Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Magnetic Lines of Force

  1. May 6, 2005 #1
    My Stupid Questions Thread

    First, if you go here to the forum archives, you will see a thread called "What is a field?" -

    The thread starter asked, very pointedly,

    Note that he didn't ask "how is a field produced", "how is a field measured/defined", or "how can a field be detected", "what are a field's mathematical properties", etc., Rather, he wanted to know WHAT IS A FIELD.

    The honest answer would have been something along the lines of "We don't exactly know", or "here's one theory..." - but instead he was double-talked (innocently enough) until finally someone, at the bottom of the thread, finally responded with "As someone said earlier, no one actually KNOWS why they work, they just do."

    This question stuck in my mind for a long time. One day, several years ago, I was experimenting in Adobe Illustrator. I made a rimless wheel of spokes. Then I duplicated it, and moved it to the right. What I saw astounded me. I cloned the wheel again, and shifted it to the right. I repeated this a few times, until finally I got this:

    Click here to see a .gif representation of the image I created.

    Tell me those lines-of-interference from the spokes (slash particles?) don't look eerily like classic magnetic Lines of Force?

    Thoughts anyone? Could there be an answer here with regard to the mechanics involved in creating lines of force?
    Last edited: May 6, 2005
  2. jcsd
  3. May 6, 2005 #2


    User Avatar
    Gold Member

    I can't answer the question, but I love the artwork. Reminds me of a black-light poster I had in high school. (Probably still have it somewhere.)
  4. May 6, 2005 #3
    Thanks. Came as a shock to me, actually, first time I saw it. Took my breath away, actually - that I could take something as mundane and overly simple as a spoked wheel, and, by laying two of them side by side, could create something as complex seeming as a magnetic field (something I've yet to see explained in terms of the mechanics of why, or "what" it is).

    I mean, take a look at the same phenomenon with only two adjacent spoked wheels. The effect is both immediate and dramatic.

    At any rate, before discovering this accident of art, I knew of only one way that a pattern like that could be created (made visible with iron filings), and that was a magnetic/electromagnetic field.

    On one website on magnets and magnetism I learned that physical particulate/granule size determined how the lines of force appeared from a common bar magnet.

    Now I can't resist but wonder if you considered the spoked arrays I made as analogues for radiating particles?
  5. May 6, 2005 #4
    In case I wasn't clear (I should have asked up front) - does anyone know of any published theories that attempt to explain the physical mechanism that creates or determines magnetic fields/lines of force (i.e., types of particles/waves and their interactions)? If so, can you point me in the right direction?

    Also, are there any published theories (reputable) that likewise attempt to explain the actual/possible physical mechanics of gravity?.
  6. May 6, 2005 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    I've read several of your postings, including in a another thread on "mass and inertia", and I see a common theme recurring in almost all of them. I am going to try and address this here.

    When you ask "what is so-and-so", if you think about it carefully, for ANYTHING that you know of, the answer to such a question ALWAYS, without fail, consists of a list of properties of that entity. If you ask, what is a tomato, then the answer to that can be that a tomato is a fruit (biologically, even though the US Supreme Court categorizes it as a vegetable), it is typicaly red, but can have a variety of colors, it has watery content, etc.. etc. All of these are its properties that together, define what it is.

    If you ask "what is an electron?", I have to list you a whole set of properties that are associated with an electron - it's charge, it's spin, it's mass, how it interacts in matter, what it does in a conductor, etc. These properties are what DEFINE an electron. There is no other way to know what it is without these properties. We only know that something is an electron due to these properties.

    Thus, when you ask "what is a field", for some reason, you're expecting an explanation devoid of the properties associated with it. Yet, we detect the existence of a field DUE to such properties. A classical field can be directly defined as the field of force experienced by a "test" entity within that field. If you have an electrostatic force, a test charge in that field will experience a coulombic force. If you have a gravitational field, a test mass will experience a gravitational force, etc... This is how the classical field is defined in the first place. There is no way to get around that. Such property IS what consitutues the identity of a "field". Note that I am not going into quantum field theory because I believe this doesn't add to the point I'm trying to get across.

    I could do the same thing with "inertia". As in cephid's correct response, an inertial "mass" that we measure IS a property of "inertia". It is a measure of it, and thus, part of what defines what "inertia" is. The resistance to change in velocity is how we defined what an "inertia" is. It's how inertia came into existence in the first place. Thus, we cannot simply dismiss or trivialize that as being inadequate because, by definition, that is what it is!

    Again, there's a chance that we can redefine what "inertial" and "gravitational" mass is in the future, depending how much we can discover about the Higgs, and other quantized fields. But it doesn't change the point that an entity is physics is defined by the properties it exhibit, because these properties are how that entity is detected in the first place.

  7. May 6, 2005 #6
    I've been having a hard time with this also... I like to understand everything to the smallest bit like wtf a field is made of. Say I have a bar magnet and I can see the field it is creating from north to south. What is going through these lines? Photons going from north to south or what? It's gotta be made of something! It can't just be nothing causing interactions between other objects. How does a magnet "know" that another magnet is nearby?
  8. May 6, 2005 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    The way you are asking this implies an answer from quantum field theory and quantum electrodynamics. I don't know to what extent that can be done clearly on here.

