Exposition on the Kelvin or Absolute Temperature Scale

Loosely speaking temperature is a measure of the mean microscopic kinetic energy of particles (including light ``particles'' still speaking loosely). If you extract all the microscopic kinetic energy, then all microscopic motion ceases. So if you want to set up a temperature scale, then choosing the condition no microscopic kinetic energy to be zero temperature is very useful. You never have to worry about negative temperatures and zero temperature has a universal physical relevance: if something is at zero temperature, then it's in a special physical condition that is the low temperature limit of all its behavior. So in the Kelvin or absolute scale, zero temperature is defined classically to be the condition of no microscopic kinetic energy. The Kelvin scale was named in honor of Lord Kelvin (William Thompson) (1824--1907) who had a lot to with setting it up one supposes.
Actually, it seems that a macroscopic object can never have be put at ABSOLUTE ZERO. But we can still define absolute zero as the limiting case where microscopic kinetic energy vanishes.
And also actually, from modern quantum mechanics we find that there is an irremovable minimum amount of kinetic energy in any microscopic system. Thus, nowadays absolute zero is the condition of having reached this minimum or ZERO-POINT KINETIC ENERGY.
The Kelvin degree (a kelvin or K) is chosen to be the same size as the Celsius degree (C): i.e.,
```
     1 K = 1 degree C  ,
     
```
where note the word degree or symbol degree is not used with the Kelvin scale. The relation between the Kelvin and Celsius scales is
```
     T_K =T_C + 273.15  ,
     
```
which implies that -273.15 degrees C is absolute zero. Recall 0 degrees C is the freezing point of water and 100 degrees C is the boiling point of water. The relation of the Celsius scale to the Fahrenheit is
```
     T_F =1.8*T_C + 32 .
    
```
In my view, we should just drop Fahrenheit and Celsius altogether and just use Kelvin all the time. This is my crank idea.
At this point you may wonder what is the macroscopic physical significance of temperature. Well macroscopically temperature is a measure of the thermal equilibrium condition of a body. If two bodies have the same temperature and you put them in thermal contact (i.e., allow heat energy to flow between them), then no heat will in fact flow spontaneously between them and the bodies do not change (e.g., expand, contract, change phase, or experience pain). Now if the bodies have a temperature difference, there will be a heat flow and the bodies will change somehow: the larger the temperature difference, the larger the heat flow and change. In statistical mechanics, the microscopic and macroscopic meanings of temperature are shown to be consistent.
Macroscopic Temperature can be related in formalue to OTHER THERMODYNAMIC QUANTITIES: e.g., pressure and density.
Typically in everyday life temperature is measured by putting an object in contact with a THERMOMETER which consists of an enclosed fluid which changes size dramatically as a function of temperature. The size of the fluid is then used as a measure of temperature. Ordinary thermometers must be calibrated against standard thermometers that measure temperature as precisely defined in physics. We will not go into those precisely defined temperature methods here.
Another important way to measure temperature is to measure the spectrum of the electromagnetic radiation a body emits. A body all at one temperature emits a radiation spectrum of a particular shape. This method is particularly important in astrophysics where all we can measure in many cases is the electromagnetic radiation we see from an astro-body
An important astrophysical point is that the universe as a whole is in profound thermodynamic disequilibrium. The stars are very hot and space is very cold: space is actually 2.73 K as known from the microwave background radiation. Heat flows from hot to cold spontaneously: this is a basic property of nature: a consequence of the 2nd law of thermodynamics which we won't discuss here. Thus the starlight streams off into empty space which it can never---so far as we know now---heat up significantly.
The universal disequilibrium is very important and its consequences are one of those STORIES that you should know: i.e., life among other things. Thermodynamic equilibrium is a timeless, lifeless state. Life is only possible as one of the small, but very complex channels in which the heat energy of the Sun passes to the cold emptiness of space. In even simpler terms LIFE AS WE KNOW IT is only possible between hot and cold baths: it is only between those baths that we can extract energy to move around and do work. No energy can be extracted if you only have a bath at one temperature.
For example, there is huge heat energy in ocean water, but we can't make use of it unless we bring up a colder heat reservoir into which that heat can flow with work extracted along the way. Open space also has a tremendous amount of energy, but only as very cold microwave radiation (T=2.73 K [Se-387]) from which no work can be extracted because there are no significant colder baths.
Most of the Sun's energy just streams away forever from the Sun as light. A small amount is captured by the Earth and then re-radiated to empty as colder infrared light. Only a tiny bit of the light captured by the Earth is processed through the biosphere, before it too is ultimately degraded to cold infrared light that escapes the Earth.

Just to get some idea of notable temperature we can present a small comparison table.


Table of Notable Temperatures

   Notable Temperature     Kelvin (K)    Celsius (^oC)    Fahrenheit (^oF)

   absolute zero              0          -273.15          not worth knowing
   coincidence              233.15        -40            -40
   water freezing           273.15          0             32
   human warmish            300            26.85          80.33
   water boiling            373.15        100            212
   pure iron melting       1808          1535            who cares
   pure iron boiling       3023          2750              "
   Sun surface             5800           who cares        "
   Sun center             15*10**6          "              "

Sources: HRW-429, Se-147, and for iron EnvironmentalChemistry.com

As the table indicates there are temperatures that just arn't worth knowing in Fahrenheit and Celsius.
Note that iron is a good REFRACTORY material: it doesn't melt or boil at low temperatures by human standard or even the standards of many materials like water, O_2, and CO_2. Materials that evaporate relatively easily are VOLATILE. As we frequently say, he/she is very volatile---and that helps you to understand what we mean.
One could go on and on, but for now enough on temperature.