Temperatura

Chapter 19

Temperature
CHAPTE R OUTLI N E

19.1 Temperature and the Zeroth Law of Thermodynamics 19.2 Thermometers and the Celsius Temperature Scale 19.3 The Constant-Volume Gas Thermometer and the Absolute Temperature Scale 19.4 Thermal Expansion of Solids and Liquids 19.5 Macroscopic Description of an Ideal Gas

L Why would someone designing a pipeline include these strange loops?Pipelines
carrying liquids often contain loops such as these to allow for expansion and contraction as the temperature changes. We will study thermal expansion in this chapter. (Lowell Georgia/CORBIS)

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In our study of mechanics, we carefully defined such concepts as mass, force, and kinetic

energy to facilitate our quantitative approach. Likewise, a quantitative description of thermalphenomena requires careful definitions of such important terms as temperature, heat, and internal energy. This chapter begins with a discussion of temperature and with a description of one of the laws of thermodynamics (the so-called “zeroth law”). Next, we consider why an important factor when we are dealing with thermal phenomena is the particular substance we are investigating. For example, gasesexpand appreciably when heated, whereas liquids and solids expand only slightly. This chapter concludes with a study of ideal gases on the macroscopic scale. Here, we are concerned with the relationships among such quantities as pressure, volume, and temperature. In Chapter 21, we shall examine gases on a microscopic scale, using a model that represents the components of a gas as small particles.19.1 Temperature and the Zeroth Law of Thermodynamics
We often associate the concept of temperature with how hot or cold an object feels when we touch it. Thus, our senses provide us with a qualitative indication of temperature. However, our senses are unreliable and often mislead us. For example, if we remove a metal ice tray and a cardboard box of frozen vegetables from the freezer, the icetray feels colder than the box even though both are at the same temperature. The two objects feel different because metal transfers energy by heat at a higher rate than cardboard does. What we need is a reliable and reproducible method for measuring the relative hotness or coldness of objects rather than the rate of energy transfer. Scientists have developed a variety of thermometers for making suchquantitative measurements. We are all familiar with the fact that two objects at different initial temperatures eventually reach some intermediate temperature when placed in contact with each other. For example, when hot water and cold water are mixed in a bathtub, the final temperature of the mixture is somewhere between the initial hot and cold temperatures. Likewise, when an ice cube is droppedinto a cup of hot coffee, it melts and the coffee’s temperature decreases. To understand the concept of temperature, it is useful to define two often-used phrases: thermal contact and thermal equilibrium. To grasp the meaning of thermal contact, imagine that two objects are placed in an insulated container such that they interact with each other but not with the environment. If the objects are atdifferent temperatures, energy is exchanged between them, even if they are initially not in physical contact with each other. The energy transfer mechanisms from Chapter 7 that we will focus on are heat and electromagnetic radiation. For purposes of the current discussion, we assume that two objects are in thermal contact with each other if energy can be exchanged between them by these processesdue to a temperature difference.
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C H A P T E R 19 • Temperature

A

B

A (b) (c)

B

(a)

Figure 19.1 The zeroth law of thermodynamics. (a) and (b) If the temperatures of A and B are measured to be the same by placing them in thermal contact with a thermometer (object C), no energy will be exchanged between them when they are placed in thermal contact with each other...