Talk:Displacement current/Archive 1

message: Dielectric constant not time independent?? Never heard of this one. Would someone care to explain?? Light current 00:27, 4 September 2005 (UTC)
replies: text: Can anyone tell me where I can get time dependent dielectric material to be used as a PCB substrate. I could really do with some. I'm not joking! Light current 00:21, 7 September 2005 (UTC)
thread_title:  Dispersion in dielectrics 

message: A definition which uses an undefined word is hardly an adequate description.  Anyone care to attempt to explain what the word pseudocurrent really means?  DFH 16:06:13, 2005-09-06 (UTC)
Not worth it. Ive just changed it! Light current 22:47, 6 September 2005 (UTC)
thread_title:  Pseudocurrent? 

message: How can current be stored (unless in a superconducting ring) para 3?? Light current 23:07, 6 September 2005 (UTC)
replies: text: Paragraph 3 repeats the commonly taught error that something is stored in the dielectric material.  Is it not rather obvious that for a capacitor in which the space between the plates is a vacuum, such an explanation is nonsense? The best that can be said is that it is a poor analogy to bring in the word elastic.  DFH 07:06:58, 2005-09-07 (UTC)
Sorry. I just couldn't stand seeing this nonsense any more - so I changed it Light current 14:03, 7 September 2005 (UTC)
thread_title:  Stored Current? 

message: Without getting into the discussion of whether there is a physical 'thing' called electric displacement and an associated flow of this stuff, a much more interesting question (to me at least) is whether the 'displacement current term'
\frac{d\vec D}{dt}
produces (generates, induces, whatever) a magnetic field or merely 'accompanies' a magnetic field.  There are mathematical formulations that give the correct magnetic field without a displacement current term (these formulations take in to account the fact that instantaneous action at a distance is incompatible with Special Relativity). Additionally, it can be shown that if Coulombs Law is made compatible with the postulates of Special Relativity, magnetic effects are simply the electric field as observed from a relatively moving reference frame.
So, does the dispacement current term generate a magnetic field? Alfred Centauri 01:35, 16 September 2005 (UTC)
replies: text: A magnetic field exists in EM waves -- no one knows what causes it, people can only guess. Is it just the relativistic effects of a moving electric field? BTW has anyone actually measured any of this flow of charge in EM waves? If so how did they do it?--Light current 01:47, 16 September 2005 (UTC)

text: I dont actually know. Probably the magnetic field. I was led to believe that an EM wave propagates by the continuous interchange of energy between the electric and magnetic fields. As a result of this, each field 'causes' the other (further down the line of course). In this model, although you can postulate a so called current causing the magnetic field, I dont think its necessary to do so. The real question is, although this current is indicated by the equations, and a magnetic field exists, does this necessarily mean that this displacemnet current actually exists? Or does it really matter whether is it really exists or not since we can't measure it anyway? The argument about displacement current was originally to do with capacitors. I fear we have strayed slightly away from this, unless, of course, you are shortly going to make a new revelation based on the previous material on EM waves?--Light current 03:31, 16 September 2005 (UTC)

text: Sorry to butt in here but I just noticed you said the charge density was assumed uniform on this parallel plate capacitor. But how does it become uniform initially? Answer: the energy has to travel somehow from the wires to the edge of the plates. So the charge density cannot be uniform on the plates at ac--Light current 16:02, 16 September 2005 (UTC)
replies: text: <sigh> If I say assume the charge is uniform then assume that it is, please.  I'm not asking you to believe that this is a realistic assumption.  If you read carefully, you'll see that my point was that IF one assumes this is true, the result is different from what is taught or expected.  Besides, I believe it is possible in principle to construct such a capacitor.  Alfred Centauri 17:36, 16 September 2005 (UTC)

text: Yes I got your point OK and its interesting. I was just restating my belief thats all. BTW Im not sure about the existence of a real magnetic field between capacitor plates (other than that due to a EM wave)--Light current 17:52, 16 September 2005 (UTC)

