What is Thermodynamics? Thermodynamics is a branch of knowledge that is heavily used in both engineering and science. This is evident from the number of courses at Berkeley that are devoted in whole or in large part to this subject. This wide coverage suggests that thermodynamics is a discipline with many facets, from which each branch of science or engineering takes what it needs most.
Thermodynamics can be approached from a microscopic or a macroscopic point of view. The macroscopic approach analyzes systems without regard to their detailed structure. In particular, macroscopic or “classical” or “macroscopic” thermodynamics does not need the knowledge that all substances are composed of atoms and molecules that store energy in their motions of translation (in a gas) and vibration (in solids). The branch of thermodynamics that explicitly recognizes these microscopic features of matter is called statistical thermodynamics.
There is of course a connection between the microscopic and macroscopic aspects of thermodynamics. Classical thermodynamics cannot provide a first-principles derivation of the ideal gas law pV = nRT, but is confined to stating that there must be a unique relation between pressure p, volume V and temperature T for any pure substance. Statistical thermodynamics, on the other hand, provides a method for deriving the ideal gas law from the basic motions of gas molecules.
Thermodynamics has its own set of terms, some of which are familiar to everyone (such as temperature and pressure) and others which are mysterious to the non-specialist (such as entropy and reversibility). Thermodynamics deals with the condition or state of the material contained inside a well-defined portion of space, called the system. The system generally contains a fixed quantity of matter, and in particular, the same matter whatever change is taking place. Such a system is said to be closed, in the sense that no mass leaves or enters the boundaries of the system. The gas in a sealed container is an example of a closed system. However, situations in which matter is flowing through a device are quite common, especially for gases and liquids. In such cases, the system is defined as the matter contained in an arbitrarily chosen fixed region of space. These systems are said to be open, with the implication that matter flows across the boundaries. An example of an open system is the steam inside a turbine, which is continually replenished by supply at the inlet and removal at the outlet. In this case, the system is the gas contained within the inner surfaces of the turbine housing and imaginary surfaces covering the inlet and outlet ports like porous meshes.
Thermodynamic systems are also classified by their degree of uniformity. A gas uniformly filling a container is an example of a homogeneous system, but homogeneity is not a prerequisite for applying thermodynamics. Ice and water in a glass can be treated as a thermodynamic system, one that is heterogeneous. However, in heterogeneous systems, each constituent or phase must be
separated from the others by a sharp interface. Systems with a gradient in concentration are not in
equilibrium, and cannot be treated thermodynamically.
All thermodynamic properties of a system in a particular state are fixed if as few as two properties are specified. For example, specification of the temperature and pressure of a gas fixes its internal energy as well as its volume. Thermodynamics cannot predict the law that relates pressure, temperature and energy any more than it can predict the p-V-T relation of the gas. It only requires that there be such a law. Thermodynamics in general can be fairly regarded as a science of relationships. It provides logical connections in a welter of seemingly unrelated properties of substances.
Thermodynamics is also concerned with what is involved when a system moves from one state to another. For such a change to occur, the system must interact with what lies outside of its confines. This exterior region is called the surroundings. The surroundings interact with the system by serving as a reservoir of energy, which can be transmitted to or received from the system in various guises. The two broad categories of system-surroundings energy exchange are called heat and work. These forms of energy in motion are manifest when they cross the boundaries, real or imaginary, that separate system from surroundings.
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