Physics - ThermodynamicsHere's a really interesting and cool one from physics
Negative Absolute Temperature. Sixty Symbols made a really good video describing what is it:
https://www.youtube.com/watch?v=yTeBUpR17RwFor anyone's that is familiar with the Kevin scale will definitely know about the concept of absolute temperature where 0K, which is -273.16 degree Celsius, is the lowest temperature that can be achieved by matter, called absolute zero.
The reason 0K is the lowest temperature is because temperature is an indirect way to determine the thermal energy contained in an object, which is the kinetic energy of the constituent particles (the formula of the thermal energy contained in a substance is
, in case you're wondering) The higher the temperature, the faster the particles are moving about, so they contain higher amounts of kinetic energy, hence higher thermal energy. So there must be a point where the particles have the lowest energy which corresponds to having absolute zero temperature.
However, there do exist certain systems where they have negative absolute temperature. How is that possible? It is like saying the particles have negative kinetic energy, which makes no physical sense.
As it turns out having negative absolute temperature require us to define temperature in an entirely different way using
entropy, and 2nd law of Thermodynamics, which states that systems will spontaneously increase its total entropy. Entropy can be thought of as the degree of disorderliness.
Objects with higher temperature have the greater tendency to lose its energy, so if a graph of energy vs entropy is plotted, higher temperature means it has a much less steep gradient, as it is more willing to lose the energy it has to an object with more steep energy vs entropy graph for it to gain total entropy. In other words, temperature can be defined as the inverse of the gradient of the energy vs entropy graph given as:
The objects we are familiar with around us will have greater entropy when it has more energy. Think of the particles getting more disorderly as it has more energy, which is why it has its usual positive temperature.
As it turns out, there exist certain systems that get more orderly when it has more energy, resulting in a negative energy vs entropy graph, one famous example are para-magnetic molecules placed in the presence of external magnetic fields. Such systems have very interesting properties such as spontaneously losing its energy when placed in contact with any object of any temperature, hence having the property of being "hotter than infinity".