Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Violation in superconductivity

  1. Oct 27, 2008 #1
    How electrons flow forever in a superconducting wire?
    From where electrons getting kinetic energy?
    Is this perpetual motion? if so, can we extract work and build a perpetual motion machine?
     
  2. jcsd
  3. Oct 27, 2008 #2
    The electrons flow forever because there is zero resistance, the kinetic energy is effectively the potential you apply to the superconductor. From then on there is no resistance so there is no loss.

    Think of it like water in a circular pipe (cliche) if you add water in to a pipe that had no resistance. The energy for the movement is the water you added in and since there is nothing disrupting this flow all the energy is conserved so the movement continues forever.
     
  4. Oct 28, 2008 #3
    why do you think that they travel forever?
     
  5. Oct 28, 2008 #4
    In a superconductor there is no losses of electric energy, so once you provide into a coil a energy it is trapped and accumulated in current. Of course there are losses, due to several factors, so the current won't flow forever. But in a perfect superconductor the current will stay forever.

    About the question of extracting work of this, I would say that it is not possible. For example an electric engine based in superconducting coils will be more efficient than a copper based one, due to the absent of resistive losses. But there are other losses besides the resistive, for examples losses in the ferromagnetic. Also if you extract some energy from the coil (by rotating another coil) it will loss energy.
    Hope this help.
     
  6. Oct 28, 2008 #5
    because of zero resistance..
     
  7. Oct 28, 2008 #6
    I got it..Thanks blindnz & cubeleg..
     
  8. Nov 11, 2008 #7
    If superconductors have any resistance, it is damned low. Measurements have been made to a huge accuracy (with superconducting hyperfrequency cavities) and no resistance has been detected. Coils are used routinely and nobody observes a resistance. So to any experimentally meaningful accuracy, zero resistance.
     
  9. Nov 28, 2008 #8
    I dont know of any proof that current flows forever or even for a short time after the emf is removed. the persistence of the magnetic field around the coil is probably due to the meissner effect.
     
  10. Nov 28, 2008 #9

    marcusl

    User Avatar
    Science Advisor
    Gold Member

    The current in superconducting magnets (such as are used for MRI machines) is routinely maintained for years after the initial drive is removed.
     
  11. Nov 28, 2008 #10
    how do they know that the current is still flowing?
     
  12. Nov 29, 2008 #11

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    By measuring the induced magnetic field!

    Zz.
     
  13. Nov 29, 2008 #12
    the magnetic field wouldnt be expected to collapse (even if the current dies) due to the meissner effect.
     
  14. Nov 29, 2008 #13

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    What Meissner effect? No one is subjecting the superconductor to an external field. So no Meissner effect.

    The "induced" magnetic field is due to the persistent current in the superconductor. Haven't you use a clip-on ammeter before? Same concept.

    Zz.
     
  15. Nov 30, 2008 #14

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2015 Award

    Like Zapperz said, there is no external magnetic field, so there is no Meissner effect.

    There are at least two other problems with this. One is that to generate a magnetic field there has to be a current somewhere. The only two possibilities are in individual atoms or in the bulk flow of electrons - i.e. what we call a current. If you decide it's not in the current, it must be in the atoms - but the problem with that is that the exact same atoms are in the exact same configuration when the superconductor is slightly above Tc as slightly below.

    The other is that the Meissner effect essentially states that superconductors are perfect diamagnets. This idea of a magnetic field unsupported by currents is not a property of diamagnets. So it doesn't have these properties to begin with. (And, like ZapperZ said, the conditions aren't right for it to apply anyway)
     
  16. Nov 30, 2008 #15
    the magnetic field is obviousely created by a current. but the current dies and the miessner effect prevents it from collapsing. the field is sustained by the spin of the electrons.

    unless you believe in perpetual motion.
     
  17. Nov 30, 2008 #16

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    This is incorrect.

    The persistent current does continue because there is nothing to dissipate the supercurrent, by definition. It is not "perpetual motion" because as soon as you try to use it to do work (i.e. power a motor), you are using it up and it will start to dissipate.

    Again, there's no Meissner effect. These is the supercurrent. The magnetic field generate by the supercurrent is external to the superconductor, not inside the superconductor.

    Zz.
     
  18. Nov 30, 2008 #17
    yes. so? what does that have to do with anything?
     
  19. Nov 30, 2008 #18

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2015 Award

    This is no more perpetual motion than "a body in motion remains in motion" is. As ZapperZ points out, if you try and extract energy from the system, the current dissipates.

    By having the electron spins generate the field in your theory, you are essentially saying a superconductor is a ferromagnet. One problem with this is that magnetic order and superconductivity are at odds with each other: for years it was thought they were mutually exclusive, and even now our exceptions are rare and exotic: UGe2 under pressure, for example. (It is also worth noting it is neither a good ferromagnet nor a good superconductor)

    The other problem is that superconductors are perfect diamagnets - that's what your Meissner effect is saying. Ferromagnets have their induced magnetism pointing in the other direction.
     
  20. Nov 30, 2008 #19
    suppose I put a wire inside a tube of regular conductor and then bend the whole thing into a ring. then I create a regular current in the outer conductor and after it is established I lower the temp of the inner wire to the point that it bocomes supercondicting. then I allow the current in the outer wire to stop. what happens to the magnetic field?
     
  21. Nov 30, 2008 #20

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2015 Award

    You just said it was generated by the electron spins inside the superconductor.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Violation in superconductivity
Loading...