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Energy flow in a DC circuit as Tsao Chang & Jin Fang see it

  1. Aug 14, 2015 #1
    From the abstract:

    "Experiments show that Poynting vector is just a mathematical definition in the DC case. Since it does not form real energy flow, there is no transmission of electromagnetic energy flow from the exterior to the interior of a metal wire. Therefore, DC power is transmitted entirely within the metal wire."

    Full article available here: ( <<Predatory publisher link deleted>> - click on the PDF link). The translation from Chinese isn't perfect, but the ideas come across clearly enough.

    Thoughts?
     
    Last edited by a moderator: Aug 15, 2015
  2. jcsd
  3. Aug 14, 2015 #2

    LvW

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    Gerry, Thank you for providing the link.
    LvW
     
  4. Aug 14, 2015 #3
    I've been thinking about the experiments performed by Chang and Fang in relation to Feynman's description of charging a capacitor. Here's an illustration (on the left) and an excerpt (in purple) from one of his lectures:

    feynman capacitor 1.jpg

    "[The theory] tells us a peculiar thing: that when we are charging a capacitor, the energy is not coming down the wires; it is coming in through the edges of the gap. That’s what this theory says! How can that be? That’s not an easy question, but here is one way of thinking about it. Suppose that we had some charges above and below the capacitor and far away. When the charges are far away, there is a weak but enormously spread-out field that surrounds the capacitor. Then, as the charges come together, the field gets stronger nearer to the capacitor. So the field energy which is way out moves toward the capacitor and eventually ends up between the plates."

    Seems to me this theory would be easy to test using the same technique as Chang and Fang: put a shield around the capacitor (as shown in red on the right diagram) and see if the capacitor still charges up. If it does, then the energy is coming down the wires, and not through the edges of the gap. I have to admit I'm surprised, considering the wide readership of Feynman on this topic, that there is no record (that I can find) of anyone testing this theory in this way.
     
    Last edited: Aug 14, 2015
  5. Aug 14, 2015 #4

    sophiecentaur

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    The sheild would need holes in it where the wires enter it and that would modify the fields right next to the capacitor. But to have any significant effect, the Capacitance between the shield and the plates would need to be in the order of the Capacitor value. The shield could be regarded (equivalent circuit) as two capacitors connected in series, connected across the plates of the original capacitor. This would effectively increase the capacitance of the arrangement. The way the diagram is drawn, the capacitance introduced by the shield would be much greater than the capacitance of the original capacitor.
    That Feynman passage doesn't describe the process fully enough, really but if you interpret it in terms of joining two pairs of switch contacts, on in each leg of the supply, you would get an EM wave travelling along the wire and that would be on or near the surface of the wire and not actually through it. Then the energy would spread in between the plates. But that's a standard idea and doesn't describe the DC condition as it's a transitional situation.
     
  6. Aug 14, 2015 #5

    Baluncore

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    The Tsao Chang, Jing Fan paper seems to confuse “DC/AC” as “unipolar/bipolar” currents with “steady/changing” currents. It studies “the Redistribution Speed of the Electric Field in a DC Circuit”. If fields are “redistributing”, then that is a variation, so AC analysis is applicable.

    Consider a pair of filamentary superconductors. The electric field exists between the two conductor's surfaces, the magnetic field is the sum of the two fields due to the two equal and opposite currents on the two filaments. Those magnetic fields reinforce between the conductors, but tend to cancel outside that inner space. It matters little to the fields how deeply into a conductor the current and the magnetic field diffuse over time. But for changing currents, any change of the flux diffused into a conductor represents an energy loss. Only the guided surface wave can be efficient, and then only if it can be kept outside the conductor, that requires a good conductor.

    Where a current flows, (on a conductor), through an insulated hole in a shield, the magnetic and electric fields are distorted or pinched by the presence of the shield. Those fields and their poynting vectors all exist initially within the insulation. Over time, the magnetic field will diffuse into and through the shielding.

