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What is magnetic force ?

  1. Feb 17, 2010 #1
    When i am shown pictures of bar magnets they show magnetic field lines with arrows diverging out the north pole, and converging back into the south pole

    1. Does the magnetic field coming out the north pole, turn back and wind back into the south pole ?

    2. what is actually carried along the lines, such that arrows need to be used ?

    Basic questions i know, but amazingly its been difficult to get an answer on this.
  2. jcsd
  3. Feb 17, 2010 #2


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    Wikipedia is often a good place to start for basic questions like this ... and it addresses both your questions.

    http://en.wikipedia.org/wiki/Magnetic_field" [Broken]
    Last edited by a moderator: May 4, 2017
  4. Feb 17, 2010 #3
    thanks...i looked there last year, but i see a lot of new stuff on this has been added.
    Last edited by a moderator: May 4, 2017
  5. Feb 18, 2010 #4


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    In wikipedia's article there is a picture on the right side where in comment it is said that "magnetic field is a pseudovector" or alternatively axial vector.
    So it is meaningless to ask what is actually carried along the lines the same way as asking what is carried along rotation axis of the body.
    It is more meaningful to ask what is carried around the lines. And to that question I would say that it seems useful to think that it's electric field that is carried around the lines or another way around is that magnetic field describes gradient of electric field perpendicular to it's direction and magnetic field lines are contour lines of electric field in that perpendicular direction.
  6. Feb 18, 2010 #5
    You might find this interesting:

    I'd suggest (inspired by the above) simply that there is a magnetic field, in the same way that there is an electric field, and that it points in a direction in the same way the electric field does. The meaning of the direction of the electric field is that a (positively) charged particle placed at a point in the field will get pushed in the direction of the arrow. The effect of the magnetic field is slightly wierder; it wants to push at right angles to both the motion of the particle and the direction of the field. The wierdness of this definition is related to the deep connection between the electric and magnetic fields, which is really a topic for the relativity forum :wink:
    Last edited by a moderator: Sep 25, 2014
  7. Feb 18, 2010 #6
    So Basically when you have magnetic force lines dominating the electric component of electromagnetism such as bar magnets, earths core field. Then these force lines represent the structure of tightly bound electrons in iron which are forced to spin in one direction ?

    Even though the electrons in the magnet are still bound in their iron atoms a generalized force has accumulated from their spins and this is the magnetic field ?
  8. Feb 18, 2010 #7

    Not really. Magnetic fields are generated by the motion of charged particles; the magnetic field due to a wire carrying an electic current is nothing to do with the spin of the electrons in the current. The final sentence is pretty much true, but it's a specific case of a general phenomena; an explanation of why iron can be made into a bar magnet, even though it's got no obvious currents associated with it. In particular, all it really does is say that individual atoms within a magnet behave... like tiny bar magnets!

    Also, the lines of the B-field aren't directly descriptive of the charge distribution; the lines don't wrap around the magnet to illustrate that the electrons inside are spinning!
  9. Feb 18, 2010 #8
    Well that was frustrating watching Feynman not explain it, but i guess he was expressing his previous frustration at trying to explain virtual photons in laymans terms.

    Im still not sure i get magnetic repulsion, except that the north and south pole forces are spinning in opposite directions, and when you push them together they are forced to squeeze out virtual photons that are spinning against each other ?

    an attractive girl is waiting here patiently for you to explain to her what virtual particles are

    Last edited by a moderator: Sep 25, 2014
  10. Feb 18, 2010 #9
    So the magnetic field is virtual photon interactions which can occur ?

    So The tiny magnets operate together in an accumulative manner towards the poles though ? Or would it be better to say that there is no actual north / south force, except when the magnet is exposed to other electromagnetic forces ? Then the electrons in the iron atoms are recruited together to push their virtual photon "field" in one direction ?

    So the b field (which is the magnetic field lines) is the virtual photon interactions that can arise which are the result of the constrained electrons zipping spining within the iron itself ?
  11. Feb 18, 2010 #10
    I found these diagrams..


    pretty clear in terms of giving you an idea of magnetic forces in a variety of scenarios,

    I couldnt find any, but if you know of some diagrams that integrate virtual photons this clearly that would be great.
    Last edited by a moderator: Apr 24, 2017
  12. Feb 18, 2010 #11
    In a bar magnet, the "tiny magnets" arising from atomic structure can be thought of as pointing in the same direction; the overall magnetic field is the sum of the magnetic fields associated with each of the atomic magnets.

    In this thread I briefly described the picture associated with quantum field theory, which might help you get some kind of idea of the relationship between fields and particles (here, the B-field and photons). It's not something you can easily picture...
  13. Feb 18, 2010 #12
    Ok thanks. I think i am getting the general gist. i.e The bar magnet magnetic field is not switched on for the individual magnetic domains electrons forces to act in a unified direction, until other electromagnetic forces approach it. Then the action begins as the magnet and approaching force start spinning their respective hard to quantify interactions against each other.

    From here it seems i will need to study, why the right hand relationship between electric and magnetic fields exist and hunt the web for easier descriptions of virtual photons.