text: I confess a great deal of ignorance the area of Vector Field Theory so I shall defer to your superior knowledge in that subject. I have heard of an experiment (and the equipment is supplied for the demonstration of such an effect for the education of students!) which  purports to demonstrate the passage of diplacement current in a parallel plate capacitor. It comprises 2 large discs (about 1ft in dia) separated by a distance of about 1ft. A sinusoidal voltage at ~ 15kHz is applied between the plates and a search coil is placed in the gap. THe search coil o/p is then displayed on a scope which shows-- well I'm not sure what it proves,-- but a sine wave at 15 kHz is diplayed. Obviously the search coil when properly oriented responds to a magnetic field, but the displacement current is so small in a capacitor of these dimensions that Im dubiuos whether that this is what's being seen. Its more likely to be coupling of the electric field to the (unbalanced) coil!
The reasons Im more comfortable with the TL representation of capacitors are numerous.
a)a pair of parallel plates fed from one end is an o/c TL
b)It is hard to think of a capacitor construction that does not conform to a transmission line structure, This is because practical capacitors always have 2 parallel plates (or paralleled combinations of these) that  are usually rectangular. THe classical description of a capacitor with 2 parallel coaxial discs and fed from wires at the centre of the discs can easily be appreciated to be a number of wedge shaped parallel plate capacitors in parallel. Each wedge shape forms an end fed TL of decreasing Z0 towards the periphery. Each wedge is of course o/c at the end.
But nobody in his right mind would design a practical capacitor that way these days would he? Actually I had to design a HV capacitor of about 1pF to work at 20kV once using that design....But then Im not in my right mind!!--Light current 16:21, 16 September 2005 (UTC)

text: Well, now that you mention it!  You might be aware that a TL also support TE and TM waves.  These modes turn out to be the superposition of TEM waves that are 'bouncing' between the conductors.  That is, the direction of propagation of the TEM wave is oblique to the 'direction' of the TL.  Now, it turns out that these TE and TM wave can also exist in a non-conductive TL such as, for example, a fiber optic cable.  For these TLs, there is no conduction current!  Further, you can't make a capacitor out of one these (as far as I know).  Alfred Centauri 18:13, 16 September 2005 (UTC)
thread_title:  Does the 'Displacement Current Term' produce a magnetic field? 

message: Displacement current was an idea invented by Maxwell when he couldn't think of another way that current could flow beween the plates of a vacuum capacitor. Maxwell did not ever come across EM radiation (as it was Hertz who discovered it) and Maxwells concern was displacement current in capacitors. THis does not exist. 
Some people today say that displacement current must exist for electromagnetic radiation to exist. This necessity has not been proven. What exists in EM waves are time varying electric and magnetic fields. Some people cannot concieve of a magnetic field without the presence of a current to cause it. This current in a EM wave has never been measured. This is because it doesnt exist and is just an imaginary concept to try to explain away EM radiation with no charge carriers. Discuss (10 marks for convincing replies)--Light current 23:00, 15 September 2005 (UTC)
replies: text: The electric field and magnetic field are 'only' mathematical constructs.  What is your point? Alfred Centauri 00:27, 16 September 2005 (UTC)

text: If that's all there is to it then you made no point at all.  There exists in the this universe phenomenon that are observable such as the electric force and the magnetic force.  These forces can be mathematically modeled as force fields but does that mean that these force fields are physical?  To say that something is an imaginary concept is saying nothing at all.  Concepts are nothing other than imagination, my friend.  The incredibly successful quantum theories we now have talk of probability currents - the flow of probability.  I hope that you do agree that EM waves carry momentum thus, EM waves represent a momentum current.  You seem to be hung up on the idea that a current must be the flow of something physical.  Well, what is an electric current?  It's not the flow of electrons or ions or protons or quarks.  It is the flow of electric charge. Electric charge, what is that? - what a concept! Alfred Centauri 01:04, 16 September 2005 (UTC)

text: Well, if space is empty, there is no charge right?  Not so fast!  It turns out that what we consider 'empty space' is chock full of ghostly charged particles.  Look at the Casimir effect.  The vacuum may even be full of something called the Higgs boson that give matter mass.  For some additional 'light' reading, get a copy of "The Elegant Universe".  In the superstring theory outlined in this popular book, electric charge is simply a particular vibrational mode of little bits on concentrated spacetime called strings (or superstrings) that live in 11 dimensions (give or take).  If you think Displacement current is difficult or impossible to imagine, you ain't seen nothin' yet. Alfred Centauri 01:58, 16 September 2005 (UTC)

text: You said: "The human brain cannot concieve of a magnetic field without the presence of a current to cause it".  Are you sure you want to go on record as having made this idiotic statement? Alfred Centauri 00:29, 16 September 2005 (UTC)

