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How does induction repell?

  1. Sep 29, 2006 #1
    I was watching a lecture and the professor was explaning how changing or moving magnetic feidls generat electricty in an object. When electricty flows through metal it creates a magnetic field.

    He took a large coild of wire and ran a current through it.He took a peice of metal that looked like a WOk and sat it on top. It generated current in the woklike metal, therefore creating a magnetic feild. But instead of attracting it repelled. What I want to know is why did it repell. As far as I know all nonmagnetic metals are attracted to magnets. IS it cause the current was alternating or does it matter.

    He also went on to explain how the maglev train works. A couple of magnets are put on the train directly above the rail. As the train and magnets move it generates current in the rail. The rail then gains a magnetic fields and the train is repelled. It then levetates.

    This is opposite of how I thought it works. I thought the had magnets lining the track to repell magnets on the train. That and magnets on the bottom of the track that attract a piece of the train that hangs down bellow the track. That's how the Transrapid tgrain in Germany works.
  2. jcsd
  3. Sep 29, 2006 #2

    Andrew Mason

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    Your professor was applying alternating voltage to the coil. The resulting alternating current produces a time dependent (ie fluctuating) magnetic field around the coil which induces an emf or voltage in the wok-like loop. The induced emf causes current to flow in the wok-like loop. The induced current opposes the applied voltage (if it didn't you would be creating ever increasing potential difference that would grow without limit). the result is that the magnetic fields oppose each other and the loop flies off.

  4. Sep 29, 2006 #3
    Do all fields produced by coils repell?
  5. Sep 30, 2006 #4
    normally a static magnetic field of a coil would induce a body to have the same direction of magnetic field.

    but its not the static magnetic field that created the opposite field in the induce body.its the change in the magnetic flux on the induced body(the prof used AC) that creates current within the induced body, current which creates a magnetic field to the opposite direction to the one that created it.(if not, energy would not be conserved, think what would happen to the induced current, if the induced current would create a magnetic field in the same direction with the one that created it)

    though, im asking myself why wouldnt we see the same phenomenun on iron rod(or an iron cylinder?) , induced by ac current in a coil? why the wok-like shape?
    Last edited: Sep 30, 2006
  6. Sep 30, 2006 #5


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    I believe the type of maglev train he was talking about was the "inductrack" style.


    The relevant details are on the second page.
  7. Sep 30, 2006 #6

    Andrew Mason

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    You need a current to produce a magnetic field. While there would be an induced emf in the cylinder or rod, there would be no current. You need a loop to have a current.

  8. Sep 30, 2006 #7
    i see... i thought that the current just make tangental loops around the body.
    do you happen to have a link about inner currents within a body? or atleast a good keyword for google?
  9. Sep 30, 2006 #8


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    permanent magnetism
  10. Sep 30, 2006 #9
    SO do coils always induce a repelling magnetic field in all other metals. DO it wouldn't matter if it was DC. He also put watter into the wok and pressed it down. It began to heat and the water oiled. He said this is the same princible that microwaves mostly use.
  11. Oct 1, 2006 #10
    one more phenomenon with changing current is the electromagnetic wave, in this case microwave, which like visible light, may heat matter(though i heard that microwave, from all home materials, heat only water).

    though there is a connection between those, which i did not learn, yet...
  12. Mar 19, 2007 #11


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    do you plan to have your maglev train just "float" and then you will just push it or will you introduce electromagnets to "push and pull" on the sides of the train?
    if so, it will take a lot of engineering and computer programing to control each individual electromanget accuratly (to my understanding)
    im also interested in building a maglev
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