Talk:Displacement current

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copied from earlier talk for ref--Light current 01:40, 10 March 2006 (UTC)
thread_title:  Earlier talk 

message: The web link for Maxwell's 1861 paper 'On Phyiscal Lines of Force' was provided. You can clearly see that displacement current was first introduced at equation (111), and you can clearly read about the context. The original Wikipedia article claimed that displacement current first appeared in the 1865 paper 'A Dynamical theory of the Electromagnetic Field'. This is not true. That's why the amendment was made. 11/2/07 (124.217.34.54 07:50, 11 February 2007 (UTC)
replies: text: I fixed the date, thanks.  But I'm not clear on the reasons for your other changes.  You removed the introduction, replacing it with some history of displacement current, so I reverted that and some other changes, like removing the mathematical necessity section.  Pfalstad 17:36, 11 February 2007 (UTC)

text: It is not important for the demonstration how the charge is distributed on the capacitor plate.  The only relevant point is that current is passing through integration surface S1 but not through  S2.  Without the displacement current term in the Ampere-Maxwell equation, the equation would be inconsistent; the result of the surface integral would depend on which surface you chose spanning the contour δS.  And (disregarding fringing fields) the electric field between the plates will be substantially uniform as long as the capacitor is not charged so quickly that the voltage changes appreciably during the light travel time between the center and the periphery of the plate.  --ChetvornoTALK 01:21, 2 March 2014 (UTC)
thread_title:  Origins of Displacement Current 

message: :Also Alfred we never did find out what you meant by a 'flow of displacement' (your apparent defn of DC). How do you define displacement? Displacement of what? Are you referring to vacuum polarisation perhaps when you talk of displacement in vacuo?--Light current 15:22, 9 March 2006 (UTC)
replies: text: Thank you for outlining your position so succinctly. I actually understand what you are saying! So if we can measure  vacuum displacement current perhaps by the exptl method I outlined above, we may have discovered or proved something. I dont think vacuum ploarisation has yet been measured/proved to exist -- or has it?--Light current 16:43, 9 March 2006 (UTC)

text: Well as you know Alfred, I always try to have an open mind and am willing to be convinced! But the real test as always is the experiment.--Light current 17:14, 9 March 2006 (UTC)

text: Unless of course the experiment cannot be performed -then we have a problem!--Light current 03:25, 11 March 2006 (UTC)
thread_title:  What is electric displacement? 

message: I removed the section 'Displacement Current & Magnetism' as it has some significant problems that must be addressed.  The most obvious problem is that the the capacitor equation is a scalar equation but Ampere's Law is a vector equation.  The step of determining a vector field from the ratio of two scalars is highly suspect except in a one dimensional problem.  However, Ampere's Law cannot be applied to a one dimensional problem  (try to define 'curl' in 1-D).  I might try to fix this but the author should get first crack.  Alfred Centauri 19:16, 14 October 2006 (UTC)
thread_title:  Removed section 

message: The equations in this article (and all Wikipedia articles) need to be GIF images -- with the equation markup as alt text.
Normal browsers just show the math markup code.
Wikipedia needs to be usable without any special plugins, etc. —The preceding unsigned comment was added by 75.6.243.176 (talk) 10:02, 23 December 2006 (UTC)
thread_title:  Need Images of Equations 

message: One needs to be very careful when criticizing Maxwell's involvement with displacement current. Maxwell conceived of the idea in conjunction with an external electromotive force acting on his sea of molecular vortices and causing a tangential stress on these vortices.
In doing so, he equated electromotive force with electrical displacement in what was essentially Hooke's law, or a simple harmonic motion equation. (105 in his 1861 paper). He then took the time derivative of this equation and conceived of the concept of displacement current, which he then added to the free current term in Ampère's circuital law.
A few years later in his 1865 paper, he used this extended version of Ampère's circuital law to derive the electromagnetic wave equation. The displacement current is an essential ingredient of electromagnetic radiation and as such it is criminal to ignore the origins of the concept when analyzing the physical nature of light.
It is not Maxwell that needs to be criticized in relation to displacement current. It's true that his explanation regarding tangential stress on his vortices needs a considerable amount of elaboration.
In all probablity, the mathematical form that Maxwell established can probably apply equally to two different varieties of displacement current. There will be a linear displacement current as occurs between the plates of a capacitor.
This is most unlikely to be the same physical effect as the angular (tangential) displacement current which occurs in magnetization and EM radiation.
However, many textbooks have analyzed Maxwell's displacement current in relation to the capacitor when in truth they should have been analyzing it in relation to the magnetic field surrounding an electric circuit.
That is not the fault of Maxwell. Maxwell was a pioneer in the field and he can hardly be blamed for not having had the picture entirely in focus. David Tombe (talk) 19:05, 20 July 2008 (UTC)
thread_title:  Maxwell and Displacement Current 

