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B Macroscopic magnetic monopoles are possible?

  1. Nov 27, 2017 #1
    School physics has always taught us that the big difference between a magnetised object and an electrostatically charged object is that the latter has on it an isolated positive or negative charge whereas the former cannot be an isolated N or S pole, ie. you cannot have a magnetic monopole on a macroscopic level.

    I have been bothered by that way of thinking because of the following thought experiment. An astronaut lands on a small planet whose outer mantle had been blown off so that it is nothing but a smooth cooled down nickel-iron core. He then places a large ordinary bar magnet upright on the surface with its N pole at the top. For all intents and purposes he now has an isolated N pole because the S pole has “vanished” and he can slide his monopole over the surface of the planet until he decides to lift it up.

    How is this different in principle from our having on Earth an isolated negative charge at the end of a dry amber rod? Of course the rod can be moved about in a vacuum chamber and its isolated charge maintained until it touches a conductor whereas that monopole always has to have its conducting bar in contact with that “ferroplanet”, but that difference is just a matter of their respective magnetic or electrostatic domains is it not?
     
    Last edited by a moderator: Nov 28, 2017
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  3. Nov 27, 2017 #2

    Drakkith

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    Not true. The south pole still exists. The field lines curve away from the N pole, enter the surface, and then re-enter at the S pole. There is no monopole here.

    The field lines from an isolated negative charge all run into the charge, whereas for a magnet they curve back around to re-enter it, even in your example.
     
  4. Nov 28, 2017 #3

    Ibix

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    This isn't true.

    You don't need a situation as outlandish as the one you describe. You are simply touching two magnets together - that one of the magnets is the size of a planet is neither here nor there. What happens is just the same as if you touch two magnets together on Earth. The two magnetic fields interact, either reinforcing each other if they point in roughly the same direction or opposing each other if they point in roughly opposite directions.
     
  5. Nov 28, 2017 #4
    Yes OK thanks for your point. I now think that the perfect electrostatic analogy should have been to use a permanently polarized bar shaped electret with its positive end sitting on an earthed conductor.
     
  6. Nov 28, 2017 #5

    Drakkith

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    So? That doesn't change anything about the existence of magnetic monopoles.
     
  7. Nov 28, 2017 #6
    Well I was’nt seriously trying to claim that such an arrangement was a meaningful magnetic monopole but I was seriously trying to see that a magnet had an exact counterpart in the electrostatic domain. From the point that you made I wish I could see what the magnetic field lines would actually look like from a bar magnet standing up on an iron planet.
     
  8. Nov 28, 2017 #7

    Drakkith

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    Ah okay. Yes, electric and magnetic dipoles are essentially identical in terms of what their field lines are doing if I remember correctly.

    They should look pretty similar to the following picture (from this site):

    feaD5X0.png
     
  9. Nov 28, 2017 #8

    jbriggs444

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    Well yeah. You can make a water fountain that sprays water in all directions -- if you don't count that little pipe coming up to where the orifices live.
     
  10. Nov 28, 2017 #9
    Thanks for this but I am not convinced that a steel plate is like the situation I described. Let’s say the whole planet is made of soft highly permeable iron. The whole planet now has become the bar magnet’s S pole . Surely the field lines from where the bar contacts the planet will be pulled well into the planet before bending round and they would not crowd just along the surface because they are not restricted as in a thin plate. Therefore the magnetic field lines will now be enormously spread out.
     
  11. Nov 28, 2017 #10

    Drakkith

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    Perhaps. Someone else will have to answer that, as I admit I don't know.
     
  12. Nov 28, 2017 #11

    ZapperZ

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    I do not understand all this. The argument seems to be based simply on hand-waving explanation.

    Just pick a region in space and show that the divergence of the magnetic field is not zero. Till then, why would all of this matter?

    Zz.
     
  13. Nov 29, 2017 #12
    I think it matters only if it helps us to narrow the conceptual gap between electrostatics and magnetics and I started this thread after musing about it. The big difference between the electrostatic and magnetic domains is obviously that there is apparently no magnetic equivalent of an isolated electrical charge……or is there? I mused. Whenever we generate say a negative electrical charge we must always leave behind the exact opposite positive charge . This positive charge will stay localized on a good insulator or distribute itself into planet Earth otherwise. To get a magnetic equivalent I then mused on what would happen if a bar magnet is placed upright on a planet made exclusively of soft iron. Whereupon in this thread Drakkith said :
    “The field lines from an isolated negative charge all run into the charge, whereas for a magnet they curve back around to re-enter it, even in your example.”

    But there is a contradiction in this statement, the electrostatic field lines must ultimately bend round to the Earth just as they do for the magnetic field lines on an iron planet, so there is an equivalence between the two domains.
     
    Last edited: Nov 29, 2017
  14. Nov 29, 2017 #13

    Nugatory

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    As ZapperZ suggested above, it would be better to say that, unlike the electric field, the divergence of the magnetic field is always zero. The lack of isolated magnetic charges follows from this more precise and generally applicable statement.
    You are allowing yourself to be confused by the similarity between an electric dipole and a magnetic dipole. Yes, if you have a positive and a negative charge near one another, the resulting electric dipole will have field lines that "curve around" in a way that looks similar to the field lines of a magnet. But if you actually calculate the field intensity at various points near the two charges you will find that they aren't the same; the dipole is just a good approximation when the distance between the charges is small. This approximation breaks down as the distance between the charges increases.
     
  15. Nov 29, 2017 #14

    Nugatory

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    There's no gap to narrow - it was closed by James Maxwell 150 years ago. The key is that electrical and magnetic fields are just different special cases of the more general electromagnetic field, the behavior of which is described by Maxwell's equations.
     
  16. Nov 29, 2017 #15
    Yes O.K., but to the vast majority of non-physicists that gap has certainly not been closed as they see the different effects of isolated electrostatic charges and magnets in their daily lives.
     
  17. Nov 29, 2017 #16

    ZapperZ

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    The vast majority of non-physicists also believe in ghosts.

    Zz.
     
  18. Nov 29, 2017 #17

    Nugatory

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    That's not where the gap comes from. The gap comes from having ended one's study with the "school physics" that you mentioned in the first post of this thread; that high school physics brings you up what we understood in 1785 (Coulomb discovered his law) or thereabouts. Thus, the first step in closing the gap is learning what's been discovered since then; only then are you able to look at the gap, see if it has been satisfactorily closed, and to suggest further steps forward if it has not been.
     
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