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AC flowing through a cap. What actually happens?

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

  1. BrownOut

    BrownOut Banned

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    You probably won't like my answers any more than what you got before, because I don't do 'binary' answers. I don't think they are helpful at all. I've would not/have not called the field 'current', but I would say that current creates the field, and can be measured by observing the field, as current has been measured for over a century. Current flows between objects if a magnetic field can be observed to exist, with the proper form and symmetry as Maxwell/Ampere requires. In the air capacitor for example. But a transformer is a completely different structure.

    Further, dispalcement current can be measured and exhibits current density and cross-sectional areas, just as conduction current does. The multitude of posts I've made, and others, as well as articles that have been linked and video taped lectures should be sufficient to answer many of the questions being posed. To just keep asking the same questions over and over, and then agitating for the same answers over and over serves no purpose. My unwillingness to anwers in someone else's perfered form, such as a binary answer, or to answer the same questions over and over when my opinions have been clearly communicatd, does not in any way prove one thing or another about the correctness of displacement current as a real, quantifiable and measureable current.
     
  2. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    I dont know if we can talk classical and quantum in the same sentence. I seriously doubt tunneling is in effect here though.
    The only thing that 'flows' is energy, and even that is hard to believe because the energy is stored.
    So far, i havent found anything that we can say that 'flows'. Charge doesnt flow, field lines dont flow, just about the only thing we can say that flows is the flow field, which is still not considered to be anything physical like the fluid flow we all know about in pipes. In a water pipe the water (obviously) flows, but in the capacitor the flow field is just a concept, not a reality like the water is.
    It is often said in classrooms that the "displacement current" flows, but that's just a concept too like the flow field, and if we dont know what displacement current is really made up out of, then we still dont know what it is (if anything) that is really flowing. We're trying to figure out the essence of nature here, not the behavior so to speak, but im sure you realize this already.
    It gets even stranger when we think about radio waves, which would have a 'displacement current' associated with them too. We know that the fields reach an antenna somewhere off in the distance, but does it really help to think about a current of some type also reaching that antenna? I have no problem believing that a current is developed in the antenna once the waves reach it, but that's a little more obvious.
     
    Last edited: Apr 11, 2011
  3. killivolt

    killivolt Well-Known Member

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    Well then if the field in (Fig,D) you mentioned was on the outside around near edges and not in the middle. It means that what ever is happening is done near the edges not in the center which is supposedly void or is there something going on with force in that region? My inclinations want to point to the edges and not the center even though it has the highest surface area and is directly exposed to the other plate.

    But, if it's true that the field strength is "higher" on the "y" axis and gets "weaker" toward the "X" axis. Then either field on the edges on both plates are at 90° in both directions. Strangely enough the field is looking more like a Faraday Cage than a capacitor once the fields are generated.
     
    Last edited: Apr 12, 2011
  4. dave

    Dave New Member

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  5. killivolt

    killivolt Well-Known Member

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    What are protons doing in a cap, there is 90° turns in the cap.

    What is the proton doing in the Cap.

    A proton is released from rest at the positive plate of a parallel-plate capacitor.

    There is a 90° electron motion.

    http://www.physicsforums.com/showthread.php?t=299350
     
    Last edited: Apr 13, 2011
  6. BrownOut

    BrownOut Banned

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    Protons aren't normally released in a capacitor. That is an achedemic question, not to be taken too literally.
     
  7. killivolt

    killivolt Well-Known Member

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    Which is why I'm here. Ask and ye shall receive. hahaha:D
     
  8. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,


    I was trying to get that point across (the electron, not the proton) earlier in this thread. the electron sort of turns 90 degrees once it gets inside the cap. Supposedly (i say that because you dont know what to believe these days on the web) there is X ray proof of this, with the electrons turning as they enter the plate and distributing across the plate. I would assume that on the other plate the electrons are exiting by the same means, turning 90 degrees more or less just before they enter the wire on that side of the cap. This is what brought about the transmission line model, because if we look inside the cap we see charge flowing parallel to each other in the opposite direction, exactly what happens in a transmission line. This isnt my theory either, it's someone else's and i just read about it so i dont want to take credit or blame (depending on how you look at it ha ha) for this theory. The first guy to publish a paper on this was Catt, which i think you can find on the web.
    I dont think the protons move do they? The nucleus may move very slightly in a dielectric, but not very far, and the protons are bound up in the nucleus right? We might think of protons moving just out of convenience perhaps.
     
    Last edited: Apr 13, 2011
  9. killivolt

    killivolt Well-Known Member

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    I fixed what I had found in Physics Forum. Here is the link about what your saying.

