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Homework Help: How can a field exert a force on soemthing magentic

  1. Nov 21, 2006 #1
    Hi there. My class has recently started on magnetism and I am facing a problem already. I understand that you get a magnetic field when a charge is moving. However, what I do not understand is why that actually happens. And how is it that this field can actually exert a force on something magnetic. Is it another of Physics unsolved mysteries, like why like charges repel? However, it is easier to understand how electric fields work since there is a substantial source... charge or even for gravity... mass. But I don't see anything quite as clear as magnetism.
  2. jcsd
  3. Nov 21, 2006 #2
    The answers you seek unfortunately lie in particle physics, quantum electro dynamics which themselves is buried in a lot of complicated maths.

    However, in a nutshell: classical physics tells us there is a 'field' that carries the magnetic and electric forces from one body to another. The source of this field is as you said either a moving charge for magnetic or simply a charge for electric. These fundamental properties are like mass, that are inherent properties of the object, ie proton whatever. this can be further broken down into other subatomic particles, namely quarks. each quark has a charge of either plus or minus a third so you combine 3 quarks in different ways to get a 1 or a 0 for proton or neutron. Beyond that, i think its fair to say, and i might be wrong here just because i dont know a whole lot myself, that fundamental charge is something that lies beyond the scope of physics, and possibly human understanding for the moment, as your really delving into philosphy and metaphysics as well, all those 'big' questions.

    Particle physics however tells us that fields dont exist, and that all forces are carried by particles. for EM its the photon. when two electrons cross paths and 'interact' with each other, photons are exhcanged in such a way that they deflect. these photons are emited and reabsorbed all the time by electrons along their path, but these are virtual photons. once an itneraction occurs they become real, according to my understanding anyway.

    hope this helps
  4. Nov 21, 2006 #3


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    Current acts as a source for magnetism.
  5. Nov 21, 2006 #4
    yarr thats right! one i missed. thnx hurk
  6. Nov 21, 2006 #5
    Yes, I have been thinking about that. Even though that actually lies way beyond my scope of learning I have been reading up on it a lot so I am somewhat familiar with quarks. If I remember correctly, when 2 charges are close together they will exchange photons and if they actually move by each other the vitual photons will become real. However, I cannot figure out how magnetism and electric force are so different when they are one and the same? How can that same photon be magnetic in one part and electric in the other? After all, its just a packet of energy...

    Thanks. Your explanation was good but it still does not help me differentiate magnetism and electric force. Perhaps the question is why a magnetic field is created in the first place when there is no other object to interact with... or is there. Maybe I am getting confused here...

    Anyway, do we really need to delve into particle physics to understand this. I don't think the existance of quarks and gluons, their own force carriers, should affect the problem since what matters is magnetism is created by a moving charge, and yet not a static one. It surely makes no difference whether we consider three quarks or the proton as a whole?

    Anyway, that certainly does not explain that electrons exert magnetic fields since they are leptons.

    P.S. thinking of quarks, does that mean that neutrons can have magnetic fields as well?
  7. Nov 23, 2006 #6
    Yes. Current is basically moving charge, be it electrons or otherwise. However I still don't get how that can possibly happen.
  8. Nov 23, 2006 #7
  9. Nov 23, 2006 #8


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    Is it really any more mystifying than how charge can act as a source for the electric field?
  10. Nov 23, 2006 #9
    Think of it like heat flow, hot air conducts into cold areas. Electrons flow across a potential difference.
  11. Nov 25, 2006 #10
    Thx for the replies.

    Ahahaha...:tongue: Based on the answers I have received it does seem that why a magnetic field is created is a mystery to physics. I said to my teacher I don't understand why magnetism works as such, and he says I am in good company. I secretly hoping otherwise.

    Anyway, is it possible for someone to explain to me or at least give me a link with simple explanations on explaining magnetism through the Quantum Theory, such as the exchange of virtual photons like that between 2 charges.
  12. Nov 27, 2006 #11
    I have recently found out that a moving charge is only indirectly involved in the creation of a magnetic field. Apparently, what Faraday and Maxwell realised is that a CHANGE in the electric field is what causes the magnetic field to be created and not the actual moving charge. You can just change the electric field, such as removing and adding charge, and still create a magnetic field. In fact this should be how we produce artificial electromagnetic waves such as in the radio.
  13. Nov 28, 2006 #12


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    That's not true. It is correct that changing electric fields create magnetic fields. But this is not how all magnetic fields are made. Maxwell's equations contain two source terms for the magnetic field: current and change in electric field.

    Incidentally, the same is true for the electric field: changing magnetic fields create electric fields, and Maxwell's equations include both charge and change in magnetic field as sources for the electric field.

    You can't "remove" and "add" charge: charge must be conserved. You can only move charge around.

    We do quite the opposite: we use a time-varying current to generate radio waves!
  14. Nov 28, 2006 #13
    Correct me if I am wrong, but the way I see it, current is moving charge, and when charge moves it changes the electric field, which relates to what I said earlier.

    By "removing" or "adding" charge that in a way is basically what I meant. And varying potential difference or current would be 1 method. Charge is still conserved, as I did not really suggest destroying or creating charge.
  15. Nov 28, 2006 #14


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    In the continuum limit, we can generate an EM-signal without changing the charge distribution at all. At each point in the circuit, charge flows in just as fast as it flows out, so the charge density remains exactly constant. Yet, the varying current is still capable of generating an EM field.

    Also, don't forget about statics; you can have magnetic fields even when the electric field is not time-varying.

    Your posts make it sound like you think the magnetic field can be explained entirely due to the fact that, when a charge moves from here to there, it is now generating an E-field over there, and not over here. But that's incorrect: the effect on the B-field of the current you get while the charge is moving cannot be explained in that fashion!

    IMO, Maxwell's equations say it all:

    \nabla \times B = \frac{4\pi}{c} J + \frac{1}{c}\frac{\partial E}{\partial t}

    In other words, the current and the change in the electric field each have their own separate contribution to the curl of the magnetic field. The wave equation for the magnetic field is interesting too:

    \nabla^2 B - \frac{1}{c^2}\frac{\partial^2 B}{\partial^2 t} = \frac{4 \pi}{c} \nabla \times J[/tex]

    note that the E-field has no contribution whatsoever to the wave equation. The curl of current density kills off the only contribution of the time derivative makes to the charge density.

    In other words, from the perspective of the wave equation, charge density is completely irrelevant.
    Last edited: Nov 28, 2006
  16. Dec 2, 2006 #15
    Honestly, I only know the basic idea of Maxwell equations and I am unable to really understand the maths or use them... yet. Pity its not covered in my sylabus.

    I am not suggesting that charge density is a vital constituent in creating a magnetic field, just that varying it might be a possible way to create one.

    But really, I am thinking in terms of the simple model of a single charge carrier now. A single electron would be best. (since protons are made of 3 quarks) For this single unit of elementary charge, if it actually mives accross space, it creates a change in electric field, and creates a magnetic field.

    Moreover, a charge is not necessary in the first place to create a magnetic field really. (Well, it is of course necessary to create the first electric field) Think of an EM wave: a varying electric field creates a varying magnetic field, which creats a varying electric field and so on...
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