text: If you had said "My brain cannot conceive..." then, though I might disagree, I wouldn't consider the statement idiotic.  Think about what you said - "The human brain cannot conceive...".  I ask once again, do you wish to go on record as making such an idiotic statement? Alfred Centauri 01:04, 16 September 2005 (UTC)

text: Think about it - who are you to say what the human brain can or cannot conceive?  First, that's highly presumptuous and second, such sweeping statements are likely to get you labeled as a 'crank' .  Take a look at the new section I added here and see what you think. Alfred Centauri 01:44, 16 September 2005 (UTC)
thread_title:  Only a mathematical construct 

message: That's correct - an EM wave is composed of discrete 'wavicles' of energy called photons.  Further, the electromagnetic force is due to charged (whatever that means) particles such as electrons exchanging what are called virtual photons.  These virtual photons cannot, in principle, be directly observed.  This may cause some to wonder if this ghostly particles exist.  However, it turns out that if calculate the magnetic moment of an electron without considering the interaction of an electron with its own virtual photons, you don't get the right answer.  However, when you do take these guys into account, you get an agreement between theory and experiment to some 6 or 7 digits.  That's pretty darn impressive to me.  Alfred Centauri 17:58, 16 September 2005 (UTC)
replies: text: Yes, I like this theory, I have few probelms with it (probably cos I dont fully understand it!). I also like the idea of particles & antiparticles suddenly appearing and disappearing for no apparent reason. As I remember saying to myself only the other day : "the sum of everything is probably nothing at all-- so why worry?!"--Light current 20:58, 16 September 2005 (UTC)

text: Exactly!--Light current 09:48, 17 September 2005 (UTC)
thread_title:  Quantum Theory of EM radiation 

message: :Yes it does: see my illustration here: http://photos1.blogger.com/blogger/1931/1487/1600/electron.gif based on experimental data:
‘All charges are surrounded by clouds of virtual photons, which spend part of their existence dissociated into fermion-antifermion pairs. The virtual fermions with charges opposite to the bare charge will be, on average, closer to the bare charge than those virtual particles of like sign. Thus, at large distances, we observe a reduced bare charge due to this screening effect.’ – I. Levine, D. Koltick, et al., Physical Review Letters, v.78, 1997, no.3, p.424.
I'd like to suggest that some background history be added to this "displacement current" article.  Some material from my page follows, which may provide ideas.  Feel free to use anything that is useful!  http://feynman137.tripod.com/ :
==== Maxwells statements ====
In his (Maxwells?) final (1873) edition of his book A Treatise on Electricity and Magnetism, Article 110:
‘... we have made only one step in the theory of the action of the medium. We have supposed it to be in a state of stress, but we have not in any way accounted for this stress, or explained how it is maintained...’
In Article 111, he admits further confusion and ignorance:
‘I have not been able to make the next step, namely, to account by mechanical considerations for these stresses in the dielectric [spacetime fabric]... When induction is transmitted through a dielectric, there is in the first place a displacement of electricity in the direction of the induction...’
First, Maxwell admits he doesn’t know what he’s talking about in the context of ‘displacement current’. Second, he talks more! Now Feynman has something about this in his lectures about light and EM, where he says idler wheels and gear cogs are replaced by equations. So let’s check out Maxwell's equations.
One source is A.F. Chalmers’ article, ‘Maxwell and the Displacement Current’ (Physics Education, vol. 10, 1975, pp. 45-9). Chalmers states that Orwell’s novel 1984 helps to illustrate how the tale was fabricated:
‘… history was constantly rewritten in such a way that it invariably appeared consistent with the reigning ideology.’
Maxwell tried to fix his original calculation deliberately in order to obtain the anticipated value for the speed of light, proven by Part 3 of his paper, On Physical Lines of Force (January 1862), as Chalmers explains:
‘Maxwell’s derivation contains an error, due to a faulty application of elasticity theory. If this error is corrected, we find that Maxwell’s model in fact yields a velocity of propagation in the electromagnetic medium which is a factor of root 2 smaller than the velocity of light.’
It took three years for Maxwell to finally force-fit his ‘displacement current’ theory to take the form which allows it to give the already-known speed of light without the 41% error. Chalmers noted: ‘the change was not explicitly acknowledged by Maxwell.’ Weber, not Maxwell, was the first to notice that, by dimensional analysis (which Maxwell popularised), 1/(square root of product of magnetic force permeability and electric force permittivity) = light speed.
Maxwell’s innovation was: Total current = electric current + displacement current. But he didn’t understand what the terms were physically! Really atoms are capacitors themselves, not solids as Maxwell thought in 1873 (X-rays and radioactivity only confirmed the nuclear atom in 1912). So the light speed mechanism of electricity is associated with ‘displacement current’ and electric current results from the electric field induced by ‘displacement current’.
:Do you mean flow of electric charges is induced by what you call displacement current? How is this displacement current measured? Can it exist without EM radiation? Is it not inextricably bound up with em radiation, and in that case, why does it need to be considered separately from em radiation. If it does not need to be considered separately, then of course its not worth considering a a separte entity. If its not a separete entity, it doesnt exist. Where is the flaw in the above logic?--Light current 01:12, 14 December 2005 (UTC)
replies: text: Id just like clarification on your last point above. Are you saying that, a capacitor being charged from a voltage or current source connected to terminal A with terminal B as common (ground), will allow passage of energy from terminal A to terminal B? If so, where does this energy go? I thought the capacitor stored the energy in its electric field. Can you explain your statement please?--Light current 00:24, 14 December 2005 (UTC)
thread_title:  Reply from Nigel Cook 

