15.1

Thermodynamic Systems and their Surroundings

We have studied heat (Chapter 12) and work (Chapter 6) as separate topics. Often, however, they occur simultaneously. In an automobile engine, for instance, fuel is burned at a relatively high temperature, some of its internal energy is used for doing the work of driving the pistons up and down, and the excess heat is removed by the cooling system to prevent overheating. Thermodynamics is the branch of physics that is built upon the fundamental laws that heat and work obey.
In thermodynamics the collection of objects on which attention is being focused is called the system, while everything else in the environment is called the surroundings. For example, the system in an automobile engine could be the burning gasoline, while the surroundings would then include the pistons, the exhaust system, the radiator, and the outside air. The system and its surroundings are separated by walls of some kind. Walls that permit heat to flow through them, such as those of the engine block, are called diathermal walls. Perfectly insulating walls that do not permit heat to flow between the system and its surroundings are known as adiabatic walls.
To understand what the laws of thermodynamics have to say about the relationship between heat and work, it is necessary to describe the physical condition or state of a system. We might be interested, for instance, in the hot air within the balloon in Figure 15-1. The hot air itself would be the system, and the skin of the balloon provides the walls that separate this system from the surrounding cooler air. The state of the system would be specified by giving values for the pressure, volume, temperature, and mass of the hot air.

Figure 15-1 The air in this colorful hot-air balloon is one example of a thermodynamic system. (William Panzer/Alamy Images)

As this chapter discusses, there are four laws of thermodynamics. We begin with the one known as the zeroth law and then consider the remaining three.

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