Hello, JustAnotherCommenter.
Superconductivity is a really fascinating phenomenon and one of the few macroscopic quantum phenomena we are aware of. As such, its interactions with the surrounding environment often fail to meet our intuitions assessment of what makes logical sense. Describing the phenomenon adequately would take many pages and a lot of math but I'll do my best to explain how certain metals are able to achieve zero resistance and how this is different from a zero friction scenario, and some of the effects of this zero resistance interaction.
First for anyone who doesn't know: all materials (that we know of so far) have some kind of resistance to carrying electric current at room temperature. That is when you attach a battery to a wire, you set up a voltage (potential difference) across the wire which imparts an electromotive force (EMF) and causes the electrons in the wire to move from high potential to low potential. This process is not 100% efficient. Like sliding a box along the ground, some of the energy you put into pushing the box is spent overcoming the friction of the floor. So too with electrons on a wire. Some of the EMF is "wasted" by making the wire heat up. This can be a good thing. Materials with high resistance are used in heaters for precisely this quality.
This heating up occurs because of the nature of the atomic structure of the metal and the interaction it has with the moving electrons. At the atomic level each metal is made up of a crystalline lattice of atomic nuclei bound together by inter-atomic electromagnetic forces. It's kind of like having a bunch of tiny marbles bound together by springs so that they form a regular lattice. In addition to these bound atoms, all metals contain a Fermi Sea (or Electron Sea) of itinerant or "free" electrons. These unbound electrons are what make it possible for metals to conduct electricity at all. Some metals conduct electricity very well (copper for instance) others do not (tin or lead). The ability to conduct electricity of a given metal (equivalently its resistance as measured in Ohms) is dependent on how strongly the electrons of the Fermi Sea interact with the bound lattice elements. Those materials with weakly interacting lattice elements and free electrons conduct electricity well, those with strong interactions do not.
This is because as the electron moves through the lattice due to the imparted force from the EMF. If it interacts strongly with the lattice it will give up some of the kinetic energy it has by warping the structure of the lattice. Due to thermal energy, the lattice is always vibrating, that is, the marbles are constantly bouncing about a little on their springs. As the electron passes by it causes changes in this vibrational motion. This causes the wire to heat up (gain additional thermal energy and so vibrate more) when the electrons are moving do to the application of an EMF. Metals where this interaction is strong get very hot and don't conduct electricity very well since much of the energy imparted to the electrons is then transferred into the atomic lattice and dissipated as heat. Metals where this interaction is weaker, however, can conduct electric current much more easily and lose less energy to heat. A very fascinating phenomena happens when we bring the temperature of certain metals below a certain "Critical Temperature."
The resistance of all metals slowly drops off as they get colder. The curve for most metals looks like an inverse hill, getting constantly lower but at lower and lower rates. For some metals though this curve drops suddenly to zero. The image on a graph is rather startling, in my opinion anyway. The resistance of the metal is dropping slowly off at a steady and predictable rate, then suddenly it bottoms out to zero abruptly. It would be like watching a toy plane spiral slowly down from the sky and then abruptly drop to the ground as though all of the air beneath it had been removed.
After this temperature the metal is said to have reached a superconducting ground state and the process is very similar to a phase transition, like water changing from liquid to solid ice. Indeed, just like a phase transition we find a certain "latent heat of fusion" at the critical temperature. That is, each degree we lower the substance takes a certain number of joules per kilogram which is independent of whether we are lowing it from 98 to 97 or 4 to 3. But when we get to a phase transition there is an additional cost. Changing a block of ice to liquid water requires us to not only impart the necessary thermal energy to raise water one degree, we must also impart additional energy to break the lattice network that exists in ice but not in liquid water, this additional energy is called the "latent heat of fusion" (or latent heat of sublimation, or whatever phase transformation we are undergoing.) A similar energy cost is paid when moving between superconducting and non-superconducting states. We find that lowering the temperature of a metal costs slightly more when the next degree will bring the metal into a superconductive state.
