Talk:Ivor Catt/Archive 16

message: I do have a very slight (almost imperceptible) feeling that there is some common ground between me and Nigel (and between me and Kevin). However because Nigel and I are talking different languages, a lot gets lost in the translation. I am trying to find the best way of exploring this common ground, but its difficult. Basically, each respondent in this discussion should respond directly to the others points rather than flying off at tangents. This will aid understanding of each others POV.
I will say this only once. Listen carefully. All DC is ultimately AC  Think about it! 8-)--Light current 21:37, 16 May 2006 (UTC)
replies: text: LC, I've no quarrel with your "DC is AC" thought, but it would shed more light on Nigel's diagram to note that both the "step" and "ramp" versions represent a transition between 2 DC situations. Ahead of the "step" no current is flowing; behind it a current of magnitude V/Zο flows. Any length of the line (up to and including the whole of the line) which contains the step has a net imbalance of current; this imbalance is, properly speaking, what "displacement current" is. What Maxwell did was to show the relationship between the accumulating charge and the electric field it causes. Where this really ties back to the original Catt et al article, is that it does not matter whether the charge progresses down the line behind Catt's step, behind Nigel's ramp, or as the extreme case where the ramp extends the whole length of of the line, so that the voltage is the same all the way along; whatever the exact location of charge is, the total change in the field is the same and satifies Maxwell's observation that the displacement current is equal to dQ/dt which is equal in magnitude to dD/dt. The "whole line" extreme is effectively the one that Maxwell used when he originally derived dQ/dt + dD/dt = 0. Catt's real error is that he does not understand that Maxwell chose the "equal voltage" case entirely because it was the simplest one to do the calculations for; it was a purely hypothetical situation, and in no way meant that Maxwell claimed that it would happen in any real situation. -- Kevin Brunt 20:52, 19 May 2006 (UTC)
thread_title:  All DC is ultimately AC 

message: Why are you trying to dismember the wave? Are you allowed to do that? I dont think you can treat the wave as two separate portions. Also, all the wave on the line is ac. DC does not exist!--Light current 22:40, 19 May 2006 (UTC)
replies: text: LC, I can't see any "wave" to dismember! Whichever way the situation is viewed, you end up with two parts. If you start with charge, you end up with voltage (due to location of charge) and current (motion of charge.) If you start with the E×H of Heaviside's energy current you end up needing to explain where charge comes in. Catt started by dealing with the energies stored in the inductance and capacitance separately, and showing that they are necessarily equal. There is some sort of basic "twoness" - two aspects of the same thing. -- Kevin Brunt 00:51, 20 May 2006 (UTC)
thread_title:  Wave dismemberment 

message: What do you mean by a net imbalance of current behind the step? The current flows to the right along the top conductor, and back to the left along the bottom one. This is not an imbalance!8-|--Light current 23:10, 19 May 2006 (UTC)
replies: text: LC, No! My point is specifically that there is no net imbalance behind the step, or in front of it for that matter. However, as long a step travelling down a TL does hit an impedance mismatch, the current flowing behind the step is related to the voltage ("height") of the step by the TL's characteristic impedance. The voltage step is also a current step, and for any length of the line that includes the step, the current flows at the two ends will differ. Any length of line which does not include the step will have identical currents at each end. This exposes one of Catt's obfuscations, because a correct analysis of Catt's TL-as-a-capacitor scenario reveals the voltage step always marks the point where the folw of current from the voltage source stops, even when the "reflected" step is travelling backwards to the input end. Thus the displacement current is always happening just at the point where the charge current stops flowing along the wire and needs to "cross over" to the other conductor (very loosely speaking.) -- Kevin Brunt 00:32, 20 May 2006 (UTC)

