Adiabatic process

An adiabatic process takes place with no heat exchange with the surroundings (Q = 0) — for example during rapid compression or expansion of a gas. It is governed by the equation:

$p_1 V_1^{\kappa}=p_2 V_2^{\kappa}$

where $\kappa=c_p/c_V$ is the adiabatic index (1.4 for a diatomic gas such as air, 1.67 for a monatomic one). The temperature changes according to $T_1 V_1^{\kappa-1}=T_2 V_2^{\kappa-1}$.

The effect: compressing a gas adiabatically warms it up (the work turns into internal energy — which is why a bicycle pump heats up), and expanding it cools it down (the principle behind refrigerators and the formation of clouds).