message: The latest edits to the main article involve a new section regarding the necessity for displacement current. This new section could be summed up in prose, simply by stating that the existence of a magnetic field surrounding the space between the plates of a charging or discharging capacitor implies that an electric current must be flowing between the plates. This does not mean that an expression in the form of Maxwell's displacement current is needed to be added to Ampère's circuital law. Maxwell's justification for adding displacement current to Ampère's circuital law was quite different. David Tombe (talk) 06:45, 22 November 2008 (UTC)
replies: text: As important as the historical record is, this example addresses a different issue: the present-day understanding of displacement current (the main topic of this article).  This standard example appears in many books along with the definition of displacement current.  It demonstrates concretely why the displacement current term is needed: in circuits with a capacitor or other break, Ampere's law gives the wrong answer without it.   As for a prose explanation, Maxwell's equations are mathematical equations, and displacement current is defined here mathematically; this justifies a mathematical explanation.  However your point that Maxwell's original justifiction was different is important, and should probably be added to the History section. --ChetvornoTALK 08:36, 22 November 2008 (UTC)

text: That was the part that Maxwell had wrong.  (I'm not sure I'm addressing your point, but here goes)  The polarization component of the displacement current, \frac{\partial \mathbf{P}}{\partial t}\, represents charge movement, but in the other component,  \frac{\partial \mathbf{E}}{\partial t}\,, there is no charge movement (current) at all.   The term 'displacement current' is misleading, it doesn't necessarily consist of a physical current (movement of charge).  In regions where there are no dielectrics (polarization \mathbf{P}=0\,), such as a capacitor with vacuum between the plates, the 'displacement current' consists of the changing electric field itself.  So there is no current of any kind between the plates; the changing electric field itself acts as a source of the magnetic field.  --ChetvornoTALK 10:58, 22 November 2008 (UTC)
thread_title:  The Necessity for Displacement Current 

message: Who first decided to split the displacement vector D into and E component and a D component? In Maxwell's original papers, there is no separate P component, because the explanation given in this wikipedia article for the P component is essentially the same explanation that is given for the E component in Maxwell's papers. What the wikipedia article here is saying, is that as well as Maxwell's displacement current, there is another displacement current that doesn't involve displacement. Somewhere along the lines, another concept that is different from Maxwell's displacement current, has emerged under the name of Maxwell's displacement current to sit alongside the actual Maxwell's displacement current. David Tombe (talk) 09:45, 22 November 2008 (UTC)
replies: text: I didn't write what you're referring to, but that is how displacement current is defined today.  --ChetvornoTALK 11:30, 22 November 2008 (UTC)
thread_title:  The Explanation section 

message: Is there anybody who knows the full details of what exactly Weber and Kohlraush discovered in 1856? I have read conflicting reports over the years. It's generally agreed that they discovered the link between the speed of light and the ratio of the electrostatic units of charge to the electromagnetic units of charge. If that was so, then surely that would have be the point in history when optics, magnetism, and electricity were unified.
However, there are conflicting reports about whether or not Weber and Kohlrausch were overtly aware of it, because Weber's constant was tied up with a factor of root 2.
Maxwell got the hold of Weber's data in late 1861, while in London. It was then that he inserted it into his calculations and saw the connection between the speed of light and the elasticity of the electromagnetic medium. That point in history, in 1861, would certainly have represented a total unity between electricity, magnetism, and optics. So what exactly is the full significance of the fact that Maxwell used displacement current in 1864 to derive the electromagnetic wave equation? Was it not already a certainty by that stage that light was an electromagnetic wave?
Then there is the additional issue regarding what Kirchhoff did in 1857. He got the speed of light linked up to electricity and magnetism too. Here is a link which includes the original 1857 Kirchhoff paper in question.
There is nothing about this in the Weber article on wikipedia. David Tombe (talk) 02:50, 25 November 2008 (UTC)
thread_title:  Weber and Kohlrausch 1856 

message: Brews, The main reason that I removed the vector theorem was because the symbol A was going to cause confusion with the magnetic vector potential A. I was going to change it to another symbol when it occurred to me that we don't really need to justify this standard vector identity in this article.
You inserted two key points surrounding this topic, both of which are mutually exclusive but equally relevant to he topic. The conservation of charge issue doesn't arise in the EM wave equation because , as you pointed out, we need zero divergence for any field involved in the equation, be it, A, B, H, or E. 
Since capacitor theory is connected with cable telegraphy and transmission lines, and since in capacitor theory, the divergence of E is not zero, this puts a question mark over the telegraphy equations of both Kirchhoff and Heaviside.
There are two theories going on side by side, which seem to have been wrongly blended together ino one. Wireless telegraphy and cable telegraphy don't share exactly the same maths. David Tombe (talk) 01:27, 26 November 2008 (UTC)
thread_title:  Removal of the vector theorem from the main article 

