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Can you magnetise an iron sheet with an electromagnet?

  1. Nov 9, 2011 #1
    This is kinda related to my other thread, but I thought I'll make another one. So, can you magnetise an iron sheet with an electromagnet? Does the electromagnet have to be touching the iron sheet or can it just be near it? Does the thickness of the sheet metal affect its magnetic field strength? How much of the magnet's strength will be transferred onto the metal? I just need some clarification.
     
  2. jcsd
  3. Nov 9, 2011 #2
    In theory, yes. Iron has a greater permeability than air, and therefore if an electromagnet is brought near the iron, lines of flux will pass through material with a greater permeability. As magnetic lines of flux pass through the iron material, magnetic moments are aligned at a submicroscopic level, and hence the randomly aligned domains are synchronized. If you now cease exposing the iron to a magnetic field, some of the domains may remain aligned and therefore exhibit residual effects. This is how permanent magnets are even able to be made.
     
    Last edited: Nov 9, 2011
  4. Nov 9, 2011 #3

    phinds

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    You could make the iron sheet into an electromagnet itself by wrapping it in wire and passing a current through that wire. Then, as in the case Sandy described, there would be some residual magnetism when the current was off
     
  5. Nov 9, 2011 #4
    yay thats good to know. But another question - does it magnetise the entire sheet metal (lets say 30cmx30cm) or just the area in which the field lines passes through?

    hmm, but would that be efficient/better than magnetising via external electromag? Wrapping the entire sheet with wire = less turns and therefore weaker magnetic field strength? I'm trying to make an electro mag that looks something like this btw.

    (-o-)
    |__ | dont mind the underscores. forum deletes excess spaces :S
    | O |

    -------

    -o- = armature
    O = electromagnet
    ( )
    | | = iron sheet
    | |
     
    Last edited: Nov 9, 2011
  6. Nov 9, 2011 #5
    This can be extremely difficult to explain. The lines of flux that pass through the iron induce magnetic dipole moments at a molecular level, which may in turn induce other dipole moments in neighbor domains. In general, I would say that the net induced magnetism would be where the lines of flux were applied...
     
  7. Nov 10, 2011 #6
    Okay thats good to know. Thanks for the explanation :D

    Gotta hope my electromagnet is hella strong then! If I end up making it... xD

    Oh yeah, which would work better at two volts - less turns or more turns? and which would produce the least resistance? Gotta make sure the wires dont melt as well lol :S
     
    Last edited: Nov 10, 2011
  8. Nov 10, 2011 #7
    yay got most of the materials for my armature today. I also saw a stainless steel sheet at bunnings. Would stainless steel be good enough to use for this or should I look for an iron sheet? :S
     
  9. Nov 10, 2011 #8
    Check if a magnet is attracted to the stainless steel. If I recall correctly, some stainless steel is non-magnetic (and that's how some people tell the difference in their metal stores).
     
  10. Nov 18, 2011 #9
    go for less turns/ thick wire (less resistance therefore high current). The thicker and shorter a wire, the lesser its resistance.
     
  11. Nov 18, 2011 #10

    phinds

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    Thick wire IS a good idea, but fewer turns might not be. More turns => more flux.
     
  12. Nov 18, 2011 #11
    hmmm the stainless steel sheet at bunnings was non-ferrous (I ran a magnet through it), but the Galvabond sheet was (when it is apparently non-ferrous). Bad labelling or bad info? :S

    When I got home, I then tried to magnetise the 60cmx30cm sheet but not even tiny screws would stick on the galv sheet. My teacher said to try sanding off the coating but that didn't work either. The screws wouldn't even stick opposite the magnet when the sheet was in-between. Could it be that the sheet is too big and that I should test it after I cut it down to size? Or can only electromagnets magnetise it? Or is Galvabond just a dud?
     
    Last edited: Nov 18, 2011
  13. Nov 18, 2011 #12
    Oh yeah, as a side question, would a lighter armature with less turns be better than a heavier one with more coils on 2-12V?
     
  14. Nov 18, 2011 #13
    Are you tying to "levitate" the armature? - fo so the way you describe will not work - the Magnetic Flux are trying to go from one Pole of the (electro)magnet to the other.

    If you place a ferrous sheet on top of a magnet - the flux lines spread out in the sheet and then leave the sheet on the same side to return to the other pole. In effect - the way I read your diagram - the magnetic sheet will actually SHIELD the armature from the electromagnet.
     
  15. Nov 19, 2011 #14
    No I am not trying to levitate the armature LOL. It'll be suspended via wooden blocks with a hole drilled through each of them for the axle.

    I also think you misinterpreted the (makeshift) diagram. The underscores are not part of the metal sheet, just something I used to make the lines line up since the forum deletes excess spaces. Therefore, there should be a clear path to the armature from the magnet, but that [STRIKE]isn't[/STRIKE] wasn't my goal. It was to (somewhat remotely) magnetise the metal sheets so that they acted like radial magnets (for a motor), which the motor we have at school does perfectly well. My main suspicion for this, is that the motor at school uses pure iron for the magnetised metal, whilst mine uses Galvabond, which may or may not be ferrous (depending on which source your looking at xD)

    But alas, I have scrapped that idea, since the net flux generated wouldn't be substantial (I think) and replaced it with effectively phinds' idea, but inspired by this:

    Go to 1:12 (the diagram on the right)


    My current design is shown on my other thread:

    https://www.physicsforums.com/showthread.php?p=3623806&posted=1#post3623806
     
    Last edited by a moderator: Sep 25, 2014
  16. Nov 19, 2011 #15
    That's true for the same level of current. But more turns => thinner wire => more resistance (and for AC, more inductance). What you want to measure is the total current cross section. So 1000 turns with 1 amp, 100 turns with 10 amps, and 10 turns with 100 amps, are all the same current cross section.
     
  17. Nov 19, 2011 #16
    Thanks :D What formula are you using?
     
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