    Consider consistency of analysis from a Fourier viewpoint. Must the coefficient of the zero frequency cosine term really be treated that differently to the other terms? The poynting vector is there for 100Hz and for 1Hz, for one RPM or once per day. If I turn on my battery powered light at dusk, then turn it off at dawn, that is not a steady flow of current. If I reverse my battery at noon each day. then I have no zero frequency term, it is all AC and higher harmonics up to the wavelength of the wires.

    The “wave – particle” duality may have a parallel here as a “magnetic field – electric current” duality. For steady currents, the electric current model holds. For changing currents, the magnetic field model holds. It is the changes to the steady state that propagate at close to the speed of light.

    Any circuit can be analysed as a current generating a magnetic field or a magnetic field inducing a current.
    When you insert an ammeter into a circuit you guide the magnetic field about one of those conductors into the meter. It is then “weighed” against the permanent magnet and the hairsprings inside the meter.

    We are first taught DC current, with the fluid flow analogy and terminology. I know that pressure from a pump is distributed through hydraulic hoses to the load where the power available is the differential pressure multiplied by the flow rate. The momentum or the KE of the fluid can be ignored. The pressure difference between the two hoses is dropped across the hose fabric as hoop stress, not the space between the hoses.
    It is therefore not surprising that the analysis of EM energy flow as a poynting vector is rejected where it is not needed due to the simplicity of the model. One mistake is to try to draw a sharp line between steady and changing currents, another is to confuse DC and AC with steady or varying currents.

    Mankind will never generate a true steady current, the changing fields due to it's initiation will continue to propagate and reflect for ever. The lowest frequency wave we have managed so far was started by Volta, circa 1800, so it has a period of only about 200 years.
     
  7. Aug 14, 2015 #6
    You folks seem to be in love with obfuscation. The point of these experiments, actual and anticipated, is simply this: If energy flow is entirely in the fields and not the wires, as both Feynman and the texts say, then a drastic change in the arrangement that drastically affects the fields ought to affect the behavior of the circuit in a correspondingly drastic way; yet this is not what we observe. In Chang and Fang's experiments, the shielded, partially shielded, and un-shielded circuits behave the same to four decimal places. Their conclusion? The transfer of energy is through the wires, not via the fields outside of them. Sounds reasonable to me. Ditto for the capacitor example. I'm pretty darn sure that however we shield (or don't shield) a capacitor, we're going to see essentially the same behavior in all cases when we hook up a battery and a couple of wires to charge it. Of course it's easy enough to prove me wrong -- just hook up a capacitor both ways, see what happens, and report your findings here.
     
  8. Aug 14, 2015 #7

    Baluncore

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    There is no such thing as a shield. If a current flows through an insulated gland then there will be a high electric and magnetic field in the gland material. The electric field will still get to the capacitor by following the wire. The magnetic field will still surround the current on the wire, all the way along the wire. Energy will still flow in a direction determined by the polarity of the EM fields, the poynting vector.

    How do you propose to shield a capacitor?
     
  9. Aug 14, 2015 #8

    Baluncore

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    The electric field involved in energy transfer to the load is that between the two conductors, it is not that along each conductor.

    There is very little voltage gradient along a good conductor. Any voltage drop = electric field along a conductor results in energy propagation into the conductor, not along it. That energy is dissipated in the resistive material as heat. It represents a loss of energy that was intended for the load. There is a small poynting vector component towards the conductive wire, but a stronger component parallel to the wire, in the direction of the load.

    Replace the resistance wire with a superconductor and the energy is transferred to the load more efficiently because the poynting vector is then truly parallel with the conductors and the full voltage appears across the load.

    Tsao Chang, Jing Fan are denying the existence of the return conductor in their analysis of the electric field. That is why they think the energy is being transferred inside the wire.

    If the KE of the electrons was important then the electrons in one wire would take KE one way, while the electrons returning in the other would bring it back. The net result would be zero energy transfer.
     