When this happens the metal has zero resistance to electric current. This has a number of profound effects. The most obvious is that it does not take a continued EMF to keep the electrons moving in the metal once they've been set in motion. Thus a current traveling along a loop of superconductive metal will do so forever (provided someone remembers to keep filling the cooling system with liquid nitrogen so the metal doesn't heat back up to room temperatures). Currents have been set up in superconductive wires and tested daily for years without any change being detected. In contrast, the current in a typical conductor disappears less than a second after the EMF is switched off. Since there is no resistance to dissipate the electrons kinetic energy there is no reason for them to stop moving. This is quite literally perpetual motion. It does not violate any laws of physics however, rather it affirms them. Mechanics states that "Objects in motion tend to stay in motion unless acted upon by an outside force." Since there is no force dissipating the electrons energy it continues to move.
This is not a recipe for free energy though. The electron has whatever kinetic energy was imparted to it by the EMF, but as soon as we try to get any energy out of the system we must reduce this energy and so break the perpetual motion. Thus we cannot have our cake and eat it too, either the electrons will continue to move about indefinitely, or we'll get some useful work out of them, but not both.
A second interesting property of superconductors is that they eliminate magnetic fields within them. Put another way a superconductor is a perfect diamagnet. That is, it perfectly repels both sides of a magnet. The magnetic field within the conductor is always zero and cannot be changed. This has lead to some useful technologies. Since no magnetic field can exist within the superconductive material no magnet can be allowed to set up such a field. Thus a magnet placed over top of a superconducting block becomes "quantum locked" and levitates in midair. You can see the effect here
http://www.boreme.com/posting.php?id=31115 . This can be used to levitate other objects also, such as high speed trains.
This has some other interesting effects as well. For instance if a metal reaches superconductivity in the presence of a magnetic field, say from a nearby ferromagnet it repels the field when it moves through the phase change into superconductivity. Since a changing field produces a current, a constant current is imparted to the Fermi electrons without the use of an applied EMF. Another really interesting effect that occurs is known as a superconducting tunnel junction. A subclass of this effect is the specific Josephson Junction which has found tremendous use and power in the creation of supercomputers. I won't take the time to go into the specifics of the effect here.
All really fascinating stuff, but on to the explanation you asked for. Something peculiar about superconducting metals (not all of them do it) is that metals which make for good conductors generally don't become superconductors at any temperature, while metals which are poor conductors tend to become superconducting if they can be lowered to the right temperature. This was eventually explained in the BCS theory of superconductivity. Before the BCS theory, explanations of superconductivity were correct, but incomplete. They were phenomenological theories which explained very well what was happening, but skimped on the why.
The phenomenological theory was made up of the dual theories of the "Two Fluid Model" and the "London Model." The two fluid model described how, past the critical temperature, certain electrons in the metal lattice were able to reach a superconducting state. As a briefly mentioned before, electron in any substance do not exist in a continuum of states, but rather in a finite number of quantum states as determined by an phenomenon known as the Pauli Exclusion Principle. This states roughly that no two particles can occupy the same quantum state. That is, they can have the same quantum energy or the same spin or neither, but never both. In a physical substance the available energy states are restricted by the nature of the atoms in the substance. This leads to available "energy bands" which the electrons can occupy with large gaps which cannot be occupied because there are no available quantum states.
In all metals there is a highest band called the valence band which contains the highest energy electrons. The highest energy level in the system is known as the Fermi Energy. The states fill bottom up so that any new electrons added to the system must be above the Fermi Energy. In a superconductor some electrons are able to free themselves and become "superconducting electrons" which are able to in some ways "cheat" the Pauli Exclusion Principle by forming a superposition of their waveforms, in this way they act more like a boson type particle than the fermions they actually are. (The difference between Bosons and Fermions has to do with quantum spin, and while interesting, isn't relevant here). How this is done was not clear until BCS. However, this effect allows the electrons to move in a coordinated fashion, rather than the ad hoc Brownian Motion (also referred to as the drunkards walk) of the non-superconducting electrons, thus by passing the difficulties involved with lattice interaction.
