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Unnecessary magnetic poles ?

by Fairfield
Tags: magnetic, poles, unnecessary
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arcnets
#19
Nov6-03, 02:40 PM
P: 513
Originally posted by QuantumNet
What's wrong with poles?
Well for one thing, I've never seen an exact definition of 'magnetic pole'. Maybe it's the point of a magnetic body's surface where the magnetism is strongest, or where the force is at right angles with the surface... but then you got to define 'body' and 'surface' which may lead to trouble.

Yes it's a relativistic effect. Please let me babble some more about teaching physics: My personal concept is sort of 'the other way round': Introducing relativity via magnetism!

How?

- We can easily show in experiment that a moving electron in a magnetic field experiences a force, called the Lorentz force. The important thing is, a stationary electron doesn't.

- Electromagnetic induction in a moving coil can easily be explained by the Lorentz force.

- Now, surprisingly (or not) this also works with a stationary coil and moving magnet. Now, do we have a Lorentz force affecting stationary electrons? - Of course not.

- If we think about it, it's the same situation, only in another frame of reference. Thus: "Physical laws are the same in each frame of reference". Furthermore: "Only relative motion counts. Absolute motion does not count, since absolute motion cannot be defined anyway".

- As a consequence: The term 'moving electron' makes no sense at all. We must ask 'moving relative to WHAT?'. The usual student's answer is: "relative to the magnetic field".

- Teacher: "How can you tell a 'moving' from a 'stationary' field?" ... That's IMO a possible entry point into relativity. From magnetism!
Hurkyl
#20
Nov6-03, 06:14 PM
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The magnetical field is a relativistic effect, it's not a part of the classical physics. What's wrong with poles?
[?]

Magnetostatics predates relativity by quite a bit...
QuantumNet
#21
Nov7-03, 06:01 AM
P: n/a
Originally posted by arcnets
Well for one thing, I've never seen an exact definition of 'magnetic pole'. Maybe it's the point of a magnetic body's surface where the magnetism is strongest, or where the force is at right angles with the surface... but then you got to define 'body' and 'surface' which may lead to trouble.

Yes it's a relativistic effect. Please let me babble some more about teaching physics: My personal concept is sort of 'the other way round': Introducing relativity via magnetism!
Don't make it harder than it is. Both the charges are relativistic,
as mass.
zare
#22
Nov7-03, 10:51 AM
P: n/a
there is no "magnetic poles" in real world. it's just there for reference point. same with electromagnetic field lines. they dont exist, but they show us the density and direction, and uniformity of that field in mathematical models or simulations.
turin
#23
Nov7-03, 11:36 AM
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Originally posted by zare
there is no "magnetic poles" in real world. it's just there for reference point. same with electromagnetic field lines. they dont exist, but they show us the density and direction, and uniformity of that field in mathematical models or simulations.
Are you saying that the field themselves don't exist!? It is pretty obvious (painfully so) that the lines are just a mathematical construct, so I'm going to assume that you meant the fields themselves (otherwise, I don't understand why you bothered to make such a vacuous statement). If the fields don't really exist, then how does electromagnetic energy propogate?
arcnets
#24
Nov7-03, 02:52 PM
P: 513
Originally posted by QuantumNet
Don't make it harder than it is. Both the charges are relativistic,
as mass.
Yes, of course you are right. But see, In a course like this I'm trying to introduce what 'relativistic' means. A magnetic field is relativistic in the following sense:
If observer A sees just a magnetic field, then observer B (moving relative to A) sees an extra electric field1. This leads to the idea that forces are relativistic. Students thought they weren't. Well if forces are relativistic, then other observables might be, too. Like space and time, themselves. That's why I think it's an entry point (at school...).

1Yes I know the magnetic field is also different - but of higher order in v/c - no need to analyze this here.
zoobyshoe
#25
Nov7-03, 08:39 PM
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P: 5,625
Fairfield,

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

"Simple rules were needed to deal with a simple action-at-distance experience, and soon the rule emerged that the lodestone has poles that pointed north and south. This was followed by the rule that `Like poles repell, unlike poles attract.'

The ready availability of permanent magnets focuses attention away from the closed-circuit nature of magnetism, which is far more useful in engineering than is the pole concept. Arguments against the circuital concept are that, unlike electric circuits, magnetis circuits cannot be insulated because the corresponding magnetic conductivity of air or empty spaces is finite and only about a thousand times smaller than that of the best magnetically conducting steel. Faraday likened the design of magnetic circuits to the design of electric circuits using bare copper wire in a bath of salt water (a good conductor of electricity). Although the electric circuits of most electrodynamic machines are complex and consist of coils with many turns of thin wire, their magnetic circuits are simple, short, fat and consist of a single turn.
The disadvantages of the pole concept are that it presupposes isolated poles in space; these have not been found, despite searches (isolated electric charges, however, do exist); and that troubles arise when trying to predict reactions between permanent magnets and other, initially unmagnetised, pieces of ferromagnetic material. For this purpose the law of induced magnetism has been invented, implying that the proximity of a primary magnet pole to a piece of soft iron induces an opposite pole in the latter nearest to the magnet and a similar pole at the point most remote from the magnet. There are a number of experiments which are very hard to explain on the basis of poles alone, while the circuit concept sees no difficulty in such arrangements: the pieces will always take up a position of minimum reluctance (that is, minimum impediment to the flow of flux) within the prevailing mechanical constraints. The circuit theory sees a magnet's pole merely as a change in reluctance between different parts of a magnetic circuit."
-The New Illustrated Science and Invention Encyclopedia
H.S. Stuttman Inc. Publishers
Westport CT 06889 1987 edition
Vol. Eleven, Pages 1510-1511, In the article: Magnetism
(Many libraries have this encyclopedia.)


So, you are by no means alone in seriously questioning the ramifications of this convention. But as this learned gentleman pointed out:

Originally posted by chroot
The two ends of a bar magnet obviously do different things to a compass needle, so they deserve two different names. The choice of north and south is arbitrary -- call them "slargbast" and "meedle" if you like.- Warren
We still very much need some handy vocabulary to distinguish the north pole from the south pole because they behave differently with respect to each other.

-Zooby
Fairfield
#26
Nov8-03, 07:04 AM
P: 27
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
zoobyshoe
#27
Nov8-03, 10:35 AM
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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
Fairfield
#28
Nov9-03, 05:43 AM
P: 27
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.
kartiksg
#29
Nov9-03, 09:21 AM
P: 13
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

Kartik
zare
#30
Nov9-03, 06:08 PM
P: n/a
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.
zoobyshoe
#31
Nov10-03, 04:28 AM
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P: 5,625
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.
Fairfield
#32
Nov10-03, 12:57 PM
P: 27
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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