text: Yes, there is displacement current at the travelling edge, more or less by definition, because voltage is a manifestation of electric field and where the voltage is changing the field is changing, and that is displacement current. Where I differ from Nigel, is that he seems to think a TL has to have parallel conductors. (Let's tie him up in knots by discussing the impedance of free space, where there are no conductors.... :) ) My position is that the electric field surrounds the voltage step in 3 dimensions; the presence of another conductor will affect the shape of the field, but not the essentials. The electric field stores energy, but does not of itself transfer energy elsewhere. However, if the field interacts with another one (such as that of the charge in the other conductor) the interaction will result in creation of a force. If that force causes motion, a transfer of energy will result (as required by basic mechanics...) Catt's obfuscation is his repeated assertion that there is no displacement current, which we are showing to be false by looking carefully at Catt's own example. -- Kevin Brunt 19:49, 20 May 2006 (UTC)
replies: text: Might I suggest that the precise definition of "displacement current" is that it is the label used by physicists to refer to Maxwell's work on the relationship between charge, electric field and magnetic field which completed the four equations describing electromagnetism, now referred to as "Maxwell's Equations". The term "displacement current" is obsolete and somewhat misleading, since it is not a concept of electromagnetism itself, but instead is associated with Maxwell's attempts to explain electromagnetism in terms of an underlying fluid "Aether". The Aether (at least as Maxwell conceived it) was abandoned after Maxwell's death as the accumulating experimental data made it increasingly untenable. -- Kevin Brunt 19:55, 22 May 2006 (UTC)
replies: text: NB What I do disagree with is the statement that the flow of current/energy/whatever in a conductor is "TEM", if by "TEM" you mean "transverse electromagnetic" in the same sense that physicists do when refering to "electromagnetic radiation". The latter usage is associated with a family of equations describing the electric and magnetic fields which satisfy the constraints of Maxwell's Equations in the specific case that the J term in the Ampere-Maxwell law is zero. This explicitly excludes the "flow in a conductor" case, because it can be easily shown (by considering the case of a TL properly terminated at each end) that the "plateau" following the initial step of a pulse is identical to the "steady-state" situation predicted by Ohm's Law and Kirchoff's Laws, and there is therefore current flow which is, by definition, a non-zero J value. -- Kevin Brunt 20:16, 22 May 2006 (UTC)
thread_title:  Imbalance 

message: :What I think we need in the main article is a section entitled Catt and "The History of Displacement Current", which discusses the article of that title which was published in Wireless World in March 1979. It would appear to be the article referenced in the December 1978 article which did not get published in Physics Education. Catt complains that the Institute of Physics "broke their contract"; it would be much fairer to say that Catt, Walton and Davidson failed to deliver the article that the IoP expected, and that the article that was delivered was simply not good enough to publish. Catt's annotation at the end of the article is illuminating; he still thought in 2004 that nobody before him had considered that charge does not spread instantaneously across the plate of a capacitor. This is, of course, nonsense; you don't need to look any further than the derivation of the Telegraphers' Equations to find a counter-example. Catt's problem is that the "spread of charge" issue is sufficiently complex and abstruse as to be an "advanced topic"; it needs the deployment of the full mathematical description of a capacitor and that required calculus. The full equation of the charge on a capacitor is not Q=CV; that is merely the result of an integration of a simple case. If you look at Nigel's diagram, you ought to be able to see that it is effectively a graph of V against x at some instant in time. It should be obvious that for any finite x the area under the curve is proportional to the charge on that length of the line, and that in consequence it can be shown that dQ/dx = C dV/dx. A little thought will show that at any point on the line, the graph of voltage against time is the mirror image of Nigel's diagram, with time along the horizontal axis; thus we can say that dQ/dt = C dV/dt. I'm not going to try to put these together, but what we'd get is a partial differential equation relating Q and V, where both are not simple variables, but functions of t and x (and y as well, for the more general case of a surface.) I would say that there is ample evidence that Catt's mathematical competence does not extend to this sort of calculus and that he simply can't follow the textbooks that do deal with arbitrary charge distribution. -- Kevin Brunt 21:41, 19 May 2006 (UTC)
replies: text: Catt's article (and footnotes) don't have very much to do with displacement current, but a lot to do with Catt's misconceptions. -- Kevin Brunt 01:15, 20 May 2006 (UTC)

text: They definitely won't think much of it, because it simply isn't about the history of displacement current. It's really the rest of the Dec 78 Wireless World article. I suspect a parallel between this article and Catt's 1969 article in New Scientist on computer architecture. Somewhere on his web sites he boasts of having deceived the NS editor about the content of the article. This time the IoP editors just said "No". -- Kevin Brunt 23:00, 20 May 2006 (UTC)
thread_title:  Catt and "The History of Displacement Current