message: In his (Maxwells?) final (1873) edition of his book A Treatise on Electricity and Magnetism, Article 110:
‘... we have made only one step in the theory of the action of the medium. We have supposed it to be in a state of stress, but we have not in any way accounted for this stress, or explained how it is maintained...’
In Article 111, he admits further confusion and ignorance:
‘I have not been able to make the next step, namely, to account by mechanical considerations for these stresses in the dielectric [spacetime fabric]... When induction is transmitted through a dielectric, there is in the first place a displacement of electricity in the direction of the induction...’
First, Maxwell admits he doesn’t know what he’s talking about in the context of ‘displacement current’. Second, he talks more! Now Feynman has something about this in his lectures about light and EM, where he says idler wheels and gear cogs are replaced by equations. So let’s check out Maxwell's equations.
One source is A.F. Chalmers’ article, ‘Maxwell and the Displacement Current’ (Physics Education, vol. 10, 1975, pp. 45-9). Chalmers states that Orwell’s novel 1984 helps to illustrate how the tale was fabricated:
‘… history was constantly rewritten in such a way that it invariably appeared consistent with the reigning ideology.’
Maxwell tried to fix his original calculation deliberately in order to obtain the anticipated value for the speed of light, proven by Part 3 of his paper, On Physical Lines of Force (January 1862), as Chalmers explains:
‘Maxwell’s derivation contains an error, due to a faulty application of elasticity theory. If this error is corrected, we find that Maxwell’s model in fact yields a velocity of propagation in the electromagnetic medium which is a factor of root 2 smaller than the velocity of light.’
It took three years for Maxwell to finally force-fit his ‘displacement current’ theory to take the form which allows it to give the already-known speed of light without the 41% error. Chalmers noted: ‘the change was not explicitly acknowledged by Maxwell.’ Weber, not Maxwell, was the first to notice that, by dimensional analysis (which Maxwell popularised), 1/(square root of product of magnetic force permeability and electric force permittivity) = light speed.
Maxwell’s innovation was: Total current = electric current + displacement current. But he didn’t understand what the terms were physically! Really atoms are capacitors themselves, not solids as Maxwell thought in 1873 (X-rays and radioactivity only confirmed the nuclear atom in 1912). So the light speed mechanism of electricity is associated with ‘displacement current’ and electric current results from the electric field induced by ‘displacement current’.
:Do you mean flow of electric charges is induced by what you call displacement current? How is this displacement current measured? Can it exist without EM radiation? Is it not inextricably bound up with em radiation, and in that case, why does it need to be considered separately from em radiation. If it does not need to be considered separately, then of course its not worth considering a a separte entity. If its not a separete entity, it doesnt exist. Where is the flaw in the above logic?--Light current 01:12, 14 December 2005 (UTC)
replies: text: Id just like clarification on your last point above. Are you saying that, a capacitor being charged from a voltage or current source connected to terminal A with terminal B as common (ground), will allow passage of energy from terminal A to terminal B? If so, where does this energy go? I thought the capacitor stored the energy in its electric field. Can you explain your statement please?--Light current 00:24, 14 December 2005 (UTC)
thread_title:  Maxwells statements 

