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Unnecessary magnetic poles ?

  1. Oct 19, 2003 #1
    Unnecessary "magnetic poles" ?

    If I run steady, same direction, currents through two close, slack parallel wires, I will see the two wires attract each other. If I reverse one of the currents I will see the two wires repel each other.

    If I now roll the wires up into coils, and place them on the same axis near to each other, and again run steady, same direction, currents through them, I will again see them attract each other. But the text books will tell me that, in the case of the coils, it is the magnetic north and south poles of the two coils which are attracting each other instead of just their parallel currents.

    Likewise, if I rotate one of the coils end for end, which causes its current to now be running opposite to that of the other coil, I will see the two coils repel each other. But the text books will tell me that the two coils are repelling each other because their like magnetic poles (north - north or south - south) are repelling each other, instead of their opposite parallel currents doing it.

    Doesn't Occam's razor suggest that we should forget the primitive, unnecessary, concept of north and south poles around current carrying coils, and, instead, acknowledge only the direct, lateral, attraction or repulsion between parallel current carrying wires, even when they are in the shape of coils?
     
    Last edited: Nov 8, 2003
  2. jcsd
  3. Oct 19, 2003 #2
    I could be wrong, but I think you got all the repel and attract parts in reference to the current wrong. I think parallel currents repel and antiparallel currents attract.
     
  4. Oct 19, 2003 #3

    chroot

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    Re: Unnecessary "magnetic" poles ?

    Well, currents DON'T attract or repel each other. What's going on in one wire doesn't directly affect the other. What happens is that a current through a wire sets up a magnetic field, and this field affects electrons in the other wire by pulling them to the side. The net result of lots of electrons being pulled to the side is that the wires attract or repel each other.

    The field around a straight wire is composed of concentric lines of magnetic field, so there really is no way to establish a "north" versus a "south" pole for that field -- just as there would not be in a magnetized donut of metal, in which the field lines never leave the surface.

    - Warren
     
  5. Oct 22, 2003 #4
    Re: Re: Unnecessary "magnetic" poles ?

    If the reality of that statement were unchallengeable, then, of course, I would have to accept your explanation. But it is exactly those alleged concentric magnetic lines of force that I am questioning.

    The concept of a concentric magnetic field around a direct current carrying wire was established by noting the action of any magnetic compass needle brought into the vicinity of any direct current carrying wire.

    But a compass needle is simply a miniature equivalent of a direct current carrying coil mounted horizontally on a pivot half way along its length, and having very flexible power leads to it.

    The current on the side of this coil (or a magnetic compass needle) which is going in the same direction as a direct current in a nearby reference wire will be attracted toward that reference wire. The coil's wires carrying current traveling in the opposite direction to that of the reference wire's current will be repelled from the reference wire.

    These two LATERAL forces will cause the coil (or a magnetic compass needle) to rotate to maximize these two responses, causing the coil (or a magnetic compass needle) to point across the axis of the straight direct current carrying reference wire. But this pointing direction is obviously not the direction of the actual forces which are operating on the turnable coil (or on a magnetic compass needle).

    The magnetic compass needle was originally given north and south pole names because it (like an easily turnable direct current coil would) tends to turn so its ends point toward the geographical north and south poles of the earth in response to the equatorial direct current in that giant current carrying "coil" called earth.

    So, although the north and south poles of a magnetic compass needle are a useful concept for navigation purposes, I don't see where there is any real thing called north and south magnetic lines of force. I only see real lines of force at 90 degrees to the axis of what's called north and south poles.
     
  6. Oct 22, 2003 #5

    chroot

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    Re: Re: Re: Unnecessary "magnetic" poles ?

    There isn't. It's a convention.
    At one end of a magnet, the lines of force go into the magnet body. At the other, the lines of force leave it. 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
     
  7. Oct 22, 2003 #6
    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.
     
  8. Oct 22, 2003 #7

    turin

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    The magnetic version of Gauss's Law in Maxwell's equations is usually called the law of "no magnetic monopoles." If you believe that Maxwell's equations are absolute (that, in fact, there are "no magnetic monopoles"), then it certainly seems that the fundamental interaction is between currents, and that referring to the magnetic force in terms of the attraction and repulsion of poles isn't a fundamental treatment. Physics certainly seems to support this view for the most part. However, this introduces an asymmetry between the Farady tensor and its dual. I think most physicists would like for nature to be symmetric. There have been several treatments of electromagnetic theory including the existence of magnetic monopoles, which, consequently, would be attracted or repelled by the magnetic field directly. Analogously, moving magnetic monopoles (magnetic current densities) would create a transverse electric field, thus provinding a symmetry between the Farady tensor and its dual. Of course, I could talk all this nonsense (or maybe it's not nonsense) for hours; the fact of the matter remains that magnetic monopoles have not been observed. This lack of observation has been so pronounced as to warrent a law of physics that basically says "there is no such thing as a magnetic monopole."

