when there are no atoms in the space, then what causes magnetic field there?

Discussion in 'Homework Help' started by PG1995, May 18, 2012.

  1. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Hi

    It is said that air has low magnetic permeability because it has low mass density and the atoms are mobile. Similarly, when we keep on increasing DC current in a coil wound around a ferromagnetic core, then there will come a time when the increase in current won't result in any change in flux - the core is saturated with flux at this point and almost all the atoms in the core are aligned. From these points we conclude that magnetic field has to do something with the atoms in a material. Correct? Then, what causes magnetic field in space? The space (or, vacuum) has magnetic permeability which means it should be capable of generating whatever little amount of flux. Am I interpreting it incorrectly? Please help me with it. Thank you.

    Regards
    PG
     
    Last edited: May 18, 2012
  2. steveb

    Senior Member

    Jul 3, 2008
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    I think you are not interpreting it correctly. Free space has an intrinsic permeability, which is just a constant that proves useful to use in electromagnetic field theory. When we say that air has low permeability, we are really saying that the relative permeability is one, and the matter (air molecules) have insignificant effect on the magnetic field. Actually, most matter has relatively small magnetic effects at the macroscopic level. Obviously, we all know of some particular materials that are affected by (and also affect) magnetic fields. These have high relative permeability much greater than one.

    The idea of using a relative permeability to multiply by the free space permeability is just a trick to easily calculate the macroscopic effects that we see with fields. The same is true for permittivity. We let the relative permeability and permittivity tell us the large scale effects we will see when magnetic and electric fields are at work. In other words, these provide a simple way to deal with the effects of matter in practical problems.

    To go deeper, you need to go to a good electromagnetic fields book that develops the theory of relative permitivity and permeability under particular assumptions about matter.

    EDIT: I've attached two pages from Electromagnetic Theory by Julius Adams Stratton. This discusses how matter has field responses to the free space fields. In looking at this you may appreciate why we often disregard this viewpoint and just define an effective relative permeability and relative permittivity. The presence of matter then is handled in a very simple way, despite the fact that fairly complicated things are happening at the atomic level.
     
    Last edited: May 18, 2012
  3. crutschow

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    Mar 14, 2008
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    Curiously it's the values for the electric and magnetic permeability of free space that determines the speed of light. Maxwell calculated that the speed of an electromagnetic wave was 1/\sqrt{\mu\epsilon} where μ is the magnetic permeability of space and ε is the electric permeability. Since that value was very close to the then measured speed of light he was the first to propose that light was an electromagnetic wave.
     
  4. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Thank you, Steve, Carl.

    @Steve: Thanks for the attached pages but I'm sorry I wasn't able to understand them. Perhaps, some day I will.

    What is there is there in free space which makes it to have intrinsic permeability? Free space is simply absence of matter. If you think I won't be able to understand the answer at this level, then just let me know. Thank you.

    Regards
    PG
     
  5. steveb

    Senior Member

    Jul 3, 2008
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    Trying to understand the deep nature of electromagnetics may not be possible for any of us. It is one of those mysterious things in nature. Physics is capable of describing things very well (once we figure the theory out), but it never explains anything very well. We only describe complex things in terms of simpler ideas, but that is a poor substitute for understanding why something must be the way it is.

    The permeability however, can be thought of as a constant needed to match units. In some unit systems, permeability of free space is equal to one, and you never see it. The speed of light has more physical meaning, and one can use permittivity and the speed of light in place of permeability and permittivity. One can even use a system of units in which the speed of light is equal to one, as physicists often do. When you delve deep into this you find that constants of nature are (almost always) just there to makes units of measure match up correctly.

    The real link here is that both electric and magnetic fields are components of a rank 2 tensor, which is governed by Lorentz invariance. Studying along this path is the right way to get a deeper understanding. You'll have to decide for yourself if you are ready for this. I'm guessing you need to first learn standard electromagnetics as taught to electrical engineers. This gives a good physical understanding. After that you can learn the subject from a physicist's point of view.
     
