Adiabatic index

From Academic Kids

The adiabatic index of a gas, is the ratio of its specific heat capacity at constant pressure (CP) to its specific heat capacity at constant volume (CV). It is denoted by the greek letter γ (gamma) or the greek letter κ (kappa).

γ = CP / CV

To understand this definition consider the following experiment:  A closed cylinder with a locked piston is filled with air.  This cylinder is heated.  Since the piston cannot move the volume is constant.  Temperature and pressure rise.   Heating is stopped and the energy added to the system, which is proportional to CV, is noted.

The piston is now freed and moves outwards, expanding without absorbing energy (adiabatic expansion).  Doing this work (proportional to CP) cools the air to below its starting temperature.  To return to the starting temperature the air must be heated.  This extra heat amounts to about 40% of the previous amount.

Thus air has an adiabatic index of approximately 1.4; a result often used in aerodynamics.de:Adiabatenexponent

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