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Another poser

  1. May 25, 2003 #1
    Was going to put this up a couple of days ago but was a bit preoccupied. So here goes.

    Imagine that you take a single crystal of Iron, a whisker, and bend it around to make a circle. How could you find out if the crystal was magnetized? Remember, all the magnetic field is contained inside the crystal. The usual way to find out if is is magnetized is to wrap a current loop and a sense loop around it and apply a current, if it was magnetized in that direction the sense loop would not generate a voltage, if not magnetized at all or in the opposite direction then a voltage would be induced, the amplitude being different for the two cases.

    Can you describe a "nondestructive" way of telling whether the crystal is magnetized?
     
    Last edited: May 25, 2003
  2. jcsd
  3. May 26, 2003 #2
    The A____-B__ effect will do it...
     
  4. May 26, 2003 #3

    marcus

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    Hall effect? just a guess

    attach leads so as to pass a current across a small segment of the ring

    attach another pair of leads for measuring Hall voltage at right angles to the current

    the sign of the voltage would tell if the magnetic field was up or down in that segment

    cant draw a picture but note that the current is perpendicular to the field and also the voltage is measured at right angles to the field
     
  5. May 26, 2003 #4

    chroot

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    The Bohm-Aharonov effect... perhaps spelled wrongly...

    and no marcus, it's a purely quantum-mechanical effect. Here's two hints: topological spaces that aren't simply connected, and quantum mechanical phases. :)

    - Warren
     
  6. May 26, 2003 #5
    Chroot got it

    There's no magnetic field outside the body of the crystal, but there is a vector potential. If you send an electron beam so that it has some amplitude to go inside and outside the circle there will be a interference effect if the crystal is magnetized. If the crystal isn't magnetized, no interference.
     
  7. May 26, 2003 #6

    marcus

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    Re: Chroot got it

    I know a little about the Aharonov-Bohm effect and it is of course quantummechanical (as chroot says) and it seems to me that your short description, Tyger, is very good---agrees with what I have seen.

    If that is the answer you wanted, fine.

    I proposed another way to detect the magnetic field inside
    a solid block of metal.

    This could, depending on dimensions and materials, be easier to carry out than a Bohm-Aharonov measurement (which I believe is quite hard---the effect is small).

    My way is to forget about the rest of the ring and just look at a small segment, which is like a small bar magnet (but with no field lines outside it).

    With the field oriented in the z direction, clamp contacts onto the sides of the magnet and send a current thru the magnet in the x direction. By the Hall effect a tiny voltage difference will be induced in the y direction, detecting, and indeed measuring, the field.

    Since everyone seems to want Bohm-Aharonov to be the answer this is fine with me! Proceed :smile:
     
    Last edited: May 26, 2003
  8. May 26, 2003 #7
    Marcus, your method

    should work fine too. I'm one of those people who finds a correct answer the test writer never thought of too. We should have more thorough test writers.
     
  9. May 26, 2003 #8

    marcus

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    Re: Marcus, your method

    I'd be satisfied if we merely had more poets like Wm Blake.

    Did you see that chroot recognized a line of Blake and
    gave the next four lines in his reply. If you dont know
    the poem referred to you are a do-do :smile: even tho you
    understand electricity!
     
  10. May 26, 2003 #9

    chroot

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    Re: Re: Chroot got it

    But marcus, it was asked that the measurement be "non-intrusive."

    - Warren
     
  11. May 26, 2003 #10
    The actual word

    was "nondestructive", my meaning was that it not destroy the existing state of magnetization.

    And I still haven't found where you quoated Blake.
     
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