    Last edited: May 18, 2012
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  6. steveb

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    You will understand it when you study electromagnetic theory in a formal way. All good text books will derive out the effect of electric fields and magnetic fields on matter. Note that km and ke are the relative permeability and permittivity. The P and M vectors are the resulting fields from a free space field interacting with matter. For simplicity, these field responses are folded back up into the theory and hidden. We are then presented with a modified form of Maxwell's Equations for electromagnetism which is valid for both free space and for matter. It is basically a nice trick to make a practical theory for engineers to work with. The new form of the equations are valid for calculating the net macroscopic field interactions, and the equations look very similar to the free space equations, but the permeability and the permittivity are modified to account for the effects of the matter.

    The modifications on permeability and permittivity can be quite extensive and often go beyond a simple scale factor. These quantities can become matrices (higher rank tensors) or can have complex values (for sinusoidal responses, basically along the lines of phasor theory you already know). These quantities can even be dependent on the fields themselves in nonlinear systems.
     
    Last edited: May 18, 2012
  7. crutschow

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    Free space has no matter but based on quantum theory there's a lot of stuff in "empty space" that we are not able to detect, some sort of quantum "soup". And then, of course there's stuff like dark energy and dark matter that also seems to be out there that we can only detect by gravitational and other secondary effects. So basically I think our knowledge of the universe is rather superficial at this point. We know how to quantify and calculate what happens, but the why or how is not known. For example we know what many of the universe's physical constants are to a high degree of accuracy, but we mostly don't know why they are those particular values. ;)
     
  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Maybe the quantum vacuum is like a "event horizon" shielding us from seeing the singularity of Infinite Transmission Speed of information. This "shield" protects the reality of this universe by forcing order on matter in spacetime by driving it only forward and speed of light is what's needed to to keep energy movement from affecting the past with the speed of light being just a random number defined when symmetry breaks during the big bang when mass and spacetime separate.

    We can see what seem like ITS effects (that do not transmit information only quantum states whose meaning must be transmitted at or below light speed) in Quantum teleportation.
     
  9. t_n_k

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    That should really help PG1995.
     
  10. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Thank you very much, everyone.

    The above part makes much sense to me. Free space does have intrinsic permeability but the answer is not known why it exists in the first place. When one mystery is solved, it gives rise to many new unsolved mysterious mysteries. The Newton's quote also holds true in general case. Apparently we seem to know too much, in reality we have stepped into a little deep water of never ending ocean.

    “To myself I am only a child playing on the beach, while vast oceans of truth lie undiscovered before me.” -- Isaac Newton

    Regards
    PG
     
  11. panic mode

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    Oct 10, 2011
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    this is one that almost got away even though it is incorrect...

    yes at some point core will saturate and hence contribution to flux/magnetic field due effects of ferromagnetic core will stop increasing. but this is not the only component that affects field and even though core may be saturated, increase in current still continues to increase field strength.
     
  12. PG1995

    Thread Starter Active Member

    Apr 15, 2011
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    Thanks.

    But once the core is saturated, an increase in current would cause the same change in flux as it would in free space. Isn't it correct? Please let me know. Thank you.

    Regards
    PG
     
  13. panic mode

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    Oct 10, 2011
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    that is correct, once the core is saturated, increasing current still increases field, although the increase is due to current alone.
     
  14. #12

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    Consider air core inductors commonly used in speaker crossovers. You only have to understand that air (or space) still exists after the high permeability core in some inductors is saturated. The idea that the core is "useless" after it reaches saturation only means it is useless for that particular application. It doesn't mean that a saturated piece of iron stops the magnetic field from expanding.

    For my own personal use, I simply declare that matter is not necessary for a magnetic field to exist. Whatever matter happens to exist in a magnetic field only modifies its shape and/or concentration.
     
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