  9. May 6, 2005 #8
    Firstly Zz, thank you very much for your well reasoned and thoughtful comments. Secondly, good eye, catching what is, indeed, a common theme in much of what interests me about general physics.

    If I seemed dismissive or trivializing of the properties we use to define such phenomena as inertia and fields, that was not my intent. On the contrary, I'm in awe when I consider them, and the efforts that went into their discoveries (and all subsequent discoveries that broadened our body of knowledge).

    I'm not quite sure how to use your tomato example to clarify my question, so please indulge me while I use a different example - an automobile.

    We could describe an automobile in terms of specs - mileage, performance, size, capacity, load, etc., - when really the primitive chief of the secluded tribe is asking, "How many squirrels must be forced to labor inside the bowels of this beast? What do you feed them, and how is it that they never seem to tire?" In short, he's asking (though not phrasing it well) for you to open up the hood and explain internal combustion engines to him - the entire mechanical process whereby hydrocarbons are compressed, exploded, converted to internal movements, and exhausted...the whole PHYSICAL cycle. With all due respect, that isn't in the least dismissive or trivializing of other important aspects of the automobile.

    Note PatPwnt's question, echoing mine -

    To my thinking, this is an extremely valid question (one which is equally deserving of defenses against dismissiveness or trivialization). Even if the answer is, "We don't know" - at least it's an answer.

    Final note - a common thread that I've noticed over the years (in ANY field, including physics) - it's almost axiomatic: The higher up you go, and the more enlightened someone is, the greater is the willingness to confess personal ignorance, or to expose flaws, holes, or weaknesses on behalf of whatever field(s) they represent.

    I know there is a whole process going on within the physical space surrounding an ordinary bar magnet - I'm not dismissive of what we do know about it - it's certainly mountains more than what we knew a century ago. But it is not, by any stretch of our understanding, a complete picture.

    The image I created - it may be pure coincidence that such a simple thing could mimmick the pattern of a magnetic field in such a way - but then again - who knows? I throw it out there. You never know how it might mesh with what someone else knows, or is thinking about. I find it intriguing.

    As Ghandi said, "It's always the simple things that catch your breath." I think Einstein said something similar, along those same lines.
  10. May 6, 2005 #9
    Actually, even that's useful. Often someone will ask me a question for which there is a definitive answer - but when I realize that they haven't sufficient foundation of basic understanding for me to explain it to them in terms they would understand, it can get very frustrating.

    Is that the case here? (with magnetic fields)
  11. May 6, 2005 #10
    Last edited: May 6, 2005
  12. May 6, 2005 #11
    Been to that site many times!

    It speaks as to:

    1. How magnetic fields are produced:

    Magnetic fields are produced by electric currents, which can be macroscopic currents in wires, or microscopic currents associated with electrons in atomic orbits.

    2. How it is defined (in terms of force on moving charge):

    The magnetic field B is defined in terms of force on moving charge in the Lorentz force law.

    3. Practical applications through interactions with charge:

    The interaction of magnetic field with charge leads to many practical applications.

    4. The general nature of fields, along with terminology and basic formulae:

    Magnetic field sources are essentially dipolar in nature, having a north and south magnetic pole. The SI unit for magnetic field is the Tesla, which can be seen from the magnetic part of the Lorentz force law Fmagnetic = qvB to be composed of (Newton x second)/(Coulomb x meter). A smaller magnetic field unit is the Gauss (1 Tesla = 10,000 Gauss).

    But it never says anything along the lines of "x particles/waves are emitted/returned/yadda, which travel blah blah, the energy of which is conserved/expended/yadda..."

    Is this because it is as yet unknown, or simply beyond the scope of basic understanding?
  13. May 6, 2005 #12


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    There is a difference here, since you are also describing the FUNCTION of an automobile AND the fact that an automobile isn't "unique". There are many forms and various "look". If you say "Oh, I bought a vehicle yesterday", the next logical question would be "oh, what KIND of vehicle". Now, compare to when I say "Oh yes, we did detect electrons in that measurment". Do you expect one to then ask "oh, what type of electron?"

    What you described with your automoble analogy is how I would describe the "energy", the "momentum", the trajectory, etc. of the electron. These are the "function" of the electron, i.e. how it is behaving in a certain condition. These "functions" are not unique, and different electrons will acquire different values. However, the spin and charge of and electron will not change and an inherent part of what DEFINE it.

  14. May 6, 2005 #13
    Point taken. Then I have to just come out with the question in the most primitive way I know how to frame it - in three parts:

    The typical magnetic field - is it comprised of

    1) particles
    2) waves
    3) both
    4) other
    5) unknown

    If not unknown:

    Can these particles/waves/both/other be classified, (e.g., electrons, photons, etc.)?

    Do these particles/waves/both/other emit without returning, or do they cycle in and out (like electrons in a closed circuit)?
  15. May 6, 2005 #14


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Classical fields do stop there. Quantum fields do not, and it starts from the fundamental assumption of the nature of symmetry of our space. I am dreading going into QFT to do this because I don't see any other way of explaining this without using analogies (and I hate analogies to explain physics principles because they are seldom accurate).

