Internal Energy, Work and Heat

Abstract

In this chapter, we formulate the first law of thermodynamics and study its implications on the example of reversible isochoric, isobaric, adiabatic and isothermal processes. The heat supplied to the system in isochoric and isobaric processes is expressed in terms of the heat capacity at constant volume and pressure, respectively. For processes at constant pressure, a new state function called the enthalpy is introduced, which plays a similar role as the internal energy for processes at constant volume. A reversible adiabatic process is studied for the ideal gas case. We also discuss irreversible adiabatic and isothermal processes at constant external pressure. In the latter case, we compare the work done by the system in the reversible and irreversible process between the same two states and conclude that the reversible process is always more efficient in changing heat into work. Then evaporation of a liquid is considered as an example of process at constant temperature and pressure. Finally, we apply the first law of thermodynamics to a simple chemical reaction.