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AC flowing through a cap. What actually happens?

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

  1. killivolt

    killivolt Well-Known Member

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    This looks interesting ?

    I found some more stuff on this site about the ether and why, how, etc. It really is about a classical model vs contemporary version of the same phenomena.

    I think I'm seeing double again. I'm just glad my feet are planted and I haven't spun off. EM fields do their job in any respect. Without a classical model I wouldn't be able to use my computer.
     
  2. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again kv,

    There's also the possibility of the magnetic monopole, which would force us to change Maxwells third equation. The real story is we just arent there yet, and Feynman suggested that we never will be.

    Since you're looking into gravity, take a look at the Higgs boson and the Higgs field. The Higgs boson is part of the standard model and yet it is just a hypothetical particle! That was the result of another 'fix' to bridge the gap in theory. I believe quantum mechanics says that we should be able to detect this particle, but we wont know for sure for about another year and a half while data is being collected and analyzed by the LHC. I cant wait.
    And how important is this field? Well it just happens to presumably give ALL MATTER its mass! Can you imagine that? Something that important yet we attribute a hypothetical particle to it in order to get some explanation. How's that for an imaginary entity giving us something incredibly real.

    Also we have to keep in mind that science is not that which proves a theory for *certain*, it is just that which proves what theory is more *likely*. We're never there and we never will be. Life and everything we know, feel, hear, taste, smell, and observe are just abstractions of reality. We only know reality based on what is most likely, not on what has been 'proved'.
     
    Last edited: Apr 1, 2011
  3. BrownOut

    BrownOut Banned

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    Every important physical phenominia in EM is quantifiable, and so the equations are supremely important. it doesn't make any sense at all to dismiss something because the math proves it. Sure, math needs to be checked by experimentation, but as the link you posted clearly stated:

    "with flying colors...":eek: Clearly we aren't just talking about equations. We are talking about something that is derivable, quantifiable and verifyable.
     
    Last edited: Apr 1, 2011
  4. dave

    Dave New Member

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  5. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    You missed post number 462.

    Also, you yourself admitted that there was no charge flow between the plates, yet the units work out to units of dq/dt. We have the units, but not the 'stuff'.
     
    Last edited: Apr 1, 2011
  6. BrownOut

    BrownOut Banned

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    The 'stuf' is irrelevant. Current is described two different ways, conduction and dispalcement. Anything else that I supposedly "admitted" is fantasy.
     
  7. The Electrician

    The Electrician Active Member

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    If you're asking me if this "something" is "real", I would say that this totally depends on what you mean by "real"; it's semantics again.

    If you mean by "real current" a current consisting of a flow of material particles, that's one thing.

    If you mean the common English definition of "real", namely "something that exists in the physical world; not just something existing only in human imagination", that's another thing. The magnetic field associated with a changing electric field is "real" in this sense. Whether or not a particular person chooses to call "displacement current" "real" depends on just what they mean by "real".

    If you're asking me if it's reasonable to note that the magnetic field associated with a changing electric field is indistinguishable from that produced by a flow of material charge carriers (a "conduction current"), and give the phenomenon the name "displacement current"--yes, I think it's reasonable, especially when what is meant by "displacement current" is carefully defined as it is by Maxwell.

    If we accept that semantics is crucial, then my take is that it depends on what is meant by the unqualified word "current". I've explained this in detail already.

    For a person who chooses to define the unqualified word "current" as only a flow of material charge carriers, the answer to the question could be "no". Then that person is left with the difficulty of reconciling Kirchoff's current law in near-field circuits containing vacuum capacitors.

    The question "Does current flow through a vacuum capacitor?" is not fully specific. Does it mean "current in any part of the capacitor", "current in the vacuum region between the plates", "current in the wire leads", "all of the previous"? Depending on which of these is meant, and what the questioner means by "current", the answer may vary. Semantics again.

    One of the things one learns in high school debate is to define your terms. Otherwise unnecessary confusion and dispute results. It's good in technical discussions for everybody to agree on definitions. Sometimes it helps to be redundant by qualifying words to be sure meanings are clear, e.g., "conduction current" and "displacement current" rather than just "current".
     
  8. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Brownout:
    You mean you didnt say that charge doesnt flow though a capacitor previously? Which is it? You believe charges flows through it or not?
    I thought we all agreed that it didnt.