    Electron Deflection in Parallel Plate Capacitor
     
    Last edited: Apr 14, 2011
  10. killivolt

    killivolt Well-Known Member

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    Hogwash, only in a perfect world.

    As this thread slowly dies.

    I chatted with one of my Professors that I Broadcast. We began to talk about a Vacuum Capacitor and he agree's with me, that this is not a "Perfect Vacuum" as we want it to be. As I watched the way they are made (Applying RF and heating the anode and cathode) I can agree with him. There has to be some residual gases as well as some other formation of matter within the chamber.

    Under the underlying theory was this ever considered? If so, how can you arrive at the term displacement.

    The above would lead to other possible electron motion due to "pos and neg" charge gain picked up within the dielectric.

    As well as maybe some residual atmosphere.

    Unless it's just an academic application to show the difference between a contemporary view vs a classical model.
     
  11. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,


    Yes, but we can argue that nothing in the real world can be perfect and that would lead to questioning what is the point of doing any theoretical work, while surely we can see the value of pure theoretical work. The other 'imperfections' that come up are capacitor plate shape, spacing, etc. Not that we should never think about this, but thinking along those lines only muddies up the picture rather than makes it more clear. Later, after we have a clear picture of the pure unscathed workings, we can then start adding imperfections to see what effect each one has.
    There's also the argument that although vacuum may not contain matter, it may still contain energy, or virtual particles that spring in and out of existence. We'd have to factor in that too somehow.

    The term 'displacement' came in a long long time ago due to something that is known not to exist now in more recent times. The 'current' was supposed to be 'displacing' the aether and that's how the term displacement came into being. It doesnt apply anymore but the meaning was held over because it was so ingrained in all the textbooks. In these recent times it's just a name, that's all it is now. This says nothing about the reality of what is happening inside the cap, just that the original meaning of 'displacement' can not possibly apply anymore.
     
  12. misterT

    misterT Well-Known Member Most Helpful Member

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  13. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,


    Thanks for that Link! That guy is cool. I posted another vid by him in this forum someplace too. I think he adds just the right amount of lightheartedness to his videos.

    The short answer to this is that the field inside the capacitor is a non conservative field so it does not have to obey the usual laws of circuits (Kirchhoff's)and have current flowing though it (electrical current as he puts it). The changing field does everything, and mind you any measurements outside the capacitor will not prove anything :) Once the field stops changing however, then it has to go back to obeying the usual laws for electrical current, and that is WHY the current goes to zero when the field stops changing (then we definitely have no current through the capacitor and everyone agrees).
     
    Last edited: Jun 22, 2013
  14. Ratchit

    Ratchit Well-Known Member

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    misterT,

    Thanks for the link, but that guy is a dork. I made notes of what he said in that video.

    1) He keeps saying current flows.

    Comment: Current does not flow, charge flows. Current flow means "charge flow flow."

    2) He keeps saying capacitors charge up.

    Comment: Charge up with what? Charge, as in Q? There is just as much Q charge in a cap at 0 volts as there is at 100 volts. Charge up with energy? Then he should say "energize".

    3) He said the power input to a cap is incredibly high when a voltage is first applied.

    Comment: It's not. The current might be high, but the voltage across it is low.

    4) When talking about current existing in a circuit branch containing a cap, he asks, "where can the (current) go?"

    Comment: He can't seem to grasp the concept that current might exist in a circuit branch containing a capacitor, even though current does not exist through the capacitor. The answer is that charge accumulates on one plate of the cap and depletes on the other plate. This action is transitory and cannot continue forever at an applied DC voltage. That principle also explains why a peak current occurs at the beginning and tapers off when a constant voltage is applied to a cap.

    5) He says that a cap is like an open switch.

    Comment: No it is not. A switch does not contain a significant dielectric, which would greatly increase its capacitance. A switch stops a current immediately, whereas a cap does not.

    6) He said that there is a difference between fundamental physics and practical applications.

    Comment: Not so. Practical applications are result oriented, whereas fundamental physics are knowledge oriented. However, the basic principles apply equally to both.

    7) He seems to think that the "displacement current" accounts for the current existing in a capacitor branch.

    Comment: That is his worst mistake. Displacement current is a mathematical artifice used to account for the magnetic effects caused by the changing electric fields in a capacitor when the voltage across it changes. It is not a real current, but instead a measure of what the current would have to be to make those magnetic effects happen. Displacement current is also very small, and does not account for the huge current that can exist when voltage is first applied to a large capacitor.

    Summary: He uses false facts and imprecise terminology to explain them.

