Al Salam 3alikom w ra7mat Allah w'barakatoh
Low Temperature Physics
(Cryogenics)
A Branch of physics that deals with the cases when temperature approaches absolute zero (−237°C or 0°K).
So, how do we achieve low temperatures?
Low temperatures are achieved by removing energy from a substance. This may be done in various ways. The simplest way to cool a substance is to bring it into contact with another substance that is already at a low temperature. Ordinary ice, dry ice (solid carbon dioxide), and liquid air may be used successively to cool a substance down to about 80K (about -190°C).
Some of these liquid gases have the property of super fluidity.
What is Super Fluidity?
The frictionless flow of a liquid at temperatures near absolute zero.
Another interesting property is Super Conductivity. It was discovered by Onnes & his assistants in 1911. This is 3 years after the liquefaction of helium.
So, What is Superconductivity?
when the temperature reaches a few degrees above absolute zero, the electrical conductivity of some metals becomes very high so the electrical resistance vanishes.
It is interesting to notice that if a permanent magnet is placed over a disk of a superconducting material, then the current in the superconductor generates a magnetic field, which is always repulsive with the external magnet, so the permanent magnet remains hanging in the air. This is called Meissner effect.
The reason for this is that superconductors belong to a class of materials (
diamagnetic materials) in which the magnetic field inside the material is zero. Therefore an external magnet induces current in the superconductor which creates a magnetic field inside the superconductor in an opposite direction, so that the net magnetic field inside the superconductor is zero.
This has been put into application by designing a very high speed train which is magnetically levitated. The speed of this train my reach 225km/h.
The idea is simulated by this small video
Sources:
General Secondary Certificate Physics Book
