Unnecessary magnetic poles ?

  • Thread starter Fairfield
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  • #26
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Originally posted by zoobyshoe
Fairfield,

It might be of some comfort to you to find out the encyclopedia I have specifically addresses this issue as a problem:

-Zooby

It still doesn't summarily dismiss magnetism as a purely secondary phenomenon, like the sound of a whistle, which is an identifiable phenonen, but which is only a rearrangement of a force which is already there (motion of air).

In the case of "magnetism", a force which is always present around a straight current carrying wire becomes more manipulatable when the wire is given a two dimensional shape in space. When the wire is formed into a loop, the tubular shaped force surrounding it has to take on the shape of a doughnut, but a doughnut with no poles of its own. Depending on the relative current directions in two such force doughnuts, when near each other, they may either attract each other or repel each other, just like straight wires. But now we may reverse the relative current of one or the other wires just by manipulating one of the loops rather than changing its electrical connection at a battery. But there is no need to give this alternative arranged attractive or repulsive force a new name, (edit=) magnetism, just because it is now in the shape of a more manipulatable doughnut.

Also see my post at the bottom of page 4 of the thread Classic Magnetism.

Fairfield
 
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  • #27
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Originally posted by Fairfield But there is no need to give this attractive or repulsive force a new name just because it is now in the shape of a doughnut.
I'm not sure which "new name" you're refering to in this sentence. The title of the thread calls the concept of "poles" into question. But do you mean "magnetism" in the above sentence? If it is the latter, then the problem arose because knowledge of the doughnut shape preceeded knowledge of the simpler shape by many centuries. All the "doughnut case" vocabulary was already in place.

In general I gree with what this Gentleman said:




Originally posted by Albrecht
Don't forget that the magnetic lines of force are nothing different than a picture we build for ourselves, to handle a magnetic field in practice. There is no correspondence in theory because magnetism is not a force by itself but only a relativistic side effect of the electric field.

So, if we want to understand a magnetic situation really, we have to look to the electric situation and then take into account the limited velocity c by which field changes are propagated, and the relativistic contraction of a moving chain of electric charges.

-Zooby
 
  • #28
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Zooby:

OK, I'll buy all that in your post above; except the second part of the quote from Albrecht brings up, again, the situation of a changing electric field, which doesn't happen much outside of an electromagnetic radiation (wave), and I think that keeps confusing you.

As for the phrase, "magnetic lines of force", I could rest easy with that phrase if only it were more commonly carefully defined. Otherwise I, and probably most people, am/are first inclined to think of it as an independent force, or an alleged independent force, such as it originally appeared to be in lodestone, as you mentioned in your post above.

With that thought in mind I would like to offer my idea of a more careful, but simplified, definition of magnetic lines of force. Please keep in mind that nothing needs to be changing to display this kind of magnetism.

(First draft)
Magnetic lines of force around a coil (I don't believe they really exist around a straight wire except as a name place holder in diagrams.) should be defined as the rearranged lines of attractive and repulsive forces that exist between any relatively parallel current components that exist, and in this case between the parallel components between adjacent DC powered coils in any of their relative orientations.

As for that Faraday experiment, I am suspicious that the current in the mercury is reacting with the magnet's field, causing the mercury to circle. Also the moving wire has a tilt which may have enough transverse component in it to react with the magnet's field, like the field winding of a motor with a rotating field.
 
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  • #29
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Hi guys,

I agree with arcnets. One must allow for the proper understanding of the principles of relativity in high school. One should not introduce it individually in itself, but must mould it with the concepts of electricity and Magnetism. In my post on Classical Electromagnetism, I was under the delusion that forces do not relatively transfrom. Thanks to ambitwistor, now it is not so. If u see my confusion and argument, they run on similar lines as that of fairfields. Do let the students know about poles. But why do u emphasise on poles and then say that poles dont exist? What is the pole?... My whole science stream class mates will respond ( thats about 200 of them ) that it would be the pole of a magnet.. or one end of a magnet. And yet, in places, we are taught that the pole is slightly behind the actual end of a magnet. We are taught about current loops creating poles and then told that no poles exist. And when it comes down to writing ur term exam.. nobody gives a damn bout if they understood or not... just put in values for B mu, I, l and pi get some stupid value.. underline it.. get 5 marks.. go to the next question. And if u do ask doubts, the general attitude in the class will be 'Shut up, sit down, and read the book'. Students must be given a first hand experience of what they are learning. Are we allowed to experiment on oersted's experiment? How many of us students have tried to see if 2 wires with parallel currents really attract? How many have seen an LCR circuit? How many have seen a van de graff generator at work? How many are allowed to use relays to really see if those half and full adders that we learn work? The list goes on and on....

These days, one must know a subject fully and thoroughly before teaching it. If not so, misconception and false ideas will prevail amongst students. No body will want to think if they are not expected to or are asked to.

Bottom line... offer peanuts and u get monkeys :smile:

Kartik
 
  • #30
zare
turin, i said electromagnetic field lines, not fields. you know, like design schematic of solenoid coil. central lines go troughout the coil from inside out, while top and bottom lines curve themselves around wire loops and make circles and eddies. the drawn density of those lines predicts em. field density. what i wanted to point out is that those lines do not exist on real model, they are only references in mathematical model for visualization of em. field.
 
  • #31
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Fairfield,

I think that by limiting your definition to current carrying coils you are barking up the wrong tree. I really think it has to be defined in terms of charge.

Also, I'm not confused about the static vs electromagnetic wave. The point of contention is much more about what constitutes the difference between an electric and a magnetic field. I had assumed it was the kinks in all cases. I wasn't married to that idea, it just seemed to be what the sources were saying. Apparently when it comes to the case of a current carrying wire they aren't saying anything, simply that is it so. I don't find that very insightful.
 
  • #32
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Originally posted by zoobyshoe
Fairfield,

"I think that by limiting your definition to current carrying coils you are barking up the wrong tree. I really think it has to be defined in terms of charge."

OK. Moving charges. But these charges have been somewhat usurped, maybe completley, by the positive charge that is moving them. So maybe moving charges is not the whole story.

Originally posted by zoobyshoe

"The point of contention is much more about what constitutes the difference between an electric and a magnetic field."

In my opinion, regarding coils and magnets, its only the effect of a different shape of the "electric" field (more focused),the term "electric field" being in, this case, a stand-in for the interactive forces between parallel wired currents (attraction or repulsion). In straight wired currents I believe people simply became hypnotized by the responses of magnetic compass needles (which have rearranged "electric" fields themselves).
 
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