Stupid electromagnetics question

On Thu, 10 May 2007, qwerty wrote:


I wouldn't call it a paradox (or a stupid question, either). The situations are not symmetric - in the first, there is a static electric field as well, producing the current.
You could say that, in the first case, the electric field driving the current produces the current and the magnetic field. You can get rid of the current in Ampere's law by substituting J = sigma E, where J is the current density and sigma is the conductivity.
Another thing that stops it from being symmetric is that the current is moving electric charge, driven by an electric field. The symmetric case would be a magnetic current, composed of moving magnetic monopoles, being driven by the magnetic field and producing a static electric field. No magnetic monopoles, so we don't observer this.
(Exercise for the reader: assume magnetic monopoles exist, and design a perpertual motion machine thereby.)
You might be interested to know that sometimes magnetic currents are assumed to exist, to simplify calculations in problems where electromagnetic waves interact with objects (ie scattering).
--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen /
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I've never heard of a photon being called a "perpetual motion machine" before, but they do seem to travel a long way.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 10 May 2007 21:42:46 GMT, "Androcles"

WOW... one kook posting to another.
Why don't you guys discuss it so we can all have a laugh?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Not really.
You sort of assume that the magnetic field you visualize around the conductor suddenly appears full strength about it.
In reality, it will have to build up to full strength as it is generated by current flowing in some other nearby conductor or from the magnetostatic field of a permanent magnets being moved towards the conductor .
As this magnetic field intensifies about your conductor, current will flow in it, but will stop flowing when the magnetic field is made to stop increasing. Current will start flowing again it you start decreasing the magnetic field.
Moving current induces magnetic field.
Moving magnetic field induces current.
No paradox.
Andr Michaud
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 10 May 2007 14:06:38 -0700, snipped-for-privacy@microtec.net wrote:

No. whenever it stops *moving*. It doesn't have to change strength, just position.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 10 May 2007 18:23:27 +0000 (UTC), qwerty

Yes, and AC current makes for continually moving flux. DC makes a standing field.

To INDUCE (key term here), the flux MUST be in motion. The field doesn't have to "change" as you put it. It HAS TO MOVE, as in lines of flux must "cut" through the conductor, and the current induced in it will always be varying, not static.

Nope.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
qwerty wrote:

No. The logic is faulty. It is like stipulating that "all sandboxes contain sand" and because of the stipulation assuming that "all sand is in a sandbox".
From Ampere's law you stipulate that a current through a wire results in a magnetic field around the wire. (That's the sandbox). You incorrectly assume, because of that stipulation, that a magnetic field around a wire must induce a current.
Ed
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

-------------------- Ah but Heisenberg's cat thinks it is a litter box. To crap or not to crap, that is the question.
Actually you put it very well but I couldn't resist :).
--

Don Kelly snipped-for-privacy@shawcross.ca
remove the X to answer
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Don Kelly wrote:

Is the cat's name Uncertain? :-)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ampere's law calculates the magnetic field produced by a flow of electric charges (in a wire). The magnetic equivalent of this statement would be: Erempa's law calculates the electric field produced by a flow of magnetic charges.
Because there are no magnetic charges (monopoles) in nature, no such calculation can be carried out.
In the original posting, the fallacy is to argue that if A causes B then B will cause A.
Bill -- Fermez le Bush--about two years to go.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
ehsjr wrote:

We're not sure. ;-)
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 5/10/07 10:15 PM, in article xZS0i.175499$aG1.168976@pd7urf3no, "Don

Don't do this at Ohm.
Bill -- Fermez le Bush--about two years to go.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The magnetic field is just one perspective of the superposed fields of the drifting charged particles. The B field is just a perspective of the current that we associate that B field with, rather than a cause of the current. Only E fields can accelerate charges, which implies that currents are produced solely by E fields.
Conservation of energy prohibits a current from generating itself.
Though it is common to speak of changing B fields as generating currents, this is not technically correct either. A B field can only change a particle's direction, not its speed, and this is why your static B field cannot produce a current. The so-called changing B field that was mentioned by others is in fact literally and exactly an E field.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
RP wrote:

Allow me to point out a few commonly and widely accepted fallacies with regard to this matter. One is that a changing E field produces a B field. I recently checked here on the latest experimental results attempting to prove the existence of Maxwell's displacement current. So far as anyone here knows no definitive direct proof of the existence of a displacement current has EVER been forthcoming!
Point two is that Amperes law is somewhat less than fundamental! It only works part of the time. Calculations by "flux linkages" will not give a correct result in all cases.
Point three is that famous "one E field" dogma is completely bogus! An electrostatic E field is a TOTALLY different animal from the "supposed" "E" field accelerating electrons in a Betatron. Proof is that if you place an electron on the pole piece of an electromagnet and then start increasing the current, the DIRECTION the electron will be accelerated will depend on it's tiny initial velocity. Electro- static E fields always provide a force direction!
Point four is that Maxwell's equations, are incomplete as they do not include the relation, qV x B. Thus Maxwell's equations in spite of commonplace wild-ass assertions on PBS, do NOT "explain" "All electro- magnetics" anymore than Newton (or Einstein for that matter) "explained" gravity.
Benj
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