    In light of the evidence, I must concede to your arguement that magnetic poles do not directly attract or repel, but that electric currents are what truly attract or repel.
     
  9. Oct 22, 2003 #8
    Fairfield,
    your question 'Is the concept of magnetic poles unnecessary?' looks very interesting to me.
    Well, IMO the standard answer is, of course, "A scientific concept is necessary as long as we need it to explain the observations".

    I tell you why I find this so interesting: Because I have to teach this stuff at school. And usually, the students who I teach it to, start off with almost no knowledge on magnetism.

    My idea is: 'Poles' are necessary when the course starts. But we make them unnecessary as the course proceeds. Like this:

    1) Some minerals found in the earth can attract objects made of iron, nickel, or cobalt. We call this 'magnetism' after the mines of Magnesia where these minerals were found in ancient times.

    2) If we examine one of these stones, we find that there are certain areas where the effect occurs, and others where it doesn't. The active areas are called 'magnetic poles'.

    3) If we take a thin long steel needle, and rub it repeatedly against a magnetic stone's pole in one direction, over its entire length, then the needle becomes magnetic with two poles at its ends.

    4) If we have two such needles, then we find that their poles may attract or repel each other. Thus, there must be two different types of poles.

    5) If we let such a needle swim on a cork (as the Chinese did ~1000 years ago), we find it will align roughly in geographical north-south direction. The pole pointing north is called the north pole, the other south. The instrument is called a compass and helps in navigation.

    6) (LAW OF MAGNETIC POLES). Equal poles repel. Opposite poles attract.
    Thus, the Earth must have a strong magnetic north pole in the south, and vice versa.

    7) Today, we have much stronger magnets than the needle (I won't tell you how they're made). Their magnetism is much stronger than the Earth's, when you're close to such a magnet.

    8) Such a strong magnet can hold up an iron nail. From the bottom of the nail, we can hang down another nail. And another... You see, a piece of iron that is close to a magnet, becomes a magnet itself. This even works at a distance. This is called magnetic influence (Just think of 'influenza'...). Obvioulsly, since the nails are oblong objects, they take on two poles, just like the needle. We can tell where each nail has which pole.

    9) We cover a magnet with a transparent foil, and scatter iron filings onto it. They align in lines (showing the effect of magnetic influence around the magnet, since the filings are oblong objects). These lines are called magnetic field lines. Which make up the magnetic field produced by the magnet. We define the direction of a line to be 'away from the north pole'. That's just a convention. We note that each line that leaves the magnet, also enters it again.

    10) We place a piece of iron close to the magnet, and then examine the magnetic field by using iron filings. We see that the field goes right through the iron, which seems to compress the lines. We know that the iron takes on influenced north and south poles. We deduce that a 'south pole' is nothing else than converging field lines going into a body, and a 'north pole' is nothing else than diverging lines going out of it. We also deduce that a field line has no beginning and no end, but goes thru any body.

    11) We observe the fields of two magnets. Firstly, when they attract each other, and secondly, when they repel. We see that field lines cannot cross each other. We also see that all magnetic forces can perfectly be explained from the....

    (drum roll)
    LAWS OF THE MAGNETIC FIELD:
    1) Magnetic field lines have no beginning and no end.
    2) The north pole of a magnet is just the area where the field lines leave it. The south pole is where they enter it.
    3) Field lines tend to be as short as possible (attraction).
    4) Field lines tend to be as far away from each other as possible (repulsion).
    5) The last 2 rules explain all magnetic forces.
    6) Magnetic materials like iron compress the field lines.

    (Of course, at this point you should also have told the students some facts about the internal structure of ferromagnetic materials, to explain why we can magnetize the steel permanently... but that uses poles and is not important here...)

    12) Once you have these laws, you can switch on the electricity. Oersted's result is something we just have to accept, the interaction of 2 wires follows easily from the above. Even electromagnets, the Lorentz force and electromagnetic induction are easy to explain once you have introduced the LAWS OF THE MAGNETIC FIELD, which indeed make the conept of poles unneccessary. Via induction and Lorentz force, you enter the domain of special relativity...

    Enough for now.
     
  10. Oct 23, 2003 #9

    turin

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    I like this.
     