    Electrician:
    I see what you are saying but you can't use the term "displacement current" if we dont believe that that is a current.
    My definition for current to start with is dq/dt, but then we have the aforementioned "displacement current", which was originally incorrectly called that because it came out in units equal to dq/dt. If it's in the same units as dq/dt, but it is not dq/dt, then we're still missing something, and every professor i have read or talked to so far says that it's not real, so that explains it.
    The example i tried to give earlier (as bad as it may be) was a setup where we would have the normal units be "cubic meters of water per second", and a fictitious current that appeared to flow between tanks but really didnt, that would also have to come out to "cubic meters of water per second". Now if the second flow came out to cubic meters of water per second but there was clearly no water flowing, we might be tempted to say this was the "displacement current" but it would not be a flow of water, even though the units came out the same. It's quite simple really. When we have something that works out to some units but it is not really that, we call it 'virtual', which implies that it is a mathematical tool, not a reality.
     
  9. BrownOut

    BrownOut Banned

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    Try reading my post. I said it is irrelevant. I've only said that like a gazillion times now. What's the point of asking the same question over and over and over? We don't care that charge doesn't flow through the vacuum, and we never did. Dispalcement current doesn't need charge. Try to get it the first time and we don't have to keep going over the same things.

    This is the last time I answer the question about charge flowing through the vacuum. I and others have already answered and explained why it doesn't matter ad nauseam. If you want to know if charge flown or not, refer back to post #468.

    Except that your current was completely imaginary, and iD is derivable, quantifiable and verifiable, and required by the laws of circuits.

    To recap, theis is what I've claimed:

    Any other supposed claim is fantasy. Any other statement I've made that's taken out of context, cherry picked or otherwise distorted is invalid. My claim is and has always been that iD doesn't care about charge.

    Do you get it now?
     
    Last edited: Apr 1, 2011
  10. BrownOut

    BrownOut Banned

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    Deleated double post.
     
    Last edited: Apr 1, 2011
  11. MrAl

    MrAl Well-Known Member Most Helpful Member

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    You really are getting quite funny here. You state something a different way just so you can say that you didnt say something before.
    By saying that there "are two types of current" you are still saying that charge doesnt flow through a capacitor, otherwise you wouldnt have to mention a different current. Do you get this now?

    "Except that your current was completely imaginary, and iD is derivable, quantifiable and verifyable, and required by the laws of circuits."

    Oh really? Then you will be surprised to know that Oliver Heaviside himself even denounced Maxwells "displacement current", and that is the guy who really wrote those equations you keep referring to. It was described as a "mathematical convenience" which is no longer needed.

    You really have to review what motivated Maxwell during his time to see how silly this is. He had no other way, and some have described it as "his only way across" and phrases similar to that. He had no other way to explain what he was seeing! He did NOT KNOW how fast electrical current flowed, and importantly he did NOT KNOW that charge spreads out at right angles to the leads of the vacuum capacitor when it enters the plates. In other words, in his time he did not have the knowledge to do ANYTHING else but to assume some kind of supposed current! That's the only thing he had to use. Today we have more.
    Also, i have read now that he expected this displacement current to be present in the capacitor, yet non existent in a transmission line. You also have to remember that was a long long time ago, and things change in scientific research almost every day.

    Like i said before, the more articles i find the more refute this displacement current.

    Since you are so determined to show that this displacement current exists, and what's more is SOMEHOW still needed, i challenge you to find one single at least partly credible reference to show indisputably that Maxwells displacement current is some sort of real current.

    If you cant find any solid reference then at least you and i do not have anything else to talk about, because as you said, i keep repeating and as i said, you keep repeating, so it's time to stop or present something solid here.
    Of course i'd be happy to talk about something else :)
     
    Last edited: Apr 1, 2011
  12. BrownOut

    BrownOut Banned

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    False. I've been completely consistant.

    Charge is irrelevant to iD. See post #468

    Oh really? You might be interested in this:

    And this:

    My take is that Heaviside, as many other, claimed that notion of the ether was no longer needed, but validated the notion of the displacement current. Besides, Heaviside's theories have already been reconciled with Maxwell.

    Maxwell has passed the test of time. He is taught in more EE and physics classes than anyone. His theroies in EM remain among the most important discoveries in the physical sciences. I see nothing silly about that.

    I find just the opposite. I've found many that validate Maxwell.

    Been there; done that. You have yet to show it doesn't exist.
     
    Last edited: Apr 1, 2011
  13. Mr RB

    Mr RB Well-Known Member

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    1. The "phenomenon" is real
    2. The "phenomenon" may be called "displacement current"
    3. Points 1 and 2 in no way prove that "displacement current" is "real current"

    Be careful of the non sequitur.

    1. Margarine is a real thing
    2. Margarine may be called "butter" for convenience ie; "butter my bread"
    3. Points 1 and 2 do NOT prove that "margarine" is "real butter" :)

    Real current is a very specific thing. For something to be "real current" that something must be very specifically matched, and this is not the case.

    If your above statement can be held to be true then in the case of a transformer you would have to agree that there is a "real current" flowing from primary winding to secondary winding.

    I agree with much of what Brownout says although only up to the point of "displacement current" being a real phenomenon that is not "real current" but we can describe it as a current because of the benefits of convenience.
     
    Last edited: Apr 2, 2011
  14. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Brownout:
    I've read many books that ask for students to calculate displacement current, but in those same books in another section they might ask a student to calculate the charge that flows through a capacitor. That's dq/dt, not displacement current. That isnt real either but it helps to know that theory.

    Hello again MrRB,

    Yeah that pretty much sums it up. For some reason Brownout wants to use statements about Maxwells equations and Maxwells equations themselves to prove something that we are trying to prove (or disprove) in the first place. That's where i dont agree at all. Even if Maxwells displ current is real we could not prove that with his equation.
    Maxwell also believed what many other people believe today: that current flow INSIDE the capacitor proper is tangent to the capacitors leads. This is absolutely not true. Current flow OUTSIDE of the capacitor proper is tangent to the wire leads, but once the charge gets inside the capacitor, it has to flow perpendicular to the wire leads across the plate. That means that nothing is flowing tangent to the wire leads any more. The acceleration required for this action generates an EM wave, but on the other plate another EM wave is being generated in the exact opposite phase, so the two EM waves cancel out. That's why the cap doesnt work very well as a transmitting antenna, because the EM waves cancel inside and so they dont show up outside.
    We can also note very interestingly that a transmission line is made up of two wires that act as inductance, resistance, and yes, capacitance, yet the transmission line equations do not require any 'displacement current'. I find that very very interesting, because here we have capacitance yet there is definitely no displacement current. Thus, the better model for a capacitor is a transmission line and that would force us to conclude that there's no displacement current in a capacitor.

    Brownout probably wont accept this either because he wont accept anything contrary to some kind of 'displacement current', but that's entirely up to him and after all that's his right to choose.

    Now back to the question of "Does current flow through a capacitor".
    As Electrician pointed out, this is not a simple question to answer, because even if we accept displ current as 'real' somehow, we'd have to include that into the answer or else we'd really have a problem when that person came back and asked about displ current :)
    In a circuits only course, we'd be taught that charge flows through the capacitor! That's because in that kind of class it would be considered a macro view, not a micro view, which is ultimately what is really happening.

    I think now we should concentrate on how to answer this question rather than continue to argue about displ current being real or not. However, after all i have read now, which includes many many papers and textbooks, i will never be able to accept displacement current as being real.

    Thus, I conclude that displacement current is not real, unless im wrong, which, you know, I'm not. :)
     
    Last edited: Apr 2, 2011
  15. killivolt

    killivolt Well-Known Member

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    Very interesting the LHC did have something occur but, nothing to support that it exists until complete test runs have been made which, ( they may find something after analyzing of course). Science needs to have some sort of explanation to support certain phenomena's. I like the description of how is plays a roll in Dark Matter as well as other formations of ("weakly interacting massive particles"). It would lend some reasoning to the why of things and their origins and when like the Higgs Low energy mass suggested as the lowest from the begging of the Universe that is held in a perfect state until it combines with other masses to form our known masses.

    This low energy mass is everywhere at all times. Just like gravity and could be a part of well known phenomena. No one really knows though, yet to be discovered as real.
     
    Last edited: Apr 3, 2011
  16. The Electrician

    The Electrician Active Member

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    "displacement current" is a phrase, a name, not a phenomenon. It's the referent, the "phenomenon", which is real. The reality of "phenomenon" is a given; giving it a name is not intended to prove that it's real. Giving it a name which no other "phenomenon" has avoids confusion and is a good practice.

    You are the one who commits a non sequitur. The phrase "real current" doesn't appear in your 1. or 2., so why would anyone think that, together, 1. and 2. even attempt to prove that "displacement current" is "real current"?

    Your example isn't representative of what I've said. You're assuming that the phrase "real butter" has an accepted definition which is different from the accepted definition of "margarine", which is indeed the case in everyday English.

    In your 2., 'Margarine may be called "butter" for convenience ie; "butter my bread"', the green "butter" is a noun and the red "butter" is a verb. Saying '"butter my bread"' where "butter" is a verb wouldn't be taken as an attempt to say that "margarine" is "butter" where both are nouns.


    I have carefully defined what I mean when I use the phrase "displacement current", but I haven't used the phrase "real current" except to say "If you mean by "real current" a current consisting of a flow of material particles, that's one thing.".

    Therefore this has no relevance to what I've said. I haven't said that "displacement current" is "real current". "real current" is a phrase that others have used, and which those others usually don't properly define.
     
  17. Mr RB

    Mr RB Well-Known Member

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    Yes. Twice, and very deliberately. :)

    You used the term "conduction current" which has been generalised by almost everyone in the thread as "real current" and I interpreted your meaning as such.

    I'll quote you again;
    I'm not saying I disagree with you on this, or that you are necessarily "wrong" with this statement. For the record I don't mind the term "displacement current" at all. I just wanted to make a point to be careful of non sequitur here.

    Maybe I have interpreted your words wrong, but to me it looks very close to this assumption (and I'll paraphrase);
    1. the mag field caused by a capacitor's plate fields
    2. is similar/identical to the mag field caused by "real current"
    3. therefore it is reasonable to use the term "displacement current"
    4. ie (for emphasis) therefore it is reasonable to use the term "current"

    So it appears non sequitur in that you appear to be implying that because two different things produce similar magnetic fields it is ok to call them both "current".

    As a heavy handed example; a bar magnet and a "real current" both produce magnetic fields but you would not call the bar magnet "current".

    If I have misinterpreted your words or your premise I apologise and look forward to an explanation of what you actually meant.
     
  18. BrownOut

    BrownOut Banned

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    A book worth it's salt would only speak of displacement current fowing through a capacitor.

    Or course not. The best model for a capacitor is a capacitor, not a transmission line. Note that the transmission line equations were developed from Maxwell-Ampere, and as such, are dependent on the dE/dT term, as are all wave propagation.

    False. I've done much more than that, as anyone who has actually read my many posts would know. However, I consider the math to be an important constituant of the theory and of the proof, and no amount of hand-waving is gonna change my mind about that.

    I don't see any reason to distort this very simple, direct and accurate statement. I see no non-sequiter, no false logic and no problem whatsoever. We've said all along that in many ways, dispacement current is indistingushable from conduction current, and so it's not a matter of what it's "called", rather what it is.
     
    Last edited: Apr 4, 2011
  19. BrownOut

    BrownOut Banned

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    Absoluely not! The fields in a transformer are formed by a conduction current. The fields in the vacuum cap are formed by a displacement current. The two senarios are not equvalent.
     
    Last edited: Apr 4, 2011
  20. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Interesting judgement on a book you couldn't have possibly ever read. You've managed to take the entire chapter out of context.

    The transmission line theory is due to the fact that the charge does NOT move tangent to the current in the wires, it moves normal to the current in the wires. That puts it on par with a transmission line when viewed say from the side.

    You call everything you dont understand magic, but yet you dont seem to realize that its just theory and theory is not nature. Physicists regularly drop nature in order to explain things with theory. Im very surprised you didnt learn that a long long time ago. All theory is just an attempt to explain nature, but it itself is not reflexive: nature does not have to obey our ever so humble descriptions of itself.

    Indistinguishable to someone who doesnt know how to distinguish the difference. We're looking inside the capacitor now, not outside.
    I'd also like to point out that it is more probable that that which is 'distinguishable' contains more intelligence content than that which is 'indistinguishable' because we can always derive the indistinguishable from the distinguishable, but not vice versa.
    Also, the phrase "displacement current" is quite obviously not the same as the one word "current", so there must be a difference. I propose that the former is not real, but even if it was real it's still not the same as the word "current".


    False. No one ever said that "fields are formed by a displacement current". Everyone said that "the fields that are formed are formed in the same way that a current would form them". Notice the future tense word "would", not the past tense word "did".
     
    Last edited: Apr 4, 2011
  21. BrownOut

    BrownOut Banned

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    Actually, it was you who took it out of context.

    Actually, the classic transmission line thoery is only an abstract. For example, it doesn't account for lateral movement of charge.

    NO, I never do.

    I understand theory and nature perfectly.

    And to everyone. I've only been talking about inside the capacitor, but have clearly stated that the capacitor is but a single case.

    False. "Everyone" didn't say that. Fields are formed by dispalcement current. Many of the references say that. The fields are coincident with the displacement current, not some 'future' current.
     
    Last edited: Apr 4, 2011

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