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North pole and south pole of a magnet.

  1. Sep 28, 2009 #1
    Hi, I am wondering what determines where these are on a magnetic object. For instance, on a sphere, why should there be a north pole and south pole at points rather than others. To make myself clearer, I am not asking why the south and north are opposite each other, but what determines those points on the sphere rather than any other way of putting it. A easier way to see it might be why with a bar magnet, normally you have a pole on the top and a pole on the bottom, but why not a pole on the left and a pole on the right? It seems like magic to me at the moment, because I do not know of any real reason in which geometry would decide where the poles should be orientated.
     
  2. jcsd
  3. Sep 28, 2009 #2

    mathman

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    The poles of a magnet are determined by the alignment of the atoms in the material, usually iron. Initially the atoms are oriented at random, but they can be lined up by an external mangetic field and under certain circumstances will retain the alignment after the external field is removed.
     
  4. Sep 28, 2009 #3
    So if all the electrons are aligned in the right way, you could make the poles turn up in any place?
     
  5. Sep 28, 2009 #4
    There are actually two different questions there:

    1) earth's magnetic field - this one i'm not sure of, but I think it has to do with earths rotation and the flow of magma and the relation to the core. Maybe a geophysicist can jump in.

    2) with respect to a bar magnet (or any other common magnet) - as mathman stated, they can be magnetized in any direction based on an external magnetic field. In their natural state, magnets from the ground or whatever, may have random magnetization or aligned with earths magnetic field. Some magnets (as in motors or generators) can actually be axially, or radially or tangentially magnetized (not in a straight line). The magnetization process now usually involves a strong permanent magnet that pushes the magnetic field past the point of saturation in the 'hard' magnetic material. To get non straight line magnetization requires fixtures that make the flux lines flow in certain orientations.
     
  6. Sep 29, 2009 #5
    I can understand that the earth's magnetic field, might be polarised in that direction because of the current inside the earth.

    But I am not too happy about the idea of getting permanent magnet, and explaining the direction of the poles with the way the electrons are aliened. I do however accept that as a reason for why it is magnetic. Telling me it is because of how the electrons are aliened says nothing in explaining it in detail because it does not say how they are aliened affects the dipole.

    I do like the sound of it being because of the way the earth's magnetic field is, and the rock has set in the same way. Is that basically what a machine would do?

    Thanks for the replies though.
     
  7. Sep 29, 2009 #6
    GrizzlyBat, take a look at this paper. It is a pretty good description in detail starting with section 4. http://www.oersted.com/magnetizing.PDF

    Basically each atom or group of atoms (domain) has a magnetic moment (dipole) because of the electon orbits. When you place a magnetic material (hard or soft) into an external magnetic field, the dipole moments of the atoms and domains will mostly (but not all) align with the magnetic field to be in the lowest energy state. In Hard magnetic materials, once they have been forced into a particular alignment, they will stay that way until forced into a different alignment by a significantly larger force. Once they have been aligned in this matter, they are basically in the lowest energy state. In Soft magnetic materials, they will align, but then after the external force is removed, they will usually fall back to a lower energy state which usually means that the domains will not be aligned.

    Let's say you have a solenoid such that the magnetic field H is in the Z direction. If you put an unmagnetized (or weakly) magnetized permanent magnet in the field, no matter what orientation the permanent magnet is set, will be magnetized in the Z direction.

    The reason why some materials will stay magentized, while some will not and some will basically not even react with magnetic fields is I believe a property of the electron configruations of each type of atom.
     
  8. Sep 30, 2009 #7
    I think it makes sense now. It sounds similar to how polarization works with electricity. I am happy with the answer.

    This made me wonder if it is possible to have a magnet that is a hollow sphere? Or a closed object that is hollow? I do not think I have seen one.
     
  9. Sep 30, 2009 #8
    Actually, you could probably make a sphere that is radially magnetized, but it would have to be made out of at least 2 parts 'glued' together. The magnetizing fixture would have to push the flux so that it is normal to the surface of the sphere at the inside surface of the sphere. It would probably require permanent fixturing to keep the 2 halves together mechanically though.

    You could make a sphere that is radially magnetized with alternating bands, as in a band of 15 degrees N, 15 degrees S....., again, each magnetized separately and then 'glued' together.

    Of course a sphere or other hollow closed object could be magnetized in a single cartesian direction and that would be trivial.
     
  10. Oct 30, 2011 #9
    what does really north and south poles in a magnet means?
    Are they positive and negative charges of all individual atoms in a magnetic material,,,,,,i,e.. a molecule.....?
    how do we practically define them?
     
  11. Oct 30, 2011 #10
    Have a look at the summary and link in my second post from Sept 09, 2009.

    The north and south poles of a magnet are basically just the ends of the magnet which happen to align with the external magnetic field.
     
  12. Oct 30, 2011 #11
    Your link has a good explanation of the atomic and domain level of permanent magnets but the questioner might benefit from a description of more macroscopic phenomena:

    Understanding why magnets exhibit "poles" requires understanding the magnetic field around a conducting wire with current going through it, which can be envisaged as concentric circles, or cylinders. The needle of a compass placed at any point on one of these 'circles' will take up a North-South orientation at right angles to the direction of current flow. That will continue right around the conductor. On one side N-S will point one direction perpendicular to the wire, on the other side of the wire N-S will be 180 degrees in the other direction perpendicular to the wire. Here's an illustration of this:

    http://whs.wsd.wednet.edu/Faculty/B...udyguides/chapter1920/graphics/Fig-19-15b.gif

    The field around an electromagnet is what results when you form a current carrying conductor into a loop:

    http://28.media.tumblr.com/tumblr_lcjno5UORq1qd7ygho1_400.jpg [Broken]

    The field around an induced magnet, or, a permanent magnet is exactly the same as the field around a current carrying conductor that has been formed into a loop, or many loops.

    A compass placed next to an electromagnet or permanent magnet will take up the same orientation with respect to the 'circles' of magnetic flux as it does in a straight line conductor. When the conductor has been wound into a solenoid configuration (one or more loops) all the "north" orientations lead to one end and all the "south" orientations lead to the other end. The "poles" are a bit illusory: the original 'circuit' of N-S orientation actually continues through the interior of the looped conductors, and you could follow it with your compass if it weren't blocked by the core material. You can follow it through a solenoid with no core that is large enough to accommodate your compass, like the one in my second linked image.

    A permanent magnet, as I said, has exactly the same magnetic field shape as a solenoid electromagnet: it's the field around a looped current carrying conductor. (Interesting is that there is no permanent version of the field around a straight line conductor. Also interesting is that, once you understand how a magnetic field represents a looped current carrying conductor's field, you understand the impossibility of ever having a magnetic monopole.)

    The north and south poles are practically defined simply by calling that pole "north" which points toward the earth's north magnetic pole.
     
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