message: Ampère's circuital law is solenoidal by nature. In the versions seen in Maxwell's 1861 paper, there is no question of any build up of charge, since charge is not even involved in the equation. The only relevant factor is the velocity of the current.
The modern day justification for displacement current, using the vacuum capacitor scenario, simply extends the concept of electric current to cater for the component of current that arises out of a change of charge density. It then instantly subtracts this component again in the form of adding the extra displacement current term. In this situation, the E in the displacement current satisfies Gauss's law. However, in the EM wave equation, the E vector satisfies one of the other components of the Lorentz force ie. E = -(partial)dA/dt. So in some respects, the modern day justification for adding displacement current is a tautology which produces a displacement current that is not even compatible with the EM wave equation. David Tombe (talk) 03:36, 26 November 2008 (UTC)
thread_title:  Conservation of Charge and Ampère's circuital law 

message: Oh no! Not this again. There are two common usages for "displacement current".
According to Jackson, page 238, the displacement current is 
:\boldsymbol{J_D} = \frac {\partial \boldsymbol D}{\partial t} \ . 
Sounds sensible because D=displacement field.
Jackson's definition is used by Slater & Frank (p. 84) and by Owen (p.286)
Bonnet and Cloude (p. 17) use 
: \boldsymbol{J_D} = \varepsilon \frac {\partial \boldsymbol E}{\partial t} \ . 
which I'd say was a version of Jackson's definition. Looking at Maxwell's paper, I'd say this was his usage inasmuch as he spent pages describing polarization contributions to the displacement current.
However, according to Griffiths, page 323, the displacement current is
:\boldsymbol{J_D} = \varepsilon_0 \frac {\partial \boldsymbol E}{\partial t} \ . 
Feynman (vol II, p. 18-2) uses Griffith's math, but does not use the term "displacement current".
Griffiths definition is also that of TL Chow, and of Billingham (p. 180). Brews ohare (talk) 15:46, 27 November 2008 (UTC)
replies: text: Brews, see my comments on this in the section below. David Tombe (talk) 02:53, 28 November 2008 (UTC)
thread_title:  Definition of displacement current is ambiguous 

message: Article says:However it is not an electric current of moving charges, but a time-varying electric field. How can an electric field equate to a current?--2.98.149.24 (talk) 21:43, 20 April 2011 (UTC)
thread_title: What??

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thread_title:  External links modified 

message: The "Current in capacitors" subsection seems to have two parts which are mostly redundant.  The second half starts at the sentence  right before the equation
:\oint_{\partial S} \boldsymbol{B} \cdot d\boldsymbol{\ell} = \mu_0 \int_S \left(\boldsymbol{J} + \epsilon_0 \frac {\partial \boldsymbol{E}}{\partial t}\right) \cdot d\boldsymbol{S} \, .
The first half assumes the existence of displacement current between the plates of a capacitor, assumes that it works just like current and proceeds to derive the fact that the displacement current density is dD/dt.  This doesn’t seem important, since it is derived rather directly in the “Explanation” sub-section and again in the second half of this section and the following section “Mathematical formulation” also provides a justification.  It is redundant, but tells the story in a different way.  The redundancy is not the issue.  The issue is that two false assertions are used in the derivation.   The false assertions are:
:*1) “a displacement current ID flows in the vacuum, and this current produces the magnetic field in the region between the plates according to Ampère's law”.  It is true that the magnetic field within the plates obeys Ampère's law, but it is untrue that the displacement current between the plates produces the magnetic field.  The magnetic field everywhere is produced by the currents everywhere.  So the magnetic field between the plates is a result of all the conduction currents and displacement currents.  The contribution that a current makes to the magnetic field is proportional to the strength of the current and its path length.  The path of the displacement current in an ordinary capacitor is very short.  It is so short that if you computed the magnetic field due to all the currents with the displacement current and without the displacement current, you would get almost exactly the same value, even between the plates.  So, we don’t have to have a displacement current between the plates to account for the magnetic field there.
:*2) “The magnetic field between the plates is the same as that outside the plates.”  This is nonsense since the magnetic field does not have a single value outside or between the plates.  Instead, the value varies with radius from the axis.  But consider the magnetic field outside the capacitor and tangent to the wire.  It has a value proportional to I/rwire where rwire is the radius is the wire.  The magnetic field inside the capacitor at the same radius is less, because a circle there with radius rwire encircles only a fraction of the total displacement current.  What is true is that the line integral of the magnetic field over a closed curve is the same whether the surface bounded by the curve goes between the plates or through the wire.  This is the statement that starts off the second section.
So, the first half of the “Current in capacitors” section is triply redundant and includes false assertions.  I propose to take it out, retaining only the symbol definitions as needed.  The only problem with dropping the first section is that the figure discussed in the second section does not explicitly show ID, the displacement current.  It may be sufficient  to simple define it as the surface integral of dD/dt.  I’ll wait a couple of days for comments. Constant314 (talk) 17:04, 11 July 2017 (UTC)
replies: text: Agree with both the above points.  --ChetvornoTALK 03:43, 22 January 2018 (UTC)
thread_title:  Untrue assumptions in the “Current in capacitors” sub section 

message: Please see Talk:Electric_displacement_field#Delete,_and_change_to_a_redirect. fgnievinski (talk) 02:50, 27 August 2023 (UTC)
thread_title:  Electric displacement field