. Explain the Essential Criterion of Expansion/Compression
The essential criterion of expansion / compression is that there should exist a difference between the internal pressure of the system and the external pressure. Suppose a gas is contained in a cylinder fitted with a piston. The latter is assumed to be weightless and frictionless. The piston can be held anywhere against a set of stops. Let the piston be initially held at stops S1
If the stops are now removed, then the position of the piston will be decided by the external pressure which can be controlled by putting different masses on the piston. If m is the mass that is put on the piston, then the force F acting downwards is F = mg and the pressure acting downwards p=F/A=mg/A where A is the area of cross section of the piston.
If the external pressure is greater than the internal pressure of the system, the piston moves downward. The volume of the system will decrease and it continues to decrease till the external pressure becomes equal to the internal pressure of the system. If the external pressure is smaller than the internal pressure, the piston moves upward resulting in expansion. Again, the volume will continue to increase till the external pressure becomes equal to the internal pressure. We can stop the expansion or compression in between by providing a set of stops at that stage. During the process of expansion or compression, we can change the external pressure. Thus, the process can be achieved either in one-stage or in multistage. In the latter, the external pressure may be different in each stage.