  10. Aug 15, 2015 #9

    sophiecentaur

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    It all depends on what you call 'drastic'. Your diagram of a 'schematic' capacitor and a 'real' screen, of it were a scale diagram of an experiment, would certainly be drastic and the measurement method would have disturbed the original situation to a drastic enough extent to give very different results (the added Capacitance would be hundreds of times the original Capacitance) which is not a good measurement philosophy and would prove very little.
    But, when we get down to it, this thread is yet another of those "What is really happening?" questions. Science doesn't answer that sort of question- ever. You are proposing an alternative analysis for a common phenomenon. To justify it, you would need to show that the approach actually yields consistent numerical results. What are the numbers involved?
    A point already made is that DC is not a real situation in any circuit. DC is only a limiting condition that will never be reached. 'How the DC condition was arrived at' is a different matter.
     
  11. Aug 15, 2015 #10
    Let's see if we can figure out where the problem lies. Please bear with me.

    Here's a circuit similar to the one discussed by Chang and Fang, with helpful indications of the electric and magnetic fields:

    ian 1.jpg

    The illustration is taken from this paper ( http://science.uniserve.edu.au/school/curric/stage6/phys/stw2002/sefton.pdf ) where the author asserts:

    "It is this combination of electric field and magnetic field in the space outside the wires that carries the energy from battery to globe."

    That's the theory. Now let's put it to the test. Our friends at the Wikipedia tell us that "electromagnetic shielding is the practice of reducing the electromagnet field in a space by blocking the field with barriers made of conductive or magnetic materials," and that "typical materials used for electromagnetic shielding include sheet metal" (among other things). So we construct two versions of this circuit, laid out as above, with the second circuit enclosed in sheet metal, inside and out, as illustrated in red in the cross-section below (the shields would, of course, extend all the way around):

    ian 2.jpg

    The electric field in this second circuit will thus be significantly reduced, especially in the critical blue area, and -- if the the theory is correct -- we should see a corresponding reduction in energy transfer from the battery to the globe. (We can allow a small peephole at the globe end to visually check, and/or small multi-meter connections at convenient spots to make more precise measurements.)

    And what do we observe? We find that we do not get a reduction in energy transfer: the bulb is equally bright in both circuits, and the voltage and current readings are equivalent. So we conclude that the energy is not carried from battery to globe by "the combination of electric field and magnetic field in the space outside the wires," but by something else.

    Now in all fairness to Ian (author of the above mentioned article) I should point out that in section 4.2 he backs off somewhat from his earlier (rather strong) assertion and says, "...to have a look at how energy gets into a light globe... we need to look at the electric and magnetic fields just inside the surface of the wire." Then he shares some math with us and concludes: "So the field model for energy transfer gives the right answer for energy dissipation in a resistive wire." Which is consistent with our experimental results: our shielding certainly did not eliminate fields "just inside the surface of the wire" and so it appears possible that it is such fields, just inside the surface of the wire, that are responsible for some, if not most, of the energy transfer. He continues:

    "As an extension to the derivation above, you could look at what happens just outside the wire. Since the axial component of the electric field is the same just outside as it is inside and the magnetic field is practically the same, we can conclude that the energy flux derived above still works out as expected. But now there is also a radial component of the electric field as well. If that radial component is not zero then, together with the magnetic field, it contributes to a component of the Poynting vector that is parallel to the wire – but only outside the wire. That means that energy may also be travelling along beside the wire (and getting in somewhere else)."
    Note that he now says, "may also be," not "is." And of course our experiment cannot conclusively refute this (tentative) conclusion: our shielding also did not eliminate fields "just outside the wire" since the shield had to be separated from the wire by at least some insulating material. It is thus conceivable that some energy may be transferred by fields "just outside" the wire, assuming that "just outside" means "as close as we can get with the shielding" -- certainly nothing like the fields in the typical drawings, and certainly not like the fields described by Feynman as "enormously spread-out."

