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Magnetism: conditions for magnetic attraction and magnetic shielding

  1. Jan 26, 2012 #1
    Hi, I have two questions that come from a unique example of magnetism. Here is the scenario: a thin and long piece of iron is placed in between a magnet and the north pole faces the outing sheet and some iron tacks. The iron tack do not get attracted to the iron even though a south pole is induced.

    1) when this happens two south poles are in the middle of the thin iron sheet however, the iron pins do not get attracted towards it. While if the iron sheet is free to move, it would be attracted towards the magnet. From this I came with a conclusions: for attraction to occur, there must be both a magnetic field and also a pole to be present. If a pole is present but no magnetic field is emitted by it, then even magnetic materials won't be attracted to it and vice versa.
    Is this conclusion correct?

    2) if the thin iron sheet is extended to a great length, will the magnetic field lines exit in the middle of the iron sheet to go back to the south pole or will it go through the whole iron sheet? I think that it shouldn't go through the whole thing as the distance all the way back when it leaves the ends will cause it to weaken and disappear before reaching the south pole. However, it seems weird that it would leave in the middle of the sheet. So I'm not very sure about this.

    3) if there is a piece of iron bar in between two magnets, and the two side of the magnet which faces the iron is north then how will the iron bar be induced? Will the one with a stronger magnetic field cause the induction but as a result the temporary magnet will be weaker due to the presence of the other magnet? And if they are of equal strength then it won't be magnetised at all? Due to the magnetic fields cancelling each other out?

    Thanks for all the help guys!
  2. jcsd
  3. Jan 26, 2012 #2


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    Your description is not clear. When you say thin long piece of iron is that the sheet?
    When you say placed in between a magnet, between the magnet and what? Do you mean between N and S poles of a horseshoe magnet?
  4. Jan 26, 2012 #3
    Oh sorry for being unclear. I meant to say that a long and thin sheet of iron is placed in between a bar magnet and some iron tacks. For the magnet, its north pole is facing the middle of the thin and ling iron sheet. And for 3) it is a completely different case from 1) and 2).

    Thanks and hope to hear your answer
  5. Jan 26, 2012 #4


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    I'm still not fully clear on your questions but let me see if I can explain the basics.

    The iron sheet is not the magnetic analog of a conductor blocking electric fields. It is more analogous to a sheet of dielectric material.

    Some of the magnetic field from the bar magnetic will penetrate the sheet and the tacks are attracted but the attraction is so reduced that it isn't enough to move them against normal friction forces. Do the experiment in freefall and you'll see the tacks slowly drift toward the plate.

    Visualize the bar magnet with its magnetic field lines looping through space from one pole to the other. The iron sheet will strongly refract the lines, so strongly that most will not pass through the other side but will loop back out of the sheet on the side with the magnet.
  6. Jan 26, 2012 #5
    Oh, then if the iron sheet is extended to let's say 5m will the magnetic field lines go all the way through the iron sheet or will it come out at mid point? Cos my understanding is that if it comes out at the two ends, then the magnetic field will be too weak to reach the south pole. However if it leaves at mid point it will still have enough strength to go back to the south pole. Thanks for the time and help!
  7. Jan 26, 2012 #6


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    There's no such thing as "too weak to reach the south pole".

    If you're visualizing magnetic flux lines then the strength of the field is the density of the lines. The lines never have any endpoints (unless you find some magnetic monopoles).
    You can rather visualize them as loops passing through the bar magnet.

    If you extend the iron sheet to infinity then...
    • some lines will loop around to the S pole before reaching the iron sheet.
    • some lines will pass through the sheet to the other side, then pass back through the sheet to get back to the S pole.
    • some lines will pass into the sheet but never cross out the other side. They will re-emerge out the same side moving on to the S pole.

    Now imagine that magnetic field lines running in parallel will repel each other... this is why they spread out. (The physics of it is that the energy density of the B field will be the square of the field line density so spreading them reduces total energy.)

    The effect of a ferromagnetic material is to reduce this repulsion of field lines so the material will want to move toward denser concentrations of field lines and the field lines will want to bend toward the ferromagnetic material.

    In your sheet example this means field lines will spread out from the N pole, some will curve a bit into the sheet and then once inside will tend to bend along the tangent direction clustering inside. Some are pushed on through and the rest turn back out.

    [Edit addendum]
    Also since not all lines pass through the field flux the strength is reduced. But also since those that do pass on through are greatly spread out the field strength is dramatically reduced.
  8. Jan 26, 2012 #7
    Oh but I don't understand why the magnetic field lines will be able to exist even in when they have to travel a distance of infinity. However, will majority of the magnetic field of the leave at mid point? Because if I take the example of a bar magnet being stuck on a iron wall, then if majority of the field lines leave from the end of the wall then there won't be a lot of magnetic domains with a north or south pole as they would cancel themselves out. if this was so then the piece of magnet won't be attracted to it at all.

    what I am referring to is something like this:

    [N SS N]
    So the N in the top is the magnet then the thing below is the sheet of iron/iron wall. So the SS will get attracted to N as it fulfills the condition I assumed.

    so if I put a magnet horizontally will it look something like this
    [N S ]
    [ S. N. ]
    so if majority will exit at mid point then the image would loke somewhat like this. So then they will attract each other and remain attached to each other.

    Sorry for the multitude of question, my school teacher doesn't really answer my questions so I resort to the forums. thanks again for the help.

    Edit: sorry the image I drew can't fit in but I think you'll get the idea. The first image N should be above the SS and for the other the N and S should be above the S and N respectively. Thanks
    Last edited: Jan 26, 2012
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