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My knowledge of superconductors is limited to the fact that they are

  1. Aug 7, 2011 #1
    my knowledge of superconductors is limited to the fact that they are substances whose resistnce bcme zero below a certain temperature and current once established continues to flow indefinitely.

    a) how can substances behave as supercondictors?(i dont know the name of any...superconducting material)...i mean how its resistance beomes zero at vry low temperature? the most probable theory that i figured out is that atomic vibrations nearly cease al low temprature....but then electrons also wont be availble for conduction.....xplain this

    b)this question is related to use of superconductor...but first i want to clear one thing
    when a fan/motor rotates where is the energy lost?what is the sink point? i mean is energy used to ovecome the back emf affect or the rotating coil acts as a resistance?

    c) now if we use a coil of superconducting material then how will it be better than an ordinary one?if we supply energy to it once then wil it last forever? but then wat about conversation of energy?

    d)what is the difference between potential drop and energy drop in an electrical circuit?is there no potential drop and energy drop across a superconductor?

    plz b patient to answer my questions...yhx
     
  2. jcsd
  3. Aug 7, 2011 #2

    phinds

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    Re: superconductors.....?

    I think the point of superconductors is more that they have zero resistance and so you can put HUGE currents through them and you can't do that with conventional conductors. So a superconducting material in a motor would still have to be fed more current if it is generating work via the motor (there is no free lunch).

    I think most metals are superconductors at temperature near 0 degrees Kelvin, but interestingly enough, two of the very best normal conductors, copper and gold, do not become superconductors.
     
  4. Aug 7, 2011 #3
    Re: superconductors.....?

    You should read about cooper pairs. If the metal was hot there would be a lot of vibrations and collisions so it would be hard for the electrons to flow across the wire, if we use the drude model. And you cant have a voltage drop across a superconductor or have a B field inside the superconductor, It will create surface currents to cancel the field . now that i mention it, I wonder if the electrons only flow on the outside in all cases with super conductors .
     
  5. Aug 9, 2011 #4
    Re: superconductors.....?

    F. Sonneman, from CERN, had put a fabulous compendium on the web, related to "quench analysis". If you can find it again, it's probably the best introduction. Telling the address would be helpful to more people. Wikipedia would also help a lot.

    You need to distinguish between type I, with zero resistance up to a very limited induction and current density, and type II, the only usable for intense fields, which keeps its resistance small over the field or current density that would make a type I fully resistive.

    Even in type II, which have losses, current density is limited, and increasing this to produce a strong induction is hard technology. The other direction to a stronger induction is to increase the dimensions at identical current density.

    Disappointing for sure. Number one dream destroyer when inventors come with a use for superconductors.

    Cooper pairs are the widely accepted explanation for low-temperature superconductors. It is known to fail at high-temp ceramics like YBaCuO.

    Even at cold, metals have mobile electrons, as these don't have to be ripped from the atoms. Maybe you refer to a band diagram of semiconductors, but metals have no energy gap, hence their bands are partially filled even at cold, allowing conduction.

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    In type I with zero resistance, current flows only at the surface, because the magnetic field is expelled from the type I - even at DC, even if the field was in the material prior to cooling, hence it's not only dPhi/dt, see Meissner.

    In type II the magnetic field can flow through the material. It self-organizes in small holes related with quantum flux. The movement of these holes creates losses; blocking them by metallurgical means allows more current and lower losses.

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    In an electric motor, the back EMF is linked with speed, the current with torque, and their product with the mechanical output power - a useful power, so the back EMF is not a loss.

    Part of the losses originate from resistive losses in conductors, hence aluminium, copper or superconductors - manufacturers wanted to sell such motors for orientable propeller pods at ships, but my electrostatic motor and electrostatic alternator may be better (saposjoint.net).

    More losses come from varying induction in the iron core, and still other losses (not small ones) from cooling flow, aerodynamic losses,and bearings.
     
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