The London Model is a bit harder to explain but basically describes the "magnetic penetration," that is achieved by an outside magnetic field. This model describes the superconducting effect more in terms of the fields created and repulsed than in terms of the moving charges. While correct and containing solid predictive power for the effects of such visible phenomena as quantum locking it does not help explain why the effects occur.
The BCS theory finally explained all of this in terms of individual quantum phenomena. As I explained above, when an electron passes through the atomic lattice the field that it produces warps the lattice itself in some way. This effect can be considered as a wave being generated along the lattice, this wave is referred to as a phonon (not to be confused of course with a photon). When an electron sets up a phonon on the lattice the structure of the lattice is different for the next incoming electron. This change in the lattice thus affects the behavior of the next incoming electron causing it to become "linked" to the first passing electron. It is this quantum superposition of states in the electrons wave function which allow for the behavior described above. Thus it is often said "superconductivity comes in pairs."
The macroscopic effects we observe in superconducting materials are the result of a quantum superposition of states which allows the superconducting electrons in the atomic lattice to coordinate their efforts and thus remove the deleterious effects of their interactions with the lattice. Indeed the interactions with the lattice in setting up the phononic waves between passing electrons are actually what allows for this superpositioning to take place. That is why good conductors (those metal where the electrons interact weakly with the lattice) do not become superconducting. It is necessary for a strong interaction between the electrons and the lattice to exist for superconductivity to arise.
The energy of this coupling is very weak. Most often orders of magnitude bellow the quantum energy of the electrons themselves. I once heard it described as connecting two cars together with paperclips. However it has been shown that in any superconducting material this linkage reduces the overall energy state of the material. This is just like when hydrogen and oxygen combine to form water. This happens spontaneously and causes an exothermic release of heat because the water molecule is in a lower energy state than the free atoms, just like a basketball falls down off a shelf because sitting on the floor is a lower energy state than sitting on a shelf.
Because the coupling is very weak it is being broken and reformed between different electrons all the time. This causes the entire system to set up a coordinating current which, rather than being retarded by the lattice, is actually helped along by it. In this situation the very interaction which caused the dissipation of the current now aids in its superconducting propagation. Thus the special phenomenon of superconductivity is in very like a frictionless surface from a phenomenological perspective, but very unlike one at the atomic and subatomic levels. Rather than being a surface in which adjacent atoms in passing materials do not interact it is actually the interaction of the superconducting electrons with the lattice which give rise to the effect.
As a passing note of interest: Superconductivity can be broken if we apply too high a current or impress too great a magnetic field on the substance. This happens when we fill up all of the quantum spaces available for the superconducting electrons to occupy. When this occurs the field set up cannot be balanced by a further increase in current and so the superconducting effect breaks down. However, the currents and fields required are enormous, much much higher than can be carried on any non-superconducting wire. Also only DC currents can be superconductive. While a superconductor carries AC much better than a traditional wire it cannot do so perfectly as is the case with DC. This is because at the onset of current both non-superconducting and superconducting electrons have a roughly equal chance of being accelerated by the EMF, over time (a very short time), the superconducting electrons take over and carry all of the current. Since in AC the current switches back and forth many times a second whenever the current switches a mix of superconducting and non-superconducting electrons will be carrying the current and thus the resistance due to lattice interaction will not be perfectly zero.
I hope this answers your question.
-- Updated April 19th, 2013, 9:21 pm to add the following --
Spiral Out wrote:Can we all please get back on track now before this thread gets shut down too? This is a good thread like Mazer's. I think we all feel bad about that fiasco already. Back to the topic! Please!
I wholeheartedly agree.
- By Bethel Saint Bright
- By Sy Borg