E fields don't come in different types. They're just a mathematical way of saying that a unit point charge Q will experience such and such force if located at such and such point in space relative to some other charge(s). Both the E and B fields are artifices, but there are real em fields. E and B are just different mathematical perspectives of the em field.
But there is a problem with common conceptions. To correct those: A changing B field doesn't cause an E field, it is the changing current that causes the E field, and that change in current can in turn be traced to changes in charge densities caused by yet other E fields, or in other words the induced E field is produced by other E fields, and all are precisely what you just said that one of them wasn't, Coulomb forces in action. Even the force associated with the B field is just superposed Coulomb forces. See Purcell if you want a mainstream derivation of these conclusions, or see Weber for a previous version of pretty much the same ideas.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The proof has been around all along. Just check the fundamental LC relation.
Andr Michaud
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@microtec.net wrote:

Sorry Andre, but you cannot use circuit concepts to prove field theory! Now if you wanted to say that the existence of EM waves proves the existence of "displacement current" by inference, Ok. That's what everyone has done since the 19th century! But the question is if displacement current is a real thing and not some mathematical error, then one should be able to come up with some clever experiment to demonstrate it directly. Apparently nobody has.
Benj
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Not my intention really. But simply to mention that displacement current is by structure part and parcel of the LC oscillation.

Well, since it is this intuition by Maxwell in 1865 that caused him to properly define em wave treatment afterwards, it seems to me that there must be some ground for its validity. For em waves to exist at all, in the absence of conduction current in vacuum, only displacement current could possibly be at play as a source of magnetic field and that the associated changing electric field in a region of space will induce a magnetic field in neighboring regions, even when no conduction current and no matter are present.
Sears, Zemansky & Young do discuss this in their "University Physics", 6th Edition, Addison Wesley, (1984), page 625.

To demonstrate it at our macroscopic level, I see only the LC circuit as an option.
At a more fundamental level, I think there would be need to experiment with individual photons (is it even possible ?)
De Broglie had an hypothesis in this regard that seems logical enough to me and that, according to him, satisfied at the same time Bose-Einstein's statistic and Planck's Law; and perfectly explained the photoelectric effect while obeying Maxwell's equations and conforming to the properties of Dirac's theory of complementary corpuscles symmetry.
His hypothesis was that discret photons would be made up not of one corpuscle, but of two corpuscles, or half-photons, that would be complementary like the electron is complementary to the positron.
According to him, "Such a complementary couple of particles (that he perceived as unsigned charges) is liable to annihilate at the contact of matter by relinquishing all of its energy, which perfectly accounts for the characteristics of the photoelectric effect. "
Furthermore, he said "The photon being made up of two elementary particles of spin h/4pi, it must obey the Bose-Einstein statistic as the precision of Planck's law for the black body requires."
Finally, he concluded that "this model of the photon allows the definition of an electromagnetic field linked to the probability of annihilation of the photon, a field that obeys Maxwell's equations and has all of the characteristics of electromagnetic light waves."
His two half-photons had to cycle towards and away from each other (displacement current), inducing a magnetic field that would increase as the two half-photons close in on each other, and would decrease as they receide from each other,
This would by the same token take care of the magnetic monopole issue, since in this configuration, both magnetic phases cannot be present at the same time. Meaning that while the charges symmetrically separate in space, the magnetic phases symmetrically separate in time.
But his hypothesis did not really take at the time.
Andr Michaud
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@microtec.net wrote:

Yes, it is.

Whoa! I've been hearing about this theory for quite some time and was interested but could never seem to find any definitive information beyond a description like that above!

Photon-pair annihilation to explain the photo-electric effect! That is totally cool! Do you have a reference to this paper?
Benj (Who has to admit that he isn't sure just what a half-photon might be!)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I'd very like to know how, since they move at c.

Yes. Well, there is a very special problem with de Broglie's work. Although he was instrumental with his 1924 thesis in giving Schrdinger his lead to develop his famous equation, the community always treated him as some sort of weird cookie, because he was a causalist.
Meaning that he was convinced that elementary particles were at all times localized, even as they are moving, which includes photons and also electrons (and positrons), and this despite his understanding of QM as a proper mathematical description (Einstein agreed with him also).
This idea was discussed at the time, but to my knowledge no formal paper ever came to be published on this issue.
To my knowledge, no causalist work was accepted for formal publication since the early 1960's, whoever promising.
He even had to set up a foundation to make certain that his work would not be lost.

It certainly is.

As I said, no formal paper was published, to my knowledge. He was however a very prolific writer and published a number of books, some of which are still being constantly re-edited in French. I don't believe there is anything available in English on this.
The one ref I can give you still in print is this well known book of his
"La physique nouvelle et les quanta", Flammarion, France 1937, Second Edition 1993, with new 1973 preface by L. de Broglie Page 277.
But if you are not convinced yourself of the permanent localization of elementary particles, I am afraid, even if you can read French, that what he says will remain just about meaningless to you.

Welcome to the club !
Andr Michaud
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.