Superconducters

A work in progress by the renowned author Ian Sebastian.

Hey Ian, I actually started this page a while ago and figured that that would be enough to go ahead and claim it as mine. The work is also mine.- Thanks, Savannah Lee.

Superconductors- superconductors are materials that can conduct electricity (or current) perfectly, meaning that no energy is lost to electric resistance. In order to understand why this is cool and see some examples, it's important to understand what electrical resistance is and why it creates problems. They also exhibit can get rid of all magnetic fields present on the inside of the material itself, called the Meisner effect. For some cool practical applications, stay tuned until the end.

Introduction to Resistance

Electric resistance is a property of all metals and conductors except superconductors. It's also the reason why your devices get hot after long periods of use, and the reason why they wear out. This is due to the resistance of the wire that the current is trying to pass through. Resistance measures, most broadly, how difficult it is for electrons to pass through the wire. It's kind of like friction from mechanics in the sense that it saps out energy from what would otherwise be a perfect system.

It's easy to think about electric resistance in the same way as you would water flowing through a pipe, and the obstacles it might meet. We know from earlier that a larger wire will produce a larger flow of current in the same way that a larger pipe would allow water to pass through more quickly than water through a smaller pipe.

However, it's not as easy to get rid of electrical resistance as it sounds. In a conductor and current situation, electrons flow between metal ions to their endpoint. How well they "hold on" to these electrons is a measure of their resistance. Depending on the material that this is made up off, the metal ions will hold onto the electrons differently and cause them to flow through at a different rate. Electrons get distracted on the way to the end of the wire and lose energy as a result. But, in a superconductor, this is eliminated and the electrons are able to march from start to finish without losing any energy. Also, this all takes place at temperatures around absolute zero, which is super hard to maintain in a lab.

How Superconductors Work and some Basic Properties

When you lower the temperature of a metal, its resistance will decrease. You could demonstrate this by taking a basic circuit and freezing it- because of ohm's law, the bulb would start to glow brighter since there is a lot more current flowing through until it heated up and the effect was nullified. (V=IR, lowering R will raise I which will raise brightness). For most materials, taking them to absolute zero (or really close) will cause the resistance to decrease to almost zero, or at least improve from where it was, but not quite zero. However, some materials, superconductors, lose all resistance to current. The difference between "almost zero" and "actually zero" is enough to give rise to some cool properties of superconductors. What is happening on the molecular level is that the atoms are not vibrating quickly enough to be attracted to electrons any more, and attractions are minimized to the point where the electrons can flow through with no resistance.

It's like running through a tunnel where everyone wants to high five you. You're the electron, and the people trying to high five you are atoms in the wire. If it's too cold, nobody will want to high five you and you will run faster.

Superconductors will only be superconductors if within a certain range beneath their critical temperature and critical magnetic field. If it is warmer than these values, or the magnetic field is stronger, they might be really good conductors, but not super conductors.

Critical Temperature- often labeled as Tc, or critical temperature. This is the temperature at which the superconductor needs to be beneath in order for it to exhibit these behaviors. Some superconductors are more useful than others because they have higher critical temperatures. To see a table of these, click here. Obviously, some have more practical applications than others because of these temperatures.

Critical Magnetic Field- often labeled as Hc. A superconductor also won't exhibit any of its properties if a magnetic field is greater than a certain value, called the critical magnetic field, even at absolute zero. Superconductors that have higher critical temperatures usually have higher critical magnetic fields, but the correlation isn't exact.

Meisner effect- unique to superconductors, this is the ability to cancel out all external magnetic fields on the inside of the superconductor below the critical magnetic field. Below the critical magnetic field, the superconductor can create mini currents on its surface to eliminate these in the same way that a block of metal can automatically generate an electric field to block out an external electric field, making its net electric field zero. Basically, you could never be asked to do a hall effect problem with a superconductor!

There are two types of superconductors, cleverly named Type 1 and Type 2. Their classification is based on how they break down once their critical magnetic field is reached. Under the critical magnetic field and critical temperature, they all behave similarly.

Type 1- The first type of superconductor is one that has been experimented with the longest. When they are raised above their critical magnetic field, they simply stop being superconductors. Most commonly, these are pure metals like Aluminum and Mercury.

Type 2- when at the critical magnetic field, type 2 superconductors will slowly lose their properties, rather than just completely becoming normal conductors like Type 1. When they break down, they form mini currents and somewhat exhibit the Meisner effect, having a mix of properties between conductors and superconductors. They exhibit 2 critical Magnetic fields, Hc1 where they are no longer complete superconductors, and Hc2 when they are no longer partial superconductors.