Superconductors are materials which can conduct electricity with no resistance and they also behave in a unique way in magnetic fields. By why does cooling certain materials cause this effect?
Superconductivity was first discovered in 1911 by Onnes when he cooled mercury to 4 kelvin. He found that the resistance of the metal suddenly disappeared. over 20 years later, in 1933, Meissner and Ochsenfeld discovered that superconductors also repel magnetic fields, leading to superconductors levitating. Since then, scientists have tried to find materials which superconduct at higher temperatures so their incredible properties can be used in a wider range of applications.
Many good conductors will conduct electricity with very low resistance and most materials will reduce in resistance as temperature drops. The sudden loss of all resistance at very low temperatures is superconductivity. This makes them useful in a range of applications (see below) where high currents are used and energy loss is to be kept at a minimum.
As discovered by Meissner and Ochsenfeld, magnetic fields will not penetrate through superconductors. This phenomenon is called the Meissner effect.
Why does this happen?
The BCS theory, named after Bardeen, Cooper and Schrieffer, is the main theory currently of how superconductors work.
This theory suggests that electrons pair up when the material is cooled, forming Cooper pairs. This prevents the electrons from being scattered by obstacles and allows resistance to be eliminated. However, we don’t fully understand why this only occurs at low temperatures and whether it could be possible in certain materials at room temperature.
Type 1 and 2 Superconductors
Type 1 superconductors are pure metals which will superconduct when below their critical temperature. This type of superconductor can be explained using cooper pairs. They are less useful than type 2 because of the extremely low critical temperatures. Also, they lose their superconductivity quickly when subject to a magnetic field.
The second type of superconductors is harder to explain. These usually have higher critical temperatures and are alloys of superconducting metals. They remain superconductors in stronger magnetic fields, so are much more useful in real-life applications.
- Maglev trains – these incredible trains levitate above the ground, eliminating most of the friction and allowing them to travel extremely fast and use less energy.
- MRI – superconductors are used to produce the strong magnetic fields used in MRI scans. Hydrogen atoms in water and fat molecules then resonate at a certain frequency which is then picked up by detectors.
- CERN – To keep the particles moving close to the speed of light on the right path, very strong magnets have to be used. These electromagnets are 100,000 times stronger than Earth’s magnetic field and use over 11000 amps to produce this current. Superconducting magnets therefore are used otherwise the resistance would be extremely high and energy lost thermally would produce a lot of heat.