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This module presents an interactive molecular simulation of a piston-cylinder apparatus. The piston-cylinder apparatus is a standard tool used to conceptualize and illustrate thermodynamic concepts involving heat, work, and internal energy. The module may be used also to study equations of state, reversibility, and corresponding states concepts. These ideas form the core of thermodynamics, and this module illustrates how they arise from the aggregate motions of many molecules, each behaving according to simple classical mechanics.

The simulated apparatus is a container (cylinder) with one movable wall (the piston), within which are spherical molecules (number adjustable) undergoing simple molecular dynamics. The piston moves in response to collisions with the molecules. The external pressure on the piston may be adjusted, and the system may be set as isothermal or adiabatic. The instantaneous and average density and pressure adopted by the system are presented. A dynamic plot is available showing graphically the response of the system to changes in state. Many different quantitative and qualitative experiments can be performed to explore the thermodynamic behavior of the system. The observed behavior is clearly seen as a consequence of the collective molecular motions.

This module is applicable to thermodynamics courses at both the undergraduate and graduate levels.