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n 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
thermal phenomena 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, gases expand
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 tempera-
ture. 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
ice tray 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 such quantitative
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 dropped into 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 at different
temperatures, energy is exchanged between them, even if they are initially not in physi-
cal 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 dis-
cussion, we assume that two objects are in
thermal contact with each other if energy
can be exchanged between them by these processes due to a temperature difference.