    Bottom line? It seems to me that the "field model of energy transfer" is, generally speaking, badly explained. First, the fields "just inside the surface of the wire" (which may very well account for the lion's share of the energy transfer in circuits such as this) are ignored in most descriptions. Secondly, the usual diagrams are misleading in both scale and scope. And thirdly, the bold, counter-intuitive assertions that are typically associated with these diagrams are unfounded. If we tell a student that energy is transferred through wires by lightning-fast fields that form "just inside the surface," and that the fields (or parts of fields) that radiate outwardly beyond the wires are unwanted side-effects that we should seek to minimize (unless, of course, we're designing an antenna, a transformer, an induction charger, etc), I don't think we'll have any unexpected problems. But if we insist that all energy transfer is accomplished via widely dispersed fields outside of the wires, we're going to find ourselves with a really hard sell on our hands.

    And we should leave the door open, I think, for "field-free" or "particle" descriptions of energy transfer as well. Maybe there's more to Weber's theories than has been thought. Maybe it's not a literal "field" just inside and outside the wire, but a lightning-fast wave formed by electrons jumping up to higher energy levels and then sitting back down as the next electrons in line jump up (like sports fans at a football game).
     
    Last edited: Aug 15, 2015
  12. Aug 15, 2015 #11

    sophiecentaur

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    The above post doesn't contain any numbers or supporting experimental evidence. So what is it worth? Without some 'numbers' Newton's Law of gravitation could never have been developed and neither could the extension into GR. Rockets would never have got to the Moon (not even up from the launchpad, I suspect). Science just doesn't work on pictures and arm (or pen) waving. You may or may not emerge from the process of this thread feeling that you now have some more understanding but you can't claim to understand this without being able to apply the idea to predict the behaviour of a circuit (measured Volts and Current variations in time, perhaps).
    I had a girlfriend, once, who was doing a Non-mathematical Physics A Level course. She knew absolutely no Physics, once she had finished the course with a Certificate. It was not until she did an Open University Degree (where she had to do some Maths) that she got a grasp of the subject - enough to teach it, in fact.
    Quoting Feynman is pointless because he based all his work on hard Mathematics.
     
  13. Aug 15, 2015 #12
    I'm pretty sure Newton was thinking, "equal and opposite reaction" before he filled in the numbers. Pictures first, then words, numbers last: that's the ticket.

    Which might explain why he often babbled on about ridiculous ideas instead of performing an obvious test to narrow the field (no pun intended).
     
  14. Aug 15, 2015 #13

    davenn

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    you are a real class act Gerry :rolleyes:

    1) this is the 3rd thread you have started on the same general topic
    2) you are still peddling these links to dubious B grade sites ( and that grade is being generous)
    3) you still wont listed to what some very educated people have been telling you across these 3 threads
    4) when you don't get anywhere, you proceed to insult one of the greatest physics minds of the 20 century

    Now it would be safe to assume you don't have a Nobel Prize hanging on the wall of your study and being
    as such, you really do need to start talking less and listening and learning more about real science

    Regards
    Dave
     
  15. Aug 15, 2015 #14

    Baluncore

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    Your red screens are shown only in two dimensions. In 3D you have reinvented the coaxial cable. By doing so you have changed the impedance of the conductors and simply constrained the fields to a smaller space. You have exchanged a parallel wire transmission line for a differential pair of coaxial cables. The shields form an equipotential surface, equivalent to a horizontal line drawn across the original diagram through the middle of the battery and globe. The fields will still carry the energy.

    P.S. Don't believe wikipedia without a good crap detector.

    Here is the real diagram with shields.
    coaxial shield.jpg
     
  16. Aug 15, 2015 #15

    sophiecentaur

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    Whatever Newton happened to be"thinking", he backed it up with or used it to account for observed data. What we use nowadays is his theory and not the hypotheses he may have started. How many of those he had, we will never know but he would have thought twice before publishing them. Of course, he did actually turn loopy when he drifted into an enthusiasm for alchemy and magic. So the greatest of us can go wrong in some directions.
    What you are discussing is no more than a hypothesis and, as Dave says, the ideas reflect minority opinions. Whilst that, in itself is not to say they are wrong, until supported by evidence, they are likely to be as wrong as what you can read in hundreds of crank sites.
    I can't help thinking that you are drawn to such speculative stuff because you want to avoid the rigour that's needed to get into Real Science. Can it be that you just can't see the difference?
    I wonder how qualified you are, to distinguish which of Feynman's statements were 'babbling' and which were pay-dirt. What he said and what he actually published are two different matters. What have you done on PF, other than 'babble'?
     
