Known Member

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    I'm not sure the question is what would "he" like to call it but, rather what "it" could be called other than the classical term.

    The answer is yet to be defined. There is a lot of discovery that will lend to a better understanding of principles not known in Physics. We are yet to find some underlying principle that will give the classical model a boost in the Topography.
     
  16. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again Electrician and killivolt,


    killivolt:
    Well it's interesting to hear what other people would like to call it too. What would you call it?

    Electrician:
    Well lets see, historically part of it was called "displacement" because Maxwell said the aether was 'displaced', so that's got to go. The other part, "current" came about because of the current flow in the *wires*. Thus the first part doesnt apply at all, and the other part is based on something measured outside the capacitor. I can see the reasoning behind "current" though, because it makes it easier to think about it in terms of a current. But can it really be a current flow? If it was a current flow it would interfere with the 'charging' of the capacitor. What would this 'current' be made up out of? The field lines dont flow, so what else is in there between the plates. We want to know what flows if anything really flows. Once we find out we might have a good name for it. So far though it just looks like a force. A force that can cause other things to happen. That same force appears in the wire but the particles acted on are closer together. In the capacitor those particles are farther apart yet they still get acted on, only seemingly more temporally. It's also of interest that Maxwells breaks down at very close distances. That proves that it is not as universal as we would like it to be, but we probably arent too concerned with that in the classical sense.
    I used the 'regular' Ampere's Law to calculate the field inside the capacitor and supposedly this is the law that confirms Maxwell. That is, the field appears as it would as if there was a real current flowing in the capacitor with distribution over the entire area of the plates. Strange though, if we break the capacitor up in to smaller capacitors in parallel it doesnt seem to calculate out the same way. One method leads to an increasing field along the radius, and the other method leads to a decreasing field along the radius. I could be doing something wrong in the calculation though so i'll look into it more as soon as i can.

    So to make a long story short, if we have to call it a current of some type i like the idea of "Temporal Capacitive Current", because during every normal use the current in the wires starts, stops, reverses, starts, stops, reverses, etc., in other words, it's a temporary current if it is a current at all.
    If we dont have to call it a current of some type, then i'd go with something like "Capacitive Force Field" or something like that.
    These both deserve much more thought of course.

    Also, did anyone ever try to calculate the mechanical force on the dielectric of a dielectric capacitor after say 1 amp had passed for 1 second? It seems that the charge would exert a huge force, but perhaps the force of the field counter acts that.
     
    Last edited: Apr 9, 2011
  17. killivolt

    killivolt Well-Known Member

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    I think this picture say's it all. If when looking at the picture of the sin, it's "Peak / Zero / Peak" with the opposing sin it's your 3 positions explained. As it's saying there is the power factor of enough energy to supply DC to move >>>>>>>>>>> which involves a hole new set of Idea's ? How is that and why and what do you call it.

    All kidding aside, Force has been a part of some things I have read about. In addition it is Capacitive but, then there is some effect happening un-known something else? It's more of a transfer. (Forced Capacitive Transfer) or something that will just stop short of force and say (Capacitive Transference)

    Edit: Maybe even (Forced Transference) because we all know it's a capacitor and would be given.

    Edit: Edit: Oh, yeah. I forgot. When I googled (Capacitive Transference) guess what was in the page.

    http://en.wikipedia.org/wiki/Wireless_energy_transfer

    Edit: Edit: Edit: I had a change in opinion on the subject.

    I think of "transition than transference" So, I'll change add something to it.( Capacitive Transition or Forced Capacitive Transition ) Capacitive is the "force" and is the given. So, you get "Forced Transition".

    tran·si·tion :   

    movement, passage, or change from one position, state,
    a brief modulation; a modulation used in passing.
     
    Last edited: Apr 9, 2011
  18. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello killivolt,


    Yes i guess that deserves more looking into also.

    I decided to take a good look at the fields and found some interesting things out about the fields around the wires and capacitors (see attachment).
    The fields at a distance d1 outside of the wire for either a long thin wire (Fig A) or a long thick wire (Fig B) are the same, and interestingly the field at that same distance OUTSIDE of the capacitor and its wires (Fig C) is also the same, and the grads are all the same too. That's pretty amazing.
    With the capacitor and long wires however, as we get closer to the wire (Fig D) the capacitor field inside looks different than the wire outside (bright green block), and it's grad is also different.
    The field calculations all come from theory, but instead of using the theories directly i decided to do a piecewise calculation which sums small contributions in B for each case, a sort of finite element method.
    All the current levels are the same in all the wires in all four figures and the capacitor.

    Note:
    The field band shown in each figure surrounds the entire wire as a cylindrical shell, so these are cross section views from the side of the wire showing only what is seen above the wires. The bands are just one part of the entire field just for illustrative purposes.
     

    Attached Files:

    Last edited: Apr 10, 2011
  19. killivolt

    killivolt Well-Known Member

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    So, same current in all. But, in (Fig D) the increase permittivity results in a green band which I would suppose would be an increase of EM between the Plates but, not an increase of current?
     
    Last edited: Apr 10, 2011
  20. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,


    Well, in the wire the field is high near the wire and low away from the wire, while inside the capacitor the field is low near the center and high near the edge of the plates (grad is reversed from that shown for the wire), until we get outside of the plate area and then it decreases the same as the wire, acting like a wire that is thicker. So the field looks like the field from a thick wire instead of a thin wire. That's the equivalent of lower current density in the thicker wire.
    Obviously something else is happening because the capacitor can not act like a thick wire for generating the field yet not conduct the same as the wire and short out the power supply with low resistance like the wire would. The only difference seems to be the buildup of the field.
     
    Last edited: Apr 10, 2011
  21. killivolt

    killivolt Well-Known Member

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    Ok, I've been thinking about this, if I look at a single wire and cut it flat with little or no metal burs extending around it. Holding it with the tip up in front of you then, spin it in your fingers

    Then apply voltage, the static EM field should be from 0 to 180° in all directions; with no field around the wire just at the end (Should their be current that would change). However, the field strength should extend out on "y" axis which would mean the field strength is shorter at the center and longer at the edges.

    Now I have 2 Plates with edges. Parallel to each other, each having surface area toward each other and a small surface area around the edges.

    If a voltage is applied you would get field that might behave like the wire and would have field strength at the edges and those fields would couple with little or no field at the center of the plates facing each other? Could this allow tunneling?

    As a rise of charge and EM builds it would begin to stack the force between the plates "attracted to the lower charge at the center Plate B" if a rise in EM around it push's in the stack of force finally reaches Plate B which is slowly exited until it suddenly discharges and the fields collapse.

    If the sudden charge moves, it would cancel the field around the edges momentarily with a radiating EM just like a wire with current flowing. This would continued with AC sin.
     

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