Low temperature:
Cryogenics may be defined as low temperature technology, and is
distinguished from refrigeration by definition as being any
temperature lower than -73.3 C. Low temperatures in the cryogenic
range are primarily achieved by the liquifaction of gases having
boiling points below -73.3 C. Cryogenic liquified gases (cryogenic
fluids) are characterized by extremely low temperatures, ranging from
a boiling point of -78.5 C for carbon dioxide to -269 C for helium.
Thermodynamics meaning of temperature:
About the same time that thermodynamics was evolving, James Clerk
Maxwell (1831-1879) and Ludwig Boltzmann (1844-1906) developed a
theory describing the way molecules moved - molecular dynamics. The
molecules that make up a perfect gas move about, colliding with each
other like billiard balls and bouncing off the surface of the
container holding the gas. The energy associated with motion is called
Kinetic Energy and this kinetic approach to the behavior of ideal
gases led to an interpretation of the concept of temperature on a
microscopic scale.
The amount of kinetic energy each molecule has is a function of its
velocity; for the large number of molecules in a gas (even at low
pressure), there should be a range of velocities at any instant of
time. The magnitude of the velocities of the various particles should
vary greatly - no two particles should be expected to have the exact
same velocity. Some may be moving very fast; others, quite slowly.
Maxwell found that he could represent the distribution of velocities
statistically by a function known as the Maxwellian distribution. The
collisions of the molecules with their container gives rise to the
pressure of the gas. By considering the average force exerted by the
molecular collisions on the wall, Boltzmann was able to show that the
average kinetic energy of the molecules was directly comparable to the
measured pressure, and the greater the average kinetic energy, the
greater the pressure. From Boyles' Law, we know that the pressure is
directly proportional to the temperature, therefore, it was shown that
the kinetic energy of the molecules related directly to the
temperature of the gas. A simple relation holds for this:
average kinetic energy of molecules=3kT/2,
