Work in a Gravitational Field

When we open the stopcock of the apparatus shown in Fig. 3.8, the gas expands from the vessel at the left into the evacuated vessel at the right. This process is called free expansion. The system is the gas. The surroundings exert a contact force on the system only at the vessel walls, where there is no displacement. Thus, there is no work in free expansion dvj = 0. 

If the free expansion is carried out adiabatically in a thermally-insulated apparatus, there is neither heat nor work and therefore no change in the internal energy AU = 0. If the gas is ideal, its internal energy depends only on temperature thus the adiabatic free expansion of an ideal gas causes no temperature change. 

Consider first the simple process in Fig. 3.9(a) in which the body falls freely through the fluid. This process is clearly spontaneous. Here are two choices for the definition of the system  

consider the arrangement in Fig. 3.9(b) in which the body is suspended by a thin string. The string is in the surroundings and provides a means for the surroundings to exert an upward contact force on the system. As before, there are two appropriate choices for the system  

Thermodynamics and Chemistry, second edition, version 3 2011 by Howard DeVoe. Latest version www.chem.umd.edu/themobook 

---