  17. Aug 15, 2015 #16

    jim hardy

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    Just what Bill Beatty said in his Amasci page, albeit with hyperbole. Once you get past his exaggerated prose he seems to agree with the textbooks.
    His graphics i thought did a god job of connecting the mental dots to a physical understanding of Poynting vector.
    http://amasci.com/elect/poynt/poynt.html

    BillBPoynting.jpg

    BillB2.jpg

    and as Balunore said, he shows energy is E cross B where E is between the conductors.

    Most comprehensibls explanation i've seen.
     
  18. Aug 15, 2015 #17
    ian 3.jpg

    Now here's what strikes a mathematical bumpkin like me. Something "drastically" changes when we shield the various components: specifically, the electric field lines connecting the top wire to the bottom wire (in the shaded blue area on the right) no longer "reach across" the circuit. One would think that such a "drastic" change in configuration (and thus the field) would result in a correspondingly drastic change in behavior; yet no significant change in behavior is observed. See my problem? It's as if you're saying (metaphorically) that whether we allow students to walk directly across the quad to get to class, or insist they stick to the sidewalks, nothing will change (just because you calculate that there are still 30 students in each class).

    Here's another way to look at my problem. If I think of the battery as a water tower, and the bulb as the faucet in my house, and the wires as pipes with little leaks, then it makes sense: enclosing the pipes concentrates the leaks, while the unwrapped pipes on the right waste more water. Yet the faucet in both cases works with roughly equal efficiency because the leaks are inconsequential relative to the flow of water in the pipes. The problem is that leads me to believe the fields are insignificant relative to the flow of current in the wires -- yet I'm told that I should believe the fields are the primary conduit of energy transfer.
     
    Last edited: Aug 15, 2015
  19. Aug 15, 2015 #18

    nsaspook

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    John von Neumann
     
  20. Aug 15, 2015 #19

    Dale

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    @Gerry Rzeppa

    First, the reference you cited in the OP is not a legitimate reference. Chang consistently publishes bad science in bad journals, such as the AASCIT which is a recognized predatory publisher:
    http://scholarlyoa.com/2014/01/02/list-of-predatory-publishers-2014/ [Broken]

    Second, the analysis of the reference cited is fundamentally flawed. The authors acknowledge that the Poynting vector does represent energy flow in the time-varying case, but reject it (for no justifiable reason) in the DC case. As Baluncore mentioned, there is no such thing as a DC circuit, all circuits have some time variance and there is little difference between a very low frequency and DC. Furthermore, even in the ideal case with a true hypothetical DC source, the same laws of physics (Maxwell's equations) govern the behavior at all frequencies and the derivation of the Poynting vector makes no assumptions about the frequencies involved. There is simply no justification to accept it as representing energy transfer at one frequency and not at another.

    Third, the data does not represent evidence in support of the wire hypothesis over the field hypothesis simply because they did not even bother to calculate what the field hypothesis actually predicts for their setup. This is basic science. In order for an experiment to discriminate between two different theories the two theories must make different predictions, and in order to determine if they make different predictions you must use both theories to calculate the expected result for the experiment. If you only calculate one theory then you have no way to choose between the theories on the basis of the evidence. You make this same basic scientific error when you claim "One would think that such a 'drastic' change in configuration (and thus the field) would result in a correspondingly drastic change in behavior" without bothering to calculate what the change in behavior is actually predicted to be.

    Fourth, another name for the configuration of shields shown in post 10 is a transmission line. The fact that the fields are guided in a smaller space does not in any way change the fact that they are present and can transport energy. Similarly for hole-type shields which would add some capacitance to the circuit but would not prevent energy from passing through the hole.

    Fifth, you clearly have no intention of learning. You have been given the correct physics in multiple ways by multiple people and continue to push your personal agenda, without even attempting to address many of the substantive objections raised. That is done. We cannot prevent you from teaching your boy fairy tales, but we do not have to permit it here.

    Thread closed.
     
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