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Theory adiabatic equation updated 26 may 2010

Theory adiabatic equation

Contrary to liquids and solids, the volume of gases depends to a large extent on pressure and temperature. The state of a gas is determined by volume, pressure, and temperature. If one of these quantities is kept constant, there are simple relations between the remaining two quantities. For constant volume (iso volumetric change of state) we have Gay-Lussac's law:

(1)                                          ,        V=const.

where p0 is the pressure at 273.15 K,  is the coefficient of expansion, and t is the temperature in degrees Centigrade. Extrapolating this law to low temperatures (t<0) we find that there is an absolute zero point of our temperature scale, at which the pressure is zero. This point is reached at -273.15C. As all gases condense before they reach this zero point, there will be deviations from equation (1) at very low temperatures. So this law is valid only for ideal gases, which by definition show no interaction between their molecules ( no condensation), and whose molecules can be considered as mass points ( no volume).

If you perform a change of state in a gas while hindering any heat exchange with the surrounding matter, compression will change both pressure and temperature. This process is called an adiabatic one. In this case we have



where is the ratio of the specific heat capacities at constant pressure, cp, and at constant volume, cV:

                                                            = cp/cV

In German it is called Adiabatenexponent and it depends on the number of atoms that form the gas molecules.




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