    QFT removes the classical field, and replaces them as interaction with a "sea" of virtual particles. These virtual particles represents the "force carriers" of that particular field. For EM field, these are virtual photons. I'm going to simply do this to a "generic" field and "generic" virtual particles.

    Now, from simple mechanics, we know that a force is the rate of change of momentum. It means that, in a vacuum with no other forces acting on an entity, dp/dt is zero. Now, if there is a change in momentum, you immediately know that a force is acting on that entity. This is how we detect the classical field. However, in quantum field, the ONLY way that can happen is when the entity has exchanged a virtual particle. This is where the analogy comes in...

    Let's say you are in the middle of a frozen pond. Assume absolutely no friction. If you are stationary with respect to the ice, how do you move? You don't! There's nothing to pull or push you to cause you to change your momentum from zero to something.

    Now, what if there is another person also on the ice with you. you have a bunch of bowling balls (don't ask me where you would stache those balls) with you, and so does he. If you two start throwing the bowling balls at each other, the act of heaving the ball and receiving the ball from the other person will cause you to move (a change in momentum). In fact, in this analogy, you two will move away from the other. You can use this to (inaccurately) picture two LIKE charges repelling each other.

    I'm not going to describe an analogy for "attraction".... But that's besides the point. The whole reason I'm explaning this is that QFT has a very detailed description (with TONS of mathematics to describe it), on how anything interacts via the exchange of the force carriers in terms of a virtual interaction. I want to make sure you do not carry the impression that we have no clue, or it is not known yet on the basic interaction. It doesn't mean there are still no questions left to be asked, because we keep looking at things deeper and deeper by continuing to ask why and how. But from the level you are asking, yes, we know the nature of such interactions.

  16. May 6, 2005 #15
    THANK YOU. Excellent (and entertaining) response.

    Also, thank you for being so patient with me.
  17. May 6, 2005 #16


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Well, I had no choice! :)

    You asked an important question in which we spent years studying and suffering through as physics majors. Even when I have formally studied such things, as a "non-theorist", I still consider myself as not an expert in this area of study. But I certainly cannot leave your question hanging, and certainly I cannot leave the impression that QFT doesn't exist as a means to describe such interactions.

    And now, I'm going to rest my fingers.......

  18. May 7, 2005 #17
    Well! Zz's responses (and several subsequent good reads on the same subject - see below) got me thinking -

    The first images that I provided links for implied graphic analogues to particles that radiate (two dimensionally) in all directions. They were aligned horizontally, adjacent to one another.

    So this morning I thought, what would it look like if the same "particles" did not emit, but instead had bands of concentric circles to represent orbits of some kind?

    Click here for the result - made me laugh...

    Note: The "particles" were placed horizontally adjacent to one another, just as before, only this time, the interference pattern it makes is 90 degrees out of phase with that of the spoked design.

    Again, I'm not drawing any conclusions, or presenting a hypothesis of any kind. I just find it all fascinating - I have a background in electronics, but as an artist, I love to visualize.

    Here are links to 'subsequent reads' I mentioned - all very informative, in line with Zz's responses in this thread:

    Last edited: May 7, 2005
  19. May 7, 2005 #18

    That was terriffic. I'm a sophmore in highschool so I'm hardly learned in this topic but I do have an intrest in it. the ideas you put forth were amazing. I'm still thinking over how each side of a bar magnet would radiate the virtual particles. I guess the dark parts of the radial drawing that steven made (which correspond to the dark parts of iron fillings) would be where the circles overlap and where the force is the strongest so naturally it would attract more iron filings. This is terrific. I've allways seen the filings and just thought "that's a nifty pattern". But I was so use to seeing magnets interact with metals that I never even stopped to think why. While my previous career goals were along the lines of computer programming and 3-d animation, this article has made what intrest I had in physics magnified by 10. Thank you.
  20. May 7, 2005 #19
    That made my day. My son is a junior in high school, and the artwork/ideas affected him in exactly the same way. He printed them out to show his physics teacher. Seeds - food for thought, if nothing else.

    Obviously, it would thrill me to no end if, one day, it could be shown that orbiting/radiating particles, virtual or otherwise, interacted in similar ways to create such patterns. Indeed, it would give knew clarity/meaning to the term "flux density". I love knowing the nitty gritty mechanics of any closed loop system. However, if it turned out to be sheer (albeit uncanny) coincidence, and served no other purpose than to inspire wonder and curiosity about the world of physics... well, again, your post made my day. Thank you.
  21. May 7, 2005 #20
    I would say your welcome but it seems we are in each other's debt so I will just let our debt cancel each other out. :biggrin: I've been reading for the past couple hours about physics and I would expect my head to hurt but it feels more like it's being massaged. I made another post in the General Physics section with some questions that have bugged me for years so if you know about physics, feel free to take a whack at it.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Magnetic Lines of Force
  1. Magnetic lines of force (Replies: 16)