    Ratch
     
  15. dougy83

    dougy83 Well-Known Member

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    I fail to see how "that guy is cool" :/

    I also fail to see how this topic is worth 13 pages of discussion.
     
  16. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Ratch:
    Doesnt he state that the so called displacement current is an "equivalent" current?
    When you say the displacement current (the way we take it here to be an equivalent thing) is small, that's only in the wire outside of the cap. Inside the cap other professors show it to be equal to the current in the wire during the time the field is changing and that can be large.
    It's a shame we have to call this current "it" sometimes, just because it was once declared a real thing. But we often call imaginary things "it" as well.
    Also, nit picking on whether or not current "flows" is not going to get you anywhere because it's an accepted way to describe it as i am sure you know.
    Same with "capacitor charges up". It's acceptable.

    dougy83:
    Ok lets see what your video on this topic looks like, then we can compare yours to his. Maybe you've done a better job.
    This topic has always been difficult for people to grasp so it required more time to talk about.
     
  17. misterT

    misterT Well-Known Member Most Helpful Member

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    The guy is cool because he does not give a rat-chit what you or anyone else thinks about his videos. Good or bad, it is his way. Mostly good.
     
  18. Ratchit

    Ratchit Well-Known Member

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    MrAl,

    I haven't heard the word "equivalent" from him. I think he said something about defined or definition. If you can find it, please give me the time marker where he said that.

    OK, I will concede that point. Mathematically, the displacement current can be shown to be equal to the conduction current. However, it is still a magnetic equivalent. The video instructor does not make a single statement about charges accumulating on the plates of the cap, which is what physically happens.

    It all depends on how it is defined.

    Yes, imprecise descriptions have become acceptable.

    misterT,

    Then there are a lot of "cool" people in this world, because so many of them don't care what society thinks of their actions. I guess that makes most criminals "cool cats", right?

    Ratch
     
    Last edited: Jun 23, 2013
  19. misterT

    misterT Well-Known Member Most Helpful Member

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    Yes.

    No.
     
  20. nsaspook

    nsaspook Well-Known Member

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    +1 for that.
    My 2c

    Saying current flows in a capacitor is like saying current flows via magnetic fields from the primary to secondary in a transformer. Current (charge) flows in to one plate and opposing current (charge) flows out from the other plate. The electric field generated from the charge seperation moves the energy across the physical dielectric, not flowing but changing (linear polarization in a dielectric) fixed atomic structures (or nothing in vacuum ) to the other side. If you see a capacitor as a black box as is common in circuit theory it's understandable because it greatly simplifies the math but it's just an imposter and a illusion with a name called (whatever) current because we are measuring the conduction current in the wiring. Displacement current was a mathematic tool invented by Maxwell to model the flow on energy in space (vacuum) in EM far-field plane waves. The meaning of the term has actually change since then from Maxwells physical molecular vortex model to the dualist theory of charges, currents , electric and magnetic fields as fundamental per Lorentz so what is happening is not a current but a displacement field generating a magnetic field while transporting energy. It is common to use "Displacement current" when talking about capacitors and displacement fields in vacuum but it's usual meaning in this context is different from the original meaning used by Maxwell.
     
  21. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    I have to agree with almost everything you said there. And i dont want to detract from that by stating that i disagree with one little point, because it is such a small point compared to the mountain of information you wrote which is all correct as far as i understand it myself too and i would have wrote the same thing.

    The only point i'd like to mention is that i think that the guy did a pretty good job of addressing the points that come up in this issue. He could have done a little better perhaps by stating outright what you just did, but i think he was coming from a point where he wanted people to ignore this a little bit and just get back to the main stream electronics where we measure the current in the leads of the capacitors. So he is saying more or less, "Leave the capacitor physics to the physicists, and get back to your main interest which is the circuit electronics". After all, the circuit theory itself does not need to acknowledge displacement current to work out just fine.

    The other thing i like is that unlike most other 'teachers' he is willing to attempt to explain this. Even MIT professors skimp over this fascinating phenomenon of nature and how it played out in history.

    I'll go over the video again and find the point where he mentions that it is an "equivalent" current.

    [LATER]
    Ok he uses the word "equivalent" at time 12:14 when he talks about the displacement current more directly. That's the way i have understood it to be too.
    The mechanism that makes it work is the field can transfer energy, it doesnt have to move electrons. Outside in the wire however the electrons flow because that is a different mechanism.

    It's always about the mechanism, and there are more than one mechanisms. Super conduction involves yet another mechanism.
     
    Last edited: Jun 23, 2013

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