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What causes magnetism?

  1. Aug 25, 2009 #1
    Alright, so magnetism, this weird force that just seems to occur at the end of a lump of magnetic metal. I understand that:
    - The movement of electrons "facilitates the occurrence" of a force called magnetism

    - Magnetic materials have unpaired electrons in their shells which all have the same "spin" (whatever that is). Since pairs have opposite spins, the magnetism produced by both "cancels out". In magnetic materials, the magnetism is not "cancelled out" and over many atoms produces a macroscopic effect

    - Electricity and magnetism can be thought of to actually be different parts of a single concept, in the theory of special relativity. Something to do with "inertial frames" (what's that?) and lots of other mumbo jumbo that it seems you need an IQ of 161 to understand.

    - A hell of a lot of maths that explains very well HOW stuff works, but never the ideas behind the calculations. Something more exists than just mathematical models - well that could be debated, but I'll assume that I think therefore I am. I'll take what I see to be self evident unless there is reasonable doubt.

    So let me ask the question:
    In terms of classical: Why does the movement of an electron cause magnetism?

    In terms of quantum (baring in mind, I don't understand it): Why does electromagnetism exhibit different aspects that are modelled vastly differently in classical physics, when effectively quantum seems to want me to believe that electrostatic charges are the sole cause for apparent magnetism?

    And well that leads to other questions:
    - Why do protons and electrons have charge and why do they repel each other?

    - Two magnets interact. What's going on there to cause attraction and repulsion in terms of electrostatic forces?

    - When two magnets attract, they are in each others' "magnetic fields". The idea intuitively implies that magnetism is present and continuous from the source outwards. I imagine that space is "filled" with "magnetism". Is this really correct, or could it be seen that there is nothing in between two interacting atoms except for empty space, and that they simply just affect each other...

    - In a ways that we can explain HOW but not WHY? (Asking why the universe works the way it works and not a different way, of course a question that belongs with philosophers and religion, at which point I choose to say "I don't know" rather than go around guessing that there might be a designer or that we might not exist bla bla bla)
     
    Last edited: Aug 25, 2009
  2. jcsd
  3. Aug 25, 2009 #2
    You diverge a lot in your thread. I'll pick one of your questions. You should be more aware of the fundamental knowledge before diving deep. Some of the bullets you described are inaccurate.

    Magnetism is essentially caused by electrostatic repulsion PLUS exclusion principle, which is unknown to classical electromagnetism.

    Unpaired electrons, if they are localized in the same region (say, in an isolated atom) , tend to correlate their motion to minimize the Coulomb potential they have between each other. This minimization of the "exchange interaction" is the cause of ferro-magnetism.

    Edit: The "movement" of the electron causing magnetism seems more classical to me. Historically, the way people discovered magnetic fields is by running large currents through wires, solenoids and electro-magnets. That's why when the atomic theory was first being developed, people tried to understand the "inherent" magnetic fields of the matter by assuming electrons precessing around the nucleus while spinning about themselves. Well, in short, that turned out to be wrong. And you need some decent dose of quantum mechanics to make peace with that. I suggest you read Feynman Lectures before anything else. It has the bare minimum math necessary.
     
    Last edited: Aug 25, 2009
  4. Aug 26, 2009 #3
    You sure do go on and on about nothing at all! What makes you think that physicists have any kind of clue what makes all this stuff work? Because they told you they did? And besides science in general even the philosophy of science does not ask "why". It may ask "how", however. So natural philosophers ponder the interactions of objects and attempt to create models that match these interactions.

    For example, it is known that a magnet will affect another magnet that comes near it. It will generate a force upon that second magnet. That force exists in magnitude and direction (vector) all about that first magnet so we can actually map the force that the second magnet feels anywhere in space about the first magnet. We call that map a "magnetic field". A field, however, is not a real object. It is a mathematical abstraction. It represents the behavior of whatever it is that creates the forces in space.

