Free expansion
. In this case, the pressure is the volume and the and are respectively the before and after. That is, as the gas expands and the volume increases, the pressure decreases. A gas that does free expansion does not work. The gas undergoes a state that is not in thermodynamic equilibrium until it reaches its final state, and in this state, none of the thermodynamic parameters of the gas can be determined. For example, since the pressure of a gas varies locally at each point in a point, the volume of gas as a whole of the particle is not an easily definable amount. Opening the cock and spreading the gas to vacuum is a representative example of free expansion. There is no heat energy coming into or out of the system, so nothing has happened. There is a change in entropy, but the well-known entropy change formula does not apply. This is because free expansion is not a reversible process. For ideal gases, entropy of free expansion is obtained as the joule-Thomson effect.