Thermal Physics

C.B.P. Finn

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351991575
• Edition: 2
• Topic: Ciencias físicas
• Subtopic: Química física y teórica

Concise yet thorough, accessible, authoritative, and affordable. These are the hallmarks of books in the remarkable Physics and its Applications series. Thermodynamics is an essential part of any physical sciences education, but it is so full of pitfalls and subtleties, that many students fail to appreciate its elegance and power. In Thermal Physics, the author emphasizes understanding the basic ideas and shows how the important thermodynamics results can be simply obtained from the fundamental relations without getting lost in a maze of partial differentials. In this second edition, Dr. Finn incorporated new sections on scales of temperature, availability, the degradation of energy, and lattice defects. The text contains ample illustrations and examples of applications of thermodynamics in physics, engineering, and chemistry.

1 Temperature

In this chapter we shall meet the concept of temperature, an idea which is at the very heart of thermodynamics. Indeed a definition which is often given of thermodynamics is that it is the study of the equilibrium properties of large-scale systems in which temperature is an important variable. We are all familiar, from our senses, of one body being hotter or colder than another. However, to put this concept on a sounder footing, we shall have to meet the so-called ‘zeroth law of thermodynamics’ which allows us to define the condition of thermal equilibrium; from this we can define temperature in an unambiguous way.

First, though, we have to define some basic concepts.

1.1 Systems, Surroundings, State Variables and Walls

In thermodynamics we confine our attention to a particular part of the universe which we call our system. The rest of the universe outside our system we call the surroundings. The system and the surroundings are separated by a boundary or wall and they may, in general, exchange energy and matter, depending on the nature of the wall. We shall consider here the exchange of energy only, restricting ourselves to closed systems, i.e. where there is no matter exchange.

A very useful example of a system is a fixed mass of compressible fluid, such as a gas, contained in a cylinder with a moveable piston as shown in Fig. 1.1. We shall develop many of our ideas using this simple system as an example.

Let us first consider our system to be completely isolated from its surroundings. The degree of isolation from external influences can vary over a very wide range and it is possible to imagine walls where the isolation is complete. In practice, the rigid walls of an ordinary vacuum flask are a good approximation to completely isolating walls. It is a fact of experience that, after a time, our gas system, or any other system contained in such isolating walls, tends to an equilibrium state in which no further changes occur. In particular, the pressure P becomes uniform throughout the gas and remains constant in time, as does the volume V. We say that the gas is in the equilibrium state (P, V). It is a further fact of experience that, specifying these equilibrium values of the pair of independent variables P and V, together with the mass, fixes all the macroscopic or bulk properties of the gas—for example, the thermal conductivity and the viscosity. A second sample of the same amount of gas with the same equilibrium values for P and V, but not necessarily of the same shape, would have the same viscosity as the first.

Figure 1.1 A gas contained in a cylinder is a useful example of a system.

These ideas can be generalized into the following definition:

An equilibrium state is one in which all the bulk physical properties of the system are uniform throughout the system and do not change with time.

We shall shortly be meeting other simple thermodynamic systems, apart from a gas, where we have to use other pairs of independent variables to specify the equilibrium state. For a stretched wire system, for example, we have to use the pair tension ℱ and length L. We shall consider such other systems in Chapters 2 and 8. The important point is that we require two variables to specify the equilibrium state of a simple system and we shall call such directly measurable variables state variables. Other common names are thermodynamic variables and thermodynamic coordinates.

Later in this book, we shall meet some new functions of the easily measurable state variables P, V and temperature T, to be introduced shortly in this chapter, which take unique values at each equilibrium state. Some examples of such functions are the internal energy, the entropy and the enthalpy. We call such functions state functions. It is important to realize that it does not matter how a particular state was reached: the value of a state function is always the same for a system in a given state and in no way depends on its past history. State property is an alternative and perhaps a more appropriate name for state function. Now it will be shown in section 1.5 that P, V and T are functionally connected at each equilibrium state by the equation of state and so it is possible to express any one in terms of the other two. Thus these quantities are themselves state functions but we give them the additional name of state variables because they are easily measured and enable us to specify an equilibrium state in a convenient practical way.

How then may we influence the system from outside if the walls are no longer isolating? We could bring about changes in the pressure and volume of our simple gas system in two different ways. Suppose we were to perform mechanical work on the gas system by pushing the piston in. Then the pressure and volume would in general both change, with the volume certainly changing, and we have an example of a mechanical interaction between the system and the surroundings. Suppose now that no mechanical interaction is allowed to occur—as would be the case if the piston were clamped, with the walls now being rigid. Consider a second cylinder, fitted with a free piston, containing the same gas with the same mass, volume and pressure as the first. Let the two cylinders be put into contact, as shown in Fig. 1.2, and let the piston of the second cylinder be pushed in. Depending on the nature of the intervening wall between the cylinders, there may or may not be changes in the pressure and volume of our gas system in the first cylinder. If there is no change, the intervening wall is said to be adiathermal or, more commonly, adiabatic: if there is a change, the wall is said to be diathermal and a thermal interaction has taken place. A wall made of metal such as copper or aluminium is a good approximation to a diathermal wall, while a good realization of an adiabatic wall is that of a vacuum flask. Two systems in contact via a diathermal wall are said to be in thermal contact.

Figure 1.2 An arrangement for determining whether or not a wall is adiabatic.

A remark should be made at this point. The reader may wonder why we do not define diathermal and adiabatic walls according to whether or not they conduct heat. The answer is that, while such walls have properties, we cannot define them in such a way as we have not yet defined heat. This has to wait until Chapter 3.

1.2 Thermal Equilibrium, The Zeroth Law of Thermodynamics and Temperature

If two thermodynamic systems such as gases are put in thermal contact, after a time no further changes in the pressures and volumes will occur, each gas being in an equilibrium state. The gases are said to ...