Review report by W.T. Bridgman and Nereid.
Quotes from the article discussed are in blue.
Article Reviewed:
Electric Currents Key to Magnetic Phenomena
Donald E. Scott
What is the point of equation 3? It is not used anywhere in the paper.
"Magnetic fields that obey equation (1) cannot exist in the absence of the current density, J, which is their cause."This equation can also be true if J is zero and dE/dt is non-zero, yet Scott ignores this case, which is strange considering Scott mentions the term, but not its implications.
Maxwell added the dE/dt term to Ampere's Law to solve a problem in configurations such as capacitors. It is sometimes called the "displacement current" (wikipedia), but does not correspond to an actual charge flow. This case is vital for the propagation of electromagnetic waves, where electric and magnetic fields repeatedly regenerate each other, far from their original source. This process continues to operate even after the original current source (antenna) is turned off! While some type of current may be required to get the field started, there are configurations where a changing magnetic field can be maintained via this term, long after the original current is gone. We can examine the interaction of electromagnetic waves with matter independent of the original source of the electromagnetic wave. See additional references:
- The Maxwell Displacement Current
- Displacement Current
- THE DISPLACEMENT CURRENT AND MAXWELLS EQUATIONS
"Magnetic fields are created by and moved around by electric currents – nothing else."This is a strange oversight for an electrical engineer to make. Even odder is this is NOT the first time Dr. Scott has made such wrong statements about Maxwell's equations (see Scott Rebuttal. IV. 'Open' magnetic field lines).
Apparently he did not check any of this with anyone with any actual expertise in using Maxwell's equations at a more fundamental level. This is made more annoying by the fact that Dr. Scott's fundamental thesis is accusing others of not understanding Maxwell's equations!
"There are no bar magnets in space."Planetary dipoles can be treated as bar magnets if the time scale is small compared to the intrinsic variation of the field. Even bar magnets are not permanent on sufficiently long time scales at temperatures above absolute zero.
III. Enigmatic Plasma motions.
In Scott's description of magnetic reconnection, he totally ignores the role of the plasma itself in the electromagnetic field. The field is created by charged particle motion (ions & electrons). But the field influences the motion of the charges via the Lorentz force. This causes the particle distribution to change, which drives changes in the electric & magnetic fields (a feedback mechanism). Dr. Scott made this same error in "The Electric Sky" and has failed to correct it ("The Electric Sky - Short Circuited" 2008 Draft, pg 36-37).
Dr. Scott's complaints about descriptions of magnetic fields are from popularizations?? Apart from the Parker reference in Physics Today, all of Scott's complaints are from popularizations. Scott concludes his (brief) criticism of Parker's paper like this:
"In light of the previous example, we offer a possible explanatory mechanism in Fig. (3), below."This is the same "possible explanatory mechanism" he presents in his book. In the years since, he has apparently done nothing to develop it, or to test it.
However, what Scott ignores is that if a conducting fluid (or solid, or gas) moves through a magnetic field, it generates an induced current which feeds back on the original field. This force will also feedback on the conductor motion. But if the conductor is getting energy from another source (thermal, hydrodynamic, etc.), this feedback loop can operate for a significant amount of time (i.e. a generator or dynamo action).
IV: Magnetically stored energy
Dr Scott never uses Equations 4-10 and they are not related to any measurable quantity of an EU model which Dr. Scott presents. These equations are not used to derive any measurable quantity in the paper. Are they only there for decoration? What field energy is needed? What is the current density or electric field measured in the configuration and how does it relate to an observed astrophysical quanity?
VI. Magnetic Fields that Reverse Polarity
Regardless of the direction of the main driving current coming into the Sun, the eleven-year reversal of the magnetic loops can be explained by transformer action as shown above.Models of stellar dynamos do this (MSFC), where some initial magnetic field combined with conductive fluid motions driven by other energy sources (in the stellar case, thermal energy from nuclear reactions) set up a feedback mechanism that can maintain a reversing global magnetic field so long as there is enough energy to drive it (see Dynamo Models of the Solar Cycle). These models are being run on commercial-grade desktop computing equipment giving actual numerical predictions we can compare to measurements (see a data-driven visualization of a solar dynamo model). Where is the EU equivalent?
Dr. Scott's model suggests the solar cycle is not driven by processes inside the Sun, but by external processes driven by a mechanism which he does not define.
How are these external processes setup?
Do they form naturally? If so, how?
Dr. Scott's model merely moves the problem to be solved from inside the Sun to some ill-defined location and mechanism outside the Sun. How does this improve the situation?
Nor is there any mention of the origin or driving energy of this external current.
using the joint European Space Agency (ESA)/NASA Solar and Heliospheric Observatory (SOHO) spacecraft, they had discovered ‘jet streams’ or what they called ‘rivers of hot, electrically charged gas’ (plasma) flowing beneath the surface of the Sun. They also found features similar to trade winds that transport this ‘gas’ below the Sun's surface. Flows of electric charges such as these are, by definition, electric currents.Because the definition of plasma is neutral or 'quasi-neutral', not all plasma flows qualify as electric currents.
I find no mention of usual EU claim that Sun's energy comes from external electric current, though figure 3 implies it.
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Note: Comments that DIRECTLY address the points in THIS post are favored. Since there will be a post on each of the five papers in the EU 'Special Issue', comments more relevant to one of those other papers should await that specific post.
4 comments:
'This equation can also be true if J is zero and dE/dt is non-zero'
That can only be true for points in time. Over any length of time with dE/dt<>0, current must flow i.e J>0.
It's therefore incorrect to say Ampere's law asserts magnetic fields can exist without electric current.
wrt your reference to displacement currents as an example of a magnetic field without current
: A displacement current is a ficticious construct used for convenience.
The magnetic field associated with the displacement current is not physically created or maintained by the ficticious displacement current, but rather by the real current in the capacitor ( or other object with displacement current ) terminals.
To gristy BSc,
I covered this case pointing out that we can examine electromagnetic waves by just considering E & B fields, without consideration of the original current which started the wave.
E&M waves are a direct consequence of this term. Propagating electromagnetic waves in free space have dE/dt nonzero and J=0.
If you want to say the displacement current is ficticious, then are you saying electromagnetic waves are ficticious?
True, an EM wave does not have to be maintained, but only created by acceleration of charge. I dont think D.S. over looked that.
wrt to displacement current being ficticious I need to correct myself. I was considering only the case of a vacuum capacitor. In a dielectric material real current is present ( electrons shift within their atomic position ) in which case magnetic fields are induced by real current.
To gristy BSc,
Scott says: "Magnetic fields are created by and moved around by electric currents – nothing else."
The 'nothing else' suggest Scott was not including such configurations.
An EM wave, and other configurations, once started, can maintain, and even increase magnetic field strength without an external current (using only induced currents and energy from other sources). This is why the professional literature will generally state that currents create magnetic fields, but not *only* currents can create magnetic fields. The driving mechanisms are much more complex to be limited by such dogmatism.
The displacement current term applies to capacitors in vacuum as well. As the current flows to the capacitor plate, the plate accumulates charge which changes the electric field in the region. This is necessary for charge to be conserved in the integral form of Ampere's Law.
In a dielectric, you get a polarization effect, which is why the displacement current is usually written in terms of D = epsilon*E. In an ideal (non-leaky) dielectric, you will get dielectric polarization, but not any significant net current flow from this charging (check the links I provided in the post).
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