Gas in a cylinder-and-piston device

A useful kind of deformation for the development of thermodynamic theory is a change in the volume of a gas or liquid. 

As a model for the work involved in changing the volume of a gas, consider the arrangement shown in Fig. 3.4 on the next page. A sample of gas is confined in a cylinder by a piston. The system is the gas. The piston is not part of the system, but its position given by the variable x determines the system s volume. Movement of the piston to the right, in the -l-x direction, expands the gas movement to the left, in the —x direction, compresses it. 

We will find it instructive to look in detail at the forces acting on the piston. There are three kinds the force Fgas exerted in the +x direction by the gas an external force Fext in the -X direction, which we can control in the surroundings and a frictional force Ffnc in the direction opposite to the piston s velocity when the piston moves. (The friction occurs at the seal between the edge of the piston and the cylinder wall.) 

Thermodynamics and Chemistry, second edition,version 3 20 by Howard DeVoe. Latest version www.chem.umd.edu/thermobook 

Let pb be the average pressure of the gas at the piston—that is, at the moving portion of the system boundary (the subseript b stands for boundary). Then the foree exerted by the gas on the piston is given by 

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