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Meissner Effect in a strong field?

  1. Jun 22, 2013 #1
    While studying the M. Effect, a lot of sources state: "If a superconductor is applied in a weak field",
    I assume the effect would work even in a strong field?

    Weak or strong I think the Meissner Effect will always be applied as long as the superconductor is below critical temp.

    Or am I wrong?
     
  2. jcsd
  3. Jun 22, 2013 #2
    Another thing, when the Meissner Effect occurs, is the levitation due to a force? Or just an effect when a magnetic field is expelled from an object?

    My understanding is that, a magnetic field is generated by the superconductor that is equally the same as the initial field, some may say it's the "perfect eddy currents", those currents would cancel the applied magnetic field, since it generates the same fields in an opposite direction. Not sure though...
     
  4. Jun 22, 2013 #3

    f95toli

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    If the field is too strong you "quench" the superconductivity and it goes normal, this happens at f field known as the critical field. This field is not very strong for most conventional superconductors (a few mT) although some materials (NbTi etc) have relatively high Bc which is why they are used for superconducting magnets.
     
  5. Jun 22, 2013 #4
    Could you please dumb that down a bit, I really didn't understand what you said :confused:(Rookie here).
    In the future can we create superconductor that can handel fields over 1T? Current now can superconductors handel strong fields? 1T+?

    What does Bc mean?


    I generally assumed that by exposing a superconductor to any field(weak/strong), the Meissner effect will always occur. I'm 100% sure that if I keep the condutor cool, as much as I can, this Quantum phenomena will not break.

    (Please do explain the ideas simply, I'm still new to this idea. I barley finished most of the materials in E&M. thank you!)
     
  6. Jun 22, 2013 #5
    Via Wikipedia

    That means there will be no force between the supercondutor and the source of the magnetic field?
     
  7. Jun 22, 2013 #6

    f95toli

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    Some superconductors can already handle well over 1T, I believe the limit for superconducting magnets is somewhere around 15-17 T.

    Critical magnetic field

    The Meissner effect will always occur as long as the field is smaller than the critical field, if the field is higher than that the superconductor will go normal and the Meissner effect will go away.

    This is one of those areas where the "basic" case is quite simple. However, it gets quite complicated if you want to understand the details. For example, the Meissner effect is quite different for type I and type II superconductors , the latter case is the most common in the real world since type II superconductor have higher critical fields meaning it is what you use in applications (and all high-temperature superconductors are type II). However, type II is also the more complicated case. I believe there is a good Wikipedia article that describes the difference between type I and II.
     
  8. Jun 22, 2013 #7
    I have no idea what so ever about Bc,
    Just the fact that if the applied field is > than the Bc it breaks the super conductivity, What is the critical magnetic field anyway?!

    I understand a superconductors Tc, but not Bc at all!
    But I do know the features of Type I and Type II conductors, I honestly favored typed II.
    + I know type I will not allow a field to penetrate it, but in some cases type II will example: Flux pinning?


    However, to sum things up.
    The Meissner effect expels the magnetic field(i.e cancels the applied field) Is there a force?! I assume B = 0 Thus, F = 0 since the force is a function of the field.


    Btw, thank you f95toli for all the help!
     
  9. Jun 23, 2013 #8
    Thanks for the move!
    Who ever is responsible :)
     
  10. Jun 23, 2013 #9
    But what confuses me the most, is how can the applied magnetic field can break the superconductor state?
    The idea of "Bc" still confuses me the most!

    If we can constantly keep the superconductor cooled can a stronger field break this Quantum phenomenon?
     
  11. Jun 23, 2013 #10

    mfb

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    Superconductivity depends on 3 factors: temperature, (external) magnetic field and current. The magnet will quench if the combination of those 3 is too large.
    The critical values can be expressed in 3D-graphs like that (from cern.ch).

    All superconductors have a maximal temperature (but then they need zero current and external magnetic field), a maximal external magnetic field (but then they need zero temperature and current) and a maximal current density (with zero temperature and external field). All 3 cases are not really interesting for real applications, so you have to consider all 3 parameters.
     
