Hello once again for the final time everyone. Yes, that’s right, it is time for the very last Physics guide
Yes I know, it’s very sad. But let’s make the most of the occasion! This guide will cover the last part of the core course, Superconductors. Some weird concepts, but on the whole, nothing overly difficult, as long as you are persistent. I’ll help you get prepared. This guide will be a little shorter than others, so a nice and easy finish.
As always, remember to register for an account and ask any questions you have below! It takes no time at all, and is an awesome chance to pick the brains of your peers.Right, before we cover superconductors, we have to revise a few little things. Recall that metals have a
crystal lattice structure , and can be described as a net of static nuclei, surrounded by a sea of electrons. It is the movement of these free electrons which allows conduction in metals. This lattice structure was discovered by the Braggs using X-ray crystallography. You don’t need to know much about this, but basically it involves bouncing X rays off of a metal and looking at the interference patterns. X rays, having a similar wavelength to the distance between atoms in a metal, give a good resolution picture of the atomic structure. The most you’ll have to do here is identify or describe the process, you won’t have to explain it.
So, we know what causes induction. But what causes resistance? Well, beyond the energy gaps covered in previous topics, resistance is actually caused by two things. Firstly,
impurities in the lattice. If external elements are present in a metal, or other disruptions are present, they will interrupt the flow of electrons in various ways. Secondly,
lattice vibrations contribute to resistance. These vibrations occur due to the temperature of the metal; heat energy causes vibration in the atomic structure of the metal. These can disrupt the flow of electrons and thus increase resistance.
How does all of this relate to superconductors? The best way to explain this is to explain the
BCS Theory , the theory which explains superconductive behaviour. I’ll note, however, that this theory is outdated and now thought to be at least partially incorrect. However, it is an effective model in most circumstances.
The BCS Theory concerns lattice vibrations. As the temperature of an superconductor decreases, these vibrations decrease in intensity. Eventually, the superconductor reaches a low enough temperature that these vibrations become negligible. This is called the
critical temperature . When this happens, the moving electrons actually are able to attract the positively charged nuclei around them. This creates a
lattice distortion , where the lattice distorts inwards towards the electron. In quantum terms, this is called a
phonon .
Now, these phonons are actually a region of focused positive charge. They attract other electrons, effectively creating electron pairs. These are called
cooper pairs , and due to quantum energy effects you aren’t required to understand, they travel through the lattice completely unaffected. These pairs continually break and reform, but result in a net flow of electrons with zero resistance.
This explains how a superconductor works. Below the critical temperature, superconductors have
zero resistance! Different materials have different critical temperatures, and physicists are continually developing complex compounds and alloys with higher critical temperatures. Typically, the critical temperatures are extremely low (usually below 150 Kelvin), and require liquid nitrogen/helium to reach this temperature.
You are also required to explain the
Meissner Effect . You would have seen this in action in an experiment; basically, a magnet placed above a superconductor below critical temperature will
levitate . But what is happening?
The Meissner Effect states that a superconductor below its critical temperature will exclude all magnetic flux. That is, magnetic fields cannot penetrate it. So, when we place a magnet above a superconductor, its magnetic field is excluded from the superconductor and so the magnet hovers in place. There are other quantum pinning effects at work too, but you don’t need to understand these.
Superconductors obviously prove extremely useful. Of course, zero resistance means that we can achieve 100% efficiency, no power loss occurs! This can be seen in the formula
. At the moment, superconductors are primarily used to create extremely powerful electromagnets, used in Magnetic Resonance Imaging, and
Maglev Trains . Explaining how superconductors are used in Maglev Trains is a common question…
Example: Explain how superconductors have allowed the development of new, more efficient means of transport, such as Maglev Trains.
Maglev Trains, currently used in parts of Asia, operate using extremely powerful electromagnets. Permanent magnetic fields are used to suspend the train above the track, while a set of changing magnetic fields are used to propel the train forward/backward. Superconductors, and their associated properties (namely, zero resistance) have allowed the development of electromagnets powerful enough to suit this purpose. This new technology allows friction to be completely removed, thus improving energy efficiency, reducing urban noise, and reducing commute times for travellers. Note: The Meisner Effect has no role in the operation of a Maglev Train. BOSTES hates this error, don’t make it yourself! However, there are serious limitations to how superconductors can be used, at least with the current technology available. For one, the extremely low critical temperature required for superconductive behaviour to occur. This requires use of dangerous liquid gases, and lots of energy to maintain the low temperature, which at the moment, completely offsets any benefits from using them beyond specific scenarios. Further, superconductors are not ductile/malleable, and thus are not suitable for things like transmission wires or similar. Finally, superconductors cannot carry AC current, only DC current. Thus, to use them, we would have to completely redesign our power infrastructure.
And that's actually about it for superconductors! Be sure you can explain the BCS Theory well. It is a little complex, so feel free to register and ask questions, head into Trials with all your questions answered! Best of luck for all your Trials, including Physics, happy study! 
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