Introduction

Experience shows that in all thermodynamic systems the state variables temperature, internal energy and pressure assume a prominent role. Many experiments have been performed whose description require the introduction of temperature. This fact is expressed in the so-called zeroth law of thermodynamics which simply states that every thermodynamic system is associated with a variable of state T, called temperature. If two systems are in thermal equilibrium, the temperature of both systems must be identical.

Before discussing the all-important first law of thermodynamics it will be necessary to describe briefly the concepts of heat and work. If two isolated systems are brought in thermal contact by a conducting wall, heat will flow from the warmer to the colder system as long as a temperature difference exists. Heat is energy in transition between the systems. It is not a property of the physical system, therefore, it is not a variable of state. Thus, it makes no sense to speak of the heat of a particular system. The amount of heat that is transferred depends on the “path” for how it is added to the system. If, for example, the temperature of unit mass of air is to be increased by a certain number of degrees, it makes a significant difference if the heat is added at constant pressure or at constant volume.