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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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    I felt we were concluding and drew something not related to water. i thought we were pass that and simply put the chain between the Plates as you suggested and called it an EM field.

    I also requested that people look at it for a final to see if there would be another way they would like to complete the phenomena. if you could bring more clarification I would be happy to have your explanation of whats happening or not?

    I would appreciate it.

    Edit: Your post #418

     
    Last edited: Mar 29, 2011
  2. BrownOut

    BrownOut Banned

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    I sincerely doubt it.
     
  3. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Well, the setup i was talking about was only able to emulate the real system to a point, and it was only to show how something that doesnt exist in nature can be used as a mathematical tool. This was best described by the "We have too much information" theory, where that basically says that we have more information (ideas, mathematics, etc.) than we have reality. We can imagine a unicorn, we can draw a unicorn, we can even measure the picture of a unicorn and calculate an approximate size for its single horn, and in fact notice that we can talk extensively about this creature that doesnt even exist! Our imagination exceeds reality, so from time to time we will make things up to help explain and functionally calculate things that really do exist.
    Talking about gravity, there are theories now that explain gravity in terms of many more dimensions than 3 or 4 as we usually accept in everyday life, yet there is, as of the time of this writing, no proof of their existence. There's another example of calculating something using entities that may not exist.
    I've found another physics book in something like its eighth edition now that talks about the problem of calling it "displacement current". They said that this is a poor choice of terms but it is too ingrained into history to remove from all the text books. They admit that it is not a current at all, even though students are asked to calculate "it" in problems in textbooks. So now we have at least one MIT professor, another college professor, and three physics books all denying the existence of a current in a vacuum capacitor. Another quote, "We have come to accept that the displacement current is not real".
     
    Last edited: Mar 30, 2011
  4. dave

    Dave New Member

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  5. killivolt

    killivolt Well-Known Member

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    So, then if I were to say something about the chain; it would be to say before the "switch" is turned on. The chain represents gravitational attraction. Then something else happens which involves EM waves ?

    Am I correct.

    kv
     
  6. BrownOut

    BrownOut Banned

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    What we really have is a college professor who said that displacement correct flows through the capacitor. That other quote that keeps popping up is taken completely out of context, and refers to the old ether idea. If the quote was ever posted in its entirety, it is clear that "current" refers to something else; the old idea of polarization. Anything can be taken out of context, and used to mislead. And then some phantom physics books that supposedly say something. I read too, and all the physics books I've read say this is an important phenomenon, and don't say it isn't real. They point to measurements that prove its existence, to the math that prove its existence. It wouldn't make any sense that something that isn't real is taught in virtually every EM class.
     
    Last edited: Mar 30, 2011
  7. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,


    Well i havent found any evidence that there would be sustained EM waves after a certain rather quick period has expired, generally speaking, when using a constant current. The E field would be increasing however, which should still cause the B field to increase also. So when i referred to gravity i was more or less equating the gravity 'field' to the E field, not really a wave of any kind, except in the first instant when lots of electrons get accelerated. The EM waves probably die out rather quickly using a constant current.
    It gets kind of complicated though, because we have electrons entering the plate area and of course most of them would have to change direction which would generate a real EM wave, so it's hard to say just how much of an EM wave we would get out of this thing. They would only change for a small interval though and then probably mostly straighten out, which may not generate much. I guess we dont have to worry about details like that right now, because the theory we are talking about doesnt go into the second derivative anyway, just the first. We have something that works out to dimensions of 'amperes' using the first derivative and that's really the part we are working with right now.
    The simple analogy was to have a changing gravity field generate a fictitious 'B' gravity field normal to the gravity field, and then calculate how much that would equate to water flowing 'through' the tank sides, and then reverse that calculation to show that we could calculate the 'B' gravity field from knowing the water flow, even though no water really was ever flowing through the tank sides. It really wasnt made to go too deep, but just illustrate the process. There's probably better analogies though.

    So i guess you are looking to build up a better model of the system?
     
  8. BrownOut

    BrownOut Banned

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    This is in response to the false idea that the changing E field was "just" multiplied by a constant, namely the free-space permittivity, so that iD would have units of current. The implication is ostensibly an attempt to suggest that Maxwell fudged and arbitrarily inserted coefficients to make the math come out. I guess this is the same as the equally absurd suggestion that iD was conjured up to explain some pretend or fantasy field that doesn't really exist. As we've already shown, the concept is derived from well established laws of electricity and magnetism, that they explain and quantify real and measured fields in real systems, and that it is necessary for certain systems to obey the laws of energy conservation. But to the point of suggesting the math was somehow manipulated so that it would come out, consider the following derivation:

    Consider the flux of the E field through two circular disks ( A disk capacitor ) If the radius is given as a, then the electric flux through a circle between the plates is:

    φ=∫E.dA= Πa²E=Πa²Q/ξoΠa²=Q/ξo

    We are using the identies E=σ/ξo And σ=Q/Πa²

    By differentiating both sides of the above eqn with respect to time ( using the far left term and far right one )

    dΦ/dt=(dQ/dt)/ξo=I/ξo.

    So, the time derivative of the electric field is equal to current divided by ξo. Note that nothing has been arbitrarily added or taken away; the equations have been developed from well known facts from E-M theory. So far, no quantities were multiplied by any constant just to make the units come out right. To summarize:

    dφ/dt=I/Q (Amps*Meters/Farads)

    Next we do a very simple and completely legitimate algebraic operation to make the equation more understandable. Again, we add nothing and we take nothing away, everything in the resulting equation comes from the one we just derived:

    ξo*dφ/dt=I.

    Note once again, nothing added, nothing taken away. This was no mathematical trick and no artifice. This proves decisively that the right hand of the equation has units of amps. No trickery is involved. Now, if anyone has any problems with the last algebraic operation, go ask any 7th grader if that is a legitimate operation, and you'll find that everything has been done above board. Further, it can be just as easily shown that the preceding development is the solution to the continuity equation and conservation of charge law. It has also been shown that when applied to the capacitor problem, the resulting displacement current in the vacuum is identically the conduction current in the capacitor, and thus satisfies and completes the circuit from a current standpoint.

    Now, that is how one must evaluate the equations that are given in the theory of Maxwell’s equations. To make the absurd claim that the derivative of flux was "just multiplied" to make the units come out right, without doing the math to find out if that is really the case, is just lazy guessing, and should never be claimed in a technical discussion. Math is the foundation that physics is built on. Those who disparage the results because they are supported by math, disparage every and all results from physics. Maxwell didn't guess, he didn't throw up arbitrary postulates, his work is among the most important contributions to the world of physics, and has been compared to Newton's laws of motion. Nobody is gonna come in here with their college understanding of physics and trump the fathers of the physical sciences. As soon as someone claims they can disprove Maxwell with sophomoric math and science, we know someone is about to fail.
     
    Last edited: Mar 30, 2011
  9. MrAl

    MrAl Well-Known Member Most Helpful Member

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    All this just to be able to say that "current flows through a vacuum capacitor" ? That doesnt make sense.

    Since you dont believe several references and at least two Professors, maybe you should ask this guy:

    Physics & Astronomy - Faculty & Staff Directory

    He's from BY University and he says that between the plates Maxwell's 'displacement current' is not real.


    You seem to be arguing that "Maxwells equation is true and correct, including the displacement current", and i dont think anything under the sun has been proved to be so true as that statement, but that's not the problem. The problem is not that Maxwells fourth 'equation' is not correct, it is, it's just that the 'displacement current' is not real in the vacuum capacitor.

    Things change over time. We've been talking about Maxwells fourth equation but Maxwells third equation may have to change in the near future in order to account for more recent discoveries.
     
    Last edited: Mar 31, 2011
  10. BrownOut

    BrownOut Banned

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    It makes perfect sense. The cornerstone of the argument against Maxwell is that the E field was multiplied by a constant "just" to make the units come out right. What I've shown is that the constant is derived from well known EM theory, which dispels that argument. It's not "all of that", rather simple math that proves the point. So, Maxwell is validated. I believe many references and professors, including those that say straight up that displacement current flows in the vacuum region. I've already linked many different references ( no, I'm not going back and finding and reposting. I expect people who want to have a discussion to follow along ) Maxwell's 4th equation works perfectly well in a vacuum. I see no new discoveries that require any of Maxwells equations to change. They perfectly describe observed EM phenomena, and just like Ohms Law and other well established laws and principles, they are important to the study and application of electronics.
     
    Last edited: Mar 31, 2011
  11. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,

    I agree in part.
    I looked at just about every reference anyone posted in this thread and i cant find anything contrary to the displacement current not being real. Some even add footnotes to clarify. Every reference i have found all say the same thing, that the displacement current in a vacuum cap is not real. I have yet to see one that says otherwise. I'd be happy to read anything else that anyone wants to post here.
    Look at a picture of a unicorn, measure the length of its horn. Take that and scale it up by how big a horse or zebra would be (for example) compared to that pic. Calculate the length of the unicorns horn, then go try to find a real live unicorn so you can measure its horn to compare and see if you made any mistake in the calculation. Let me know the error you found in the calculation :)
     
    Last edited: Mar 31, 2011
  12. BrownOut

    BrownOut Banned

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    There are references that say it's real, including one references that claims it was measured. On the contrary, I see no references that proves it's not real. The calculations that have been given are of well known EM principles, not of mystical creatures.
     
  13. MrAl

    MrAl Well-Known Member Most Helpful Member

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  14. BrownOut

    BrownOut Banned

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    From "The Ether"

    What I've said all along.
     
  15. The Electrician

    The Electrician Active Member

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    And, of course, the part of that article that MrAl will post in rebuttal is "...we now accept Maxwell's displacement current term in Ampere's law without believing that it represents any sort of real current in the ether."

    I still maintain that this whole dispute is really about semantics.

    If a person chooses to define the unqualified word "current" as requiring a flow of material particles, charge carriers, then they are inviting this sort of disputation.

    Nobody is saying that there is a flow of material particles in between the plates of a vacuum capacitor. However, it has been understood since Maxwell that what goes on in that region of space produces effects in other regions separated from the inter-plate region, that are indistinguishable from what would happen if that inter-plate region contained a flow of material particles carrying charge. It is those other effects, the production of a magnetic field for example, that are useful to us humans.

    That phenomenon, which produces the magnetic field which is indistinguishable from that which would be produced by a flow of charge carrying particles, is given the name "displacement current". The word "displacement" is there in recognition of the fact that the phenomenon is not a flow of material particles; the word "current" is there in recognition of the fact the the external effect is the same as would be produced by a flow of charge carrying particles. When the external magnetic field is produced by an flow of charge carrying particles, the name "conduction current" is used in recognition of the existence of the flow of material particles.

    A clamp-on ammeter, if one could enclose the region between the plates of a vacuum capacitor with it, would read exactly the same "current" as it would read if clamped around the wire leads connecting to the plates of the capacitor.

    Most everyone who has studied electrodynamics understands that "displacement current" is not a flow of material particles (as far as current knowledge of physics goes), but there is such exact similarity in the external effect of "displacement current" and "conduction current" that it is a common english usage to refer to both as the unqualified word "current". Those in the know understand from the context what is being described. It is a trope, and for those with expert knowledge, it causes no problem to refer to both as "current".
     
  16. BrownOut

    BrownOut Banned

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    Hello TheElectrician, thanks for your contributions. Personally, I take it a step further, and since the displacement current completes the theory of circuit current, and satisfies the principles of circuit analysis in many cases, and other reasons, then it is if fact a real current, on par with conduction current. So, current comes from two sources, charge flow and changing electric flux. It may not presently be popular to think of it that way, but I go by what the math and analysis tell me.

    (besides, it gets the trolls, who want to try to force everyone to think their way, really worked up. And that's the best part about it Heh!)
     
    Last edited: Mar 31, 2011
  17. The Electrician

    The Electrician Active Member

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    Of course, the part of your post in red is just what I mean when I say the effects of the two "kinds" of current are indistinguishable.

    But, using the word "real" to qualify "current" is itself a semantic issue, and will lead to objections from others. Sigh! :)
     
  18. BrownOut

    BrownOut Banned

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    Well, in that case, I'd say your position and mine are indistinguishable :)
     
  19. Ratchit

    Ratchit Well-Known Member

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    The Electrician,

    Right you are Electrician. I would call it a virtual current, i.e., the effects of a changing current are present (changing electromagnetic fields), but a real current is not producing those effects at that location (within the dielectric).

    Ratch
     
  20. killivolt

    killivolt Well-Known Member

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    The discussion have now disbanded from 2007.

    The Ether

    All questions in the rye. Answer's caught and questions in the rye.

    2007/
     
  21. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Brownout:
    Good, and that's what i have said all along too. We discussed the 'equations' several times, but as i have said numerous times now, that is not what this is about. I think Electrician summed it up nicely and at least he is willing to look at this from both sides.

    Electrician:
    Yes, i agree it is mostly just some semantics, so i would be happy to leave it at that. I am happy to see that you understand the argument from both sides, unlike some people. I have to accept Ratch's idea of a 'virtual' current, where everything works as if a current dq/dt was really there, even though there isnt. It's interesting that it is equated to dq/dt, but really isnt dq/dt. We've all agreed that it is not dq/dt, yet it is equated to that. That i think sums up the whole point. If we equate something to dq/dt and it is not dq/dt but only acts like that, then it isnt real, or at the very least is something else entirely.
    So anyway, accepting the 'semantics' postulate we are then back to the original question which remains to be answered:
    "Does current flow through a vacuum capacitor?"
    What's your take on this?

    Also:
    I am sure we all agree on many many other topics, lets not let this one little thing get us into a bad mood or something :)
    One thing is for sure i think we can all agree on now, is that when someone asks about current flow through a capacitor a one line answer isnt good enough.
     
    Last edited: Apr 1, 2011

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