  12. Jun 23, 2013 #11
    Sorry but I don't understand you're point...

    Why do you need all 3 cases?

    So far, I understand the most important thing to do is, cool down the material under Tc and achieve the superconducting state, hence the Mesinner effect is achieved(Thats how I simply understood it) why all the complications? Of those three?

    Having zero resistance is the most important interest(imo). And is the most IMPORTANT application that is currently under (R&D).
    Having zero resistance means a superconductor can and will induce eddy currents that creates a magnetic field equally the same as the one applied and cancels it, its amazing(i.e no force of attraction or repulsion). Can you imagine the amazing applications coming from that too?
    I mean, we can make indoor flying rooms lol thats one!
     
    Last edited: Jun 23, 2013
  13. Jun 23, 2013 #12

    marcusl

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    The Meissner effect can be analyzed in terms of internal energy in the superconductor (SC). The superconducting state results from a special pairing of electrons (Cooper pairs) that occurs only in SC's. This pairing results in lower energy than the normal state, which is advantageous, but also results in expulsion of magnetic field from the interior which costs energy. So long as the external field is weak, it is energetically favorable overall to be in the SC state. As the field increases, a point is reached where it costs more energy to expel field than is gained by pair formation, so the SC reverts to a normal state.
     
  14. Jun 23, 2013 #13
    Advantageous indeed!!

    However, the external magnetic field breaks that low state of energy this the electrons will not be paired and starte to move around and resistance is no longer zero?
    I believe if we can constantly keep the supercondutor cool, I think applying stronger fields will not break the effect.

    Is the expulsion due to a force? Or cancelation of the forces generated by both :confused:?!
     
    Last edited: Jun 23, 2013
  15. Jun 23, 2013 #14
    marcusl, mfb, f95toli,

    My understanding so far is good?
    I feel like I might have a misconception of something...
     
  16. Jun 23, 2013 #15

    mfb

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    It is not that simple.

    Right.

    No, that is not right.
    Colder temperatures allow to cancel stronger fields, but there is always a limit on the field strength.

    No.

    Magnetic fields influence electrons with a momentum, how is that surprising?
    I don't understand that question.
     
  17. Jun 23, 2013 #16
    By looking at multiple attempts of the experiment, it really looks simple... In fact,One of the simplest things ever, I dont understand why it "might" be complicated? I fear of missing out on something important.

    Sorry! There will always be a limit you're right forgot to state that.
    There is a limit as for how much we can cool the SC, But I believe by increasing the amount of matter the SC has, it might handel a stronger magnetic field at the same cooling temperature maybe?


    Is there even a force? Because, when the effect occurs "the levitation" some say its due to the superconductor repelling the external magnetic field :confused:, I believe it only expels the field and blocks it from penetrating it. I don't think there are forces involved...

    Its not, I forgot.

    Forgot it about it. I stated something foolish :tongue:
     
    Last edited: Jun 23, 2013
  18. Jun 23, 2013 #17

    marcusl

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    No, motion is not a very useful concept here. SC is a quantum mechanical phenomenon involving electron wavefunctions.
    It is curious that someone who doesn't understand the concepts is holding and expressing firm beliefs. Your belief is completely wrong.

    I don't think so--maybe someone else knows this one. The common viewpoint is from considering energy.
     
  19. Jun 23, 2013 #18
    Interesting.

    I don't understand it fully, It's not harm sharing firm beliefs/theories in order to be corrected, hey its sounds really simple and basic. It's not hard to connect the dots sometimes, thus new and new ideas pop easily because of its beautiful simplicity.

    Fair enough.
    I rather avoid considering energy, tends to confuse me.
     
  20. Jun 24, 2013 #19
    I noticed a lot of people prefer type II (SC), why so?
     
  21. Jun 24, 2013 #20

    Its somewhat confusing... Some sources say that the superconductor expels the applied magnetic field because it generates currents to cancel the applied fields.
    And some say, lenz law is applied. And a magnet or superconductor is repelled... Which one is right?
    If the field is cancelled or expelled there shouldn't be any force what so ever... Why use the term "repled" unless there is a force?
     
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