    Last question for today (i hope).. in a magnet there are lots of little magnetic domains, isolated from each other. Is it correct that there no south and pole in a piece of iron until something electromagnetic comes into the scene to make them start pushing together in one direction ?

    BTW i dont know if should have put this in homework. I am working in cognitive neuroscience..and need to understand how TMS (transcranial magnetic stimulation) works...

    Should i still have put this in homework section ?
    Last edited: Feb 18, 2010
  14. Feb 18, 2010 #13
    Be careful when you're on the web and type 'virtual photon'... you can get some real crap. Do yourself a favour and go for 'virtual photon feynman diagram' or 'virtual photon momentum' to rule out the quacks.
  15. Feb 18, 2010 #14
    OP - do not be too confused by the responses you are getting. the thing to remember is that NOBODY knows how a magnet works, or what a field is.
  16. Feb 18, 2010 #15
    Having googled TMS (and come across this, I'd strongly advise against looking too much into the virtual photon picture; UK students don't tackle it in any detail until their fourth year of study, and as far as I can tell US students don't until their 2nd year of PhD study.

    Unless you really need to consider the effect of the magnetic field on individual molecules (say), I'd stick to the classical picture- I suspect the neurophysiology will keep you busy enough! Unless, of course, you find it that interesting... :wink:

    jnorman, I'm not quite sure of the reason for your scepticism...

    Post script: for a question that is largely conceptual in nature, here is just fine. (It shouldn't have been in the QM forum though!) The homework fora are really for maths/physics homework questions of the form "determine this..." or "show that..." involving some level of mathematical calculation; it's perfectly appropriate discussing conceptual topics in here (so long as you don't just copy and paste forum posts int your homework :tongue:)
  17. Feb 22, 2010 #16
    well muppet, can you explain how a magnet works? or how a magnet imparts force to iron particles? or what a "field line" is? i would love to know...
  18. Feb 23, 2010 #17
    Magnetic fields occur when electrons are added to an iron molecule. In an atom the electrons need to be equal or less than the number of protons. If the atoms electrons exceed the amount of protons, the atom becomes unstable. But if that atom was to be apart of a strong enough molecular structure, it can stabilise by dispersing the uneven electrons over the molecule. This creates poles because the electrons are forced apart due to the unstable atom and forced into separate obits. This occurs because the unstable molecular structure forces a subset of quarks to change the atomic structure. This atomic structure invokes action at a distance witch occurs because the molecule is still unstable and the magnetic field is trying to orbit\disperse into more iron so fields reach out to stabilise the subset of quarks.
  19. Feb 23, 2010 #18
    Post 14 is good to remember, but should not be taken to mean we can't understand a lot about magnetic interactions. And as you have likely figured out there are different mechanisms for producing magnetic forces (fields). Some are static, like a bar magnet and others dynamic..like an electric current and/or motion relative to a static electric field.

    One could spend a lifetime and career studying the REAL answer to your question....but there is one great concept that will help....


    So electron spin, a fundamental characteristic of sub atomic particles, is key!!! What is spin?? Another avenue of possible study...
  20. Feb 23, 2010 #19
    You know, the concept that a particles spin could be thought of (I was a kid at the time) as needing to rotate a classical object (such as a billiard ball) more than 360 degrees to return to the original state. non-integer spins... the whole field sucks you right in. That, Black Holes, and QCD are like candy to budding physics duffers :smile:
  21. Feb 23, 2010 #20
    I agree there are many ways to create magnetism. But i was under the impression irrelevant to the atomic structure and any magnetic induction, that electrons create the magnetic properties.

    Meanwhile, in 1913, physicist Niels Bohr suggested that the electrons were confined into clearly defined, quantized orbits, and could jump between these, but could not freely spiral inward or outward in intermediate states.[27] An electron must absorb or emit specific amounts of energy to transition between these fixed orbits
  22. Feb 23, 2010 #21
    To answer the original question on what is magnetic force (excluding the Lorentz force).

    The stored magnetic energy W in a magnetic field B(x,y,z) is

    W = ½∫B·H·dVvolume = (1/2μμ0) ∫B2·dVvolume

    The force in a direction z is the partial derivative of W in the direction z:

    Fz = ∂W/∂z

    Bob S
  23. Feb 28, 2010 #22
    Atomic structure IS relevant; and keep in mind any electric field also has a magnetic field and vice versa...so a proton also has "magnetic" properties in some frames of reference.
  24. Feb 28, 2010 #23
    ...for example, Ferromagnetic doping!
  25. Feb 28, 2010 #24
    Meanwhile some of the elements that can produce magnetism are Co,Fe,MnBi,Ni,Gd,Dy,EuO,FeOFe2O3. All have one thing in common, the first and second set of electrons in the atomic shells are 2 and 8. That’s strange.

    And in regards to electric fields, The electricity travelling through an insulated Ferromagnetic element, called wire. Is able to produce a magnetic field because of the electric charge. You give me one example of an electric field existing without a conducting source.
  26. Feb 28, 2010 #25
    Quantum Vacuum.
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