  11. Oct 23, 2003 #10
    turin,
    I'm glad you like this. That you like this, even though you said:
    ... which is of course the deeper physical principle behind all this.
    But at school level, I think my concept is a good one.

    Or are you being cynical? No, I guess you're not...
     
  12. Oct 23, 2003 #11

    turin

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    I was being sincere. I totally agree with your perspective on teaching, especially science. IMO, for instance, students should learn about the aether before they learn that it is invalid. Why not, since they learn Newton's law for gravity, with which even he himself was uncomfortable. Science seems to start with what is readily observable, and then, if warranted by deeper investigation, the theory is adjusted. This is a development process which seems to have been robbed from students in the modern education system, and has ultimately led to great confusion in certain areas earning them the distinction of being "counterintuitive." It is one of my pet-peeves for someone to say that relativity is so.
     
  13. Oct 24, 2003 #12
    I can understand that, as a commonly encountered secondary phenomenon of circular currents (requiring at least 4 parallel currents to demonstrate any magnetic effects), magnetism needs the nomenclature that is commonly attributed to it.

    But I am troubled because this secondary phenomenon is always treated as the most primary phenomenon wherever there are magnetic effects occurring.

    But even where there is not a truly magnetic effect occurring, this false concept gets extrapolated backward thusly. Any DC current carrying wire is claimed, on account of the action of any nearby magnetic compass, to have magnetic lines of, something, circling the wire. But the compass needle is being influenced by strictly lateral forces acting on it, as any DC coil in the same situation would be. But because the compass needle already has names on its ends, from its use for navigation purposes, we declare, without any rational reason that I can see, that there are north - south magnetic lines of, something, circling the wire. Isn't science supposed to be more critical than that?
     
  14. Oct 24, 2003 #13

    turin

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    Charge is not a scalar quantity. There is a four vector that is a current density. The timelike part of this four vector is basically responsible for the electric field. The spacelike part is basically what is responsible for the magnetic field. From what I have gathered so far in my studies, the components of the magnetic field are just as real as the components of the electric field, but they are situated in the Farady tensor so that you don't see their effect on the timelike part of the current density; you see their effect on the spacelike part.
     
  15. Oct 25, 2003 #14
    I still feel inclined to ask, what if Oersted had had, instead of a compass placed near his experimental current carrying wire, a pivotal tubular coil which was also carrying current? How would he have explained the turning action of the coil in that situation other than I have explained it above, as strictly, and only, as 2 lateral forces in operation.

    The length of the coil would point to absolutely nothing in particular that exists in nature. Only to something that exists in the imagination due to reckless backward extrapolation from the interaction of two DC curent carrying coils, or the impproper interpretation of the forces at work on a compass needle.

    The true forces at work on the above coil would be more visually
    indicated if the coil had no axial length at all but, instead, was just one loop with a wide oval shape.

    I'll re mention that I don't see where you can even have the commonly characterized magnetic phenomena without having at least 4 parallel currents present, 2 in one direction and 2 in the opposite direction. To me this means magnetism is only a secondary, situational phenomenon which merely combines the lateral force vectors so that they appear to be coming from between the wires (and therefore around them), when they really aren't.
     
    Last edited: Nov 13, 2003
  16. Oct 26, 2003 #15

    krab

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    Re: Unnecessary "magnetic" poles ?

    What you are proposing is viable, but does not really make for a simpler description. Two current-carrying coils, some distance apart are most easily described as two magnetic dipoles. The forces they exert on each other is much more easily found from formulas describing dipoles, than by integrating the currents through the (possibly very many) turns of wire in each coil.

    I suppose one of the reasons many of the presentations in textbooks are based on "poles" is that elementary particles themselves appear to be point dipoles instead of current loops.
     
  17. Oct 26, 2003 #16
    I understand the practicality of dealing with magnetism "as it is", but science is also supposed to concern itself with discerning the ultimate substructures of the presentations of nature in order to better understand them, explain them, and use them.
     
  18. Oct 26, 2003 #17

    Hurkyl

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    It's the same reason the electric charge is positive for a proton and negative for an electron; a name was needed for the two opposite polarities.
     
  19. Nov 6, 2003 #18
    Re: Unnecessary "magnetic" poles ?

    The magnetical field is a relativistic effect, it's not a part of the classical physics. What's wrong with poles?
     
  20. Nov 6, 2003 #19
    Re: Re: Unnecessary "magnetic" 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!
     
  21. Nov 6, 2003 #20

    Hurkyl

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    [?]

    Magnetostatics predates relativity by quite a bit...
     
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