Internal Energy, Work and Heat

Abstract

In the previous chapter, we introduced the concept of the internal energy of a system as one of the extensive parameters of state. It follows from the energy conservation principle that the internal energy of an isolated system is a constant quantity. In ther-modynamics, however, we are mainly interested in interactions between the system and surroundings, therefore, we have to specify possible ways of energy transfer between them. One of these ways is mechanical work. If the system performs work its energy decreases. For example, if we squeeze a spring it can perform work later, decreasing its potential energy. If an external agent (surroundings) performs work on a mechanical system the energy of the system increases. We already know that thermodynamics deals with macroscopic systems consist-ing of a great number of atoms or molecules. The internal energy of such a system, as the energy of a mechanical system, can be changed by means of mechanical work. An example of mechanical work is compression of a gas in a vessel with a movable piston. We can determine such work if at any moment we know the force acting on the piston and its displacement, however, we do not need to know how individual molecules interact with the piston. Obviously, it is not the only way to perform work on the system. For instance, when an electric mixer is immersed in a liquid we can perform work by mixing the liquid. Due to the viscosity there exists friction between the liquid and a rotating mixer. Thus, the electric current, which drives the mixer, performs a definite amount of work on the system, which can be easily measured. In the case of a simple mechanical system, such as the spring, the work done by an external agent is stored in the form of potential energy, which does not change as long as the system does not perform any work. On the other hand, if some work is performed on a thermodynamic system, the energy stored in it can escape to the surroundings in the form of heat. To prevent it, we have to insulate thermally the system with adiabatic walls. Then the increase in the internal energy of the system, U , is exactly equal to the work performed on the system: