THERMO Spoken Here! ~ J. Pohl © ~ 2019 (B2200-086)

2.05 Classical Piston-Cylinder

The classical "piston and cylinder" is an artifice often used to assist in understanding physical reality. The geometry is a circular piston of finite mass that fits snuggly into a circular cylinder to contain a fluid therein. Leakage is assumed zero and the piston is idealized such that it moves freely without friction. While no such arrangement exists, this approximation of physical reality is useful and widely used by thermodynamics.

The immediate use of a "frictionless piston/cylinder assembly" is to hypothecate a constant pressure environment for some fluid system and its event at a constant pressure greater than (upright) or less than (inverted) atmospheric pressure.

The sketch (right) shows two alignments of the Classical Piston/Cylinder. In either alignment, the system pressure (fluid within) is obtained by application of the momentum equation to the piston.

For the upright position the piston mass contributes to the system pressure.


With the piston in its inverted position, the piston mass diminishes the system pressure.

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