message: Is the time derivative of a sinusoidally varying electric field always going to produce a current?? (or just a mag field)--Light current 19:16, 23 December 2005 (UTC)
replies: text: I don't understand your question, but a changing electric field will always produce a mag field, and will also produce a current in any nearby conductors.  Pfalstad 20:38, 23 December 2005 (UTC)
replies: text: You've changed the question.  At first, you asked about a sinusoidal variation.  Then, you asked about a varying electric field without specifying how it varies.  This is not nitpicking.  There is a difference.  Alfred Centauri 03:49, 24 December 2005 (UTC)

text: I'm not sure I see the difference.  Unless it's another one of those crazy step functions.  Anyway, LC, yes, a varying electric field can produce a magnetic field without producing a current, in free space anyway.  That's how waves propagate.  Pfalstad 09:31, 24 December 2005 (UTC)

text: (bangs head on wall) No current needed for propagation.  Displacement current is needed, of course.  Because displacement current is not a current, but a varying E field.  Is current needed to produce a wave in the first place?  Well maybe not, since a rotating magnet produces a wave.  Not sure if electrons spins count as current.  Pfalstad 16:06, 24 December 2005 (UTC)

text: Well sure, but too bad, that's what it's called.  The terms are defined by physicists, not by people on wikipedia.  Pfalstad 22:26, 24 December 2005 (UTC)

text: No, we're not.  Displacement current is varying E field.  Varying E field is needed for propagation of a wave.  Displacement current is not a true current, no.  Maxwell may have thought it was, but this is an article about current terminology and usage in the physics community.  Pfalstad 22:26, 24 December 2005 (UTC)

text: How's this for a health warning: "However, it is universally accepted that displacement current does not exist as a real current (movement of charge)" and right in the first sentence, " it is not a real current (movement of charge)".  I put those there, and certainly don't want them removed.  Pfalstad 03:11, 25 December 2005 (UTC)

text: Well, do you want to go read that?  Notice that it says "two major traditions of interpretation".  So the dispute is over interpretation, not physics.  That's what I want to make clear.  Also, you'll notice that I made very clear changes to this article stating that displacement current is not a current, not a flow of anything.  All I'm trying to avoid is the confusion created by saying "displacement current does not exist", because that isn't clear about what is being said: is it a dispute over physical law?  over interpretation?  Just try to phrase it less ambiguously, that's all I ask. Pfalstad 03:11, 25 December 2005 (UTC)
thread_title:  Innocent question on displacment current 

message: "Kirchhoff's current law also requires displacement current to be included in order to be valid for time-varying currents; for example, it is required when applying that law to one of the plates of a capacitor unless one uses the relatively recent transmission line equivalent capacitor interpretation of Catt".
The stuff about Catt is not correct.  Kirchoff's law definitely needs a correction to be valid for time-varying currents.  That correction is made by adding the displacement current term (whatever you want to call it); this must be done when dealing with any time-varying fields, including a transmission line.  Catt's model doesn't use Kirchoff's law, but that doesn't mean Kirchoff's law doesn't need displacement current.  So the Catt stuff should be taken out.
Catt doesn't talk about Kirchoff's law, does he?  If he does, you could say, "Ivor Catt claims that displacement current does not need to be included in Kirchoff's law."  Or you could say "According to Ivor Catt, Kirchoff's law is not necessary for an understanding of the capacitor," if this article were about Kirchoff's law, which it isn't.
:Kirchhoff's current law also requires displacement current... Yes you are right! because Catts interpretation is all about energy flowing up and down the space between the plates and Kirchoff doesnt enter into it. But we all know that at high frequencies you dont use Kirchoffs laws because in the limit, Kirchoff is wrong- but lets not get into that!
:So we need an imaginary current to prop up an erroneous law!. Thats physics for you!--Light current 02:21, 30 December 2005 (UTC)
replies: text: Now now no need for anger!! Stay calm and enjoy the discussion!  I think its right to question these things and look at them afresh. As I think i said before somewhere-- open your mind! and let it all flow through!--Light current 05:57, 30 December 2005 (UTC)

text: We have to use our brains to interpret others findings and not take them on face value. otherwise, the human race would make no progress!--Light current 06:06, 30 December 2005 (UTC)
thread_title:  Kirchoff's laws