    But this doesn't mean we know nothing. We do know that magnetic fields (forces in space on magnets) are created by electric currents. And electric currents represent a flow of charges. This always seems to be true even for magnets. The idea being that the magnetic fields of magnets are created by the circulating currents (flow of charges) in the atoms of the magnet. We do not know how a flow of charges creates a magnetic force field, but we do know that this field is not an electrostatic force field as seen between like and unlike charges. We know this because it's properties are different. So charges seem to be able to create both electrostatic forces on other charges as well as magnetic forces on charges and circulating currents.

    And we know a little bit more as well. We know for example, that if one turns on a current in a wire, the force on a magnet at some distance does not instantaneously appear. This idea was called "action at a distance" in the old days and today except for quantum mechanics, has been rejected as totally bogus. In truth it has been measured that a magnetic field travels outward in straight lines from the source at the speed of light. Hence if a source current turns on, a magnet a couple of feet away will actually only begin to feel the force a couple of nanoseconds after the current turns on.

    So we know that currents and magnets (also circulating currents) will produce forces on other currents and magnets in space about them. We can map the forces and we call that map a magnetic field. We know the forces travel outward from their sources at the speed of light. But we do not have any idea what actual objects are creating the forces or whose behavior is described by the fields.

    From here it gets even more complex. We know for example that while a constant current creates a static magnetic field, we also know that a changing current creates not only a changing magnetic field but also simultaneously an electric field capable of creating currents in another wire. This is called "induction" and is the action used in transformers. Again we know that this electric field travels at the speed of light. And again we have no idea just what mechanism is the basis of a changing current causing an electric field at a distance. We do know however that the induced electric field is in a direction either parallel or anti-parallel to the source current and it's strength is proportional to the rate of change of the source current. We also know that the strength of the induced electric field falls off as 1/R where R is the distance to the source current.

    So the answer to your question is that "why" isn't even asked and "how" is only known to a certain degree with respect to electromagnetic behavior. However fundamental mechanisms are unknown and have no logical models. The abstract "field" is generally taken as a physical model of the phenomena even though it actually does not provide any hint with regard to the actual mechanisms of the underlying transmission of forces. Physicists have a bad habit of regarding mathematical abstractions as more real than reality. So you will hear people referring to "fields" as if they were real objects doing various things, but that is an error. The force maps are indeed real, but the maps only describe an unknown mechanism.

    Consider the number 2. If I say two oranges, you know what I mean. But what kind of "object" is the number 2 all by itself when it has no other real objects to describe the nature of? It does not exist in reality. It is a mental construct without physical basis. So it is with fields.

    OK?
     
  5. Aug 27, 2009 #4
    bjacoby: Then in fact, everything we know about in science, is just a model that fits the data? So atoms might not exist, the theory of electromagnetism might be completely wrong etc etc etc?

    Ah this is what I wanted to know. Does quantum theory explain this? I'll have a read of the names provided by sokrates.

    It seems if I am to understand science, I've got to get my head around quantum theories.
     
  6. Aug 27, 2009 #5
    Actually Quantum Mechanics doesn't "explain" a whole lot! But it does try to deal with problems that have been discovered with electromagnetics. The "classical" model of electromagnetics is based upon a smooth continuous fluid flow model. But it's been discovered that down at the microscopic level reality isn't like that. It seems to be made of tiny little discrete pieces of something. Hence the previous electromagnetic theory is wrong. But that doesn't mean that it won't work just fine to get answers to engineering problems. Just as Newtons laws though incorrect at high speeds work just fine to navigate rockets. So if you want to understand science, you've got to get your head around ALL the theories even the "wrong" ones!

    Sokrates advice to study the Feynman lectures is very good. Feynman was a genius but at the same time never lost sight of practical reality. His lectures spell things out for you, but at the same time he pulls no punches on the advanced stuff. He was an American Physics icon best known for his testimony in figuring out the space shuttle disaster. In fact, while you are looking into Feynman, see if you can find his popular stories as well. One is called: "Surely you are Joking Mr. Feynman". There are a couple of them and are totally great collections of fun stories from his life. Yeah, you can't go wrong there.
     
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