Working through this chapter of the study guide will teach you to:

1. Distinguish between heat and temperature and explain the units involved with each.
2. Learn how temperature scales are set up and how to convert temperature readings between scales.
3. Work with the concepts of specific heat and latent heat to determine heat requirements for increasing a material's temperature or changing its phase.
4. Tell how heat can be transferred through various forms of matter or even through empty space.
5. Explain how solids, liquids, and gases differ, and be able to describe how the behavior of gases can be explained using the kinetic theory of molecules.
6. Discover how kinetic energy in gases can serve as a model for understanding heat and pressure effects in all phases of matter.
7. Understand the basis for the three laws of thermodynamics and how they apply to such diverse processes as heat engines and refrigerators.
8. Show how entropy can be used to explain thermodynamic processes.

Discussion

Continuing our discussion of energy, we now move into what appears at first to be an entirely new topic. The study of heat and energy developed independently until the early 1800s, and because of this we have two sets of units that can be used for heat. The original units of heat were the calorie and the British thermal unit (Btu). Once heat and energy were found to be equivalent quantities, it became evident that the joule and the foot-pound could also apply. Conversion factors that allow you to change from one heat unit to another can be found inside the back cover of your textbook. Today we are turning more and more to the joule as a standard for heat measurement, but it is still necessary to study both sets of units or it will not be as easy to see how the important aspects of heat contained in this chapter apply to our everyday lives.


Section   5.1Temperature

Temperature is often thought of as a relative measure of the hotness or coldness of a material. In physical science, however, it must be more accurately measured by thermometers using temperature scales such as those developed by Fahrenheit, Celsius, and Kelvin. These scales are all defined in terms of two basic reference temperatures — the ice point and the steam point of pure water at one atmosphere of pressure. Both the Celsius and Kelvin scales have the same size degree, dividing the interval between the ice and steam points of water into 100 equal parts. The Fahrenheit scale, although originally defined quite differently, today is defined by assigning a value of 32^° to the ice point and 212^° to the steam point and dividing this span into 180 equal degrees.

The Kelvin temperature scale is more fundamental in that it uses the coldest theoretical temperature, absolute zero, as its zero point.
Absolute zero occurs at approximately -273^° on the Celsius scale. This places the ice point of pure water at 273 K and the steam point at 373 K. The Kelvin scale will play a key role in our discussion of the kinetic theory of gases and in the ideal gas laws later in this chapter. The Kelvin temperature scale is the most scientifically relevant scale, but we must be conversant in all three scales if we are to understand how temperature measurements are made and recorded throughout the world today.

Temperature is related to the average kinetic energy of the atoms or molecules making up the substance under study. This means that hot gases must have fast-moving molecules, while cooler ones have molecules moving at lower speeds. Liquids and solids also have higher-energy molecules, but they take on higher modes of vibration and/or rotation rather than higher linear velocities. This internal motion constitutes a form of kinetic energy that is called heat energy, as defined in the next Section.


Section   5.2Heat

Heat is a measure of the total internal molecular energy of a material. Remember that there are two sets of units that can be used for heat. The units of heat are the calorie (cal) and the British thermal unit (Btu), but because heat is a form of energy we can also use the joule (J) and the ft-lb. To make things even more complex, the unit kilocalorie (kcal) came into rather widespread use in the dietary and nutritional fields, and common practice is to replace the "kilo" with a capital C so that a food Calorie (or "big" calorie) refers to 1 kcal or 1000 regular, or "little," calories. Once again, water is the substance used in the definition of heat units for both the SI and the British system of units, as discussed in the next Section .


Section   5.3Specific Heat and Latent Heat

Specific heat is the amount of heat required to change the temperature of a given quantity of some material by one degree. This concept is used for defining the basic units of heat. One calorie is the amount of heat required to change the temperature of 1 g of pure water by 1 C^°. One Btu is the amount of heat necessary to change the temperature of 1 lb of pure water by 1 F^°. One kilocalorie is the amount of heat necessary to change the temperature of 1 kg of water by 1 C^°. Specific heat varies from one substance to another, but it is nearly constant for a given material (in a given phase) over a fairly wide temperature range. Table 5.1 in the textbook shows the specific heats of some common materials in both kcal/kg-C^° and J/kg-C^°.

Another thing that can occur when heat is added to or removed from a substance is a phase change. When heat is added, a solid can melt into the liquid phase or a liquid can vaporize and become a gas. When heat is removed, a gas can condense into a liquid or a liquid can fuse into a solid. A definite amount of heat is required to change 1 kg of any particular substance from one phase to another at the melting point or boiling point. Changes that occur at the melting point of the material (melting and fusion) involve an amount of heat called the latent heat of fusion. The heat involved in the vaporization or condensation of any substance at its boiling point is called the latent heat of vaporization. Each substance has its own unique values for latent heats, and each one changes phase at different melting point and boiling point temperatures. Table 5.2 in the textbook lists the latent heats and phase transition temperatures for several common substances, including water. Figure 5.5 in the textbook is helpful in understanding these processes. Notice that during a phase change, the temperature of the substance remains the same (at either the melting point or boiling point temperature) until the entire sample of material has been converted into the new phase.


Section   5.4Heat Transfer

The transfer of heat from one place to another is often important to our understanding of the world. By using one or more of the basic heat transfer processes, conduction, convection, and radiation, heat can travel through solids, liquids, gases, or even a complete vacuum. This last aspect of heat transfer is crucial to our lives on Earth because nearly all the energy that we use comes to us, either directly or indirectly, through space by radiation from our nearest star, the Sun. If it were not for this heat-transfer method, our entire planet would be a cold, lifeless place, and we would not even be alive to concern ourselves with the study of physical science.

Conduction occurs primarily in solids and involves the physical transfer of thermal energy (heat) from one molecule or atom to the next within a sample of material. This is possible because in a solid the atoms and molecules are constrained to fixed positions called lattice sites. Their thermal vibrations pass heat energy along to other nearby molecules, and thus heat is transferred from the hot end of the solid substance to the cooler end.

Convection takes place in fluids (liquids and gases) and is accomplished by the movement of the heated fluid itself from one location to another. Warm fluids expand and rise because their density becomes less than the density of the rest of the surrounding fluid. The hottest portion of the heated fluid then rises, causing convection loops to form, by means of which the circulating fluid carries heat energy to other parts of the fluid. Winds on the surface of Earth are also driven by complex and powerful convection processes within the atmosphere.

Radiation can take place through any transparent medium. You may have noticed how radiated heat from a campfire can be felt when it is transferred through the air to your body if you are standing close enough to the fire. The actual heat transfer is made by a form of electromagnetic waves called infrared radiation. This type of thermal energy is emitted by all "hot" objects, is transferred by the process of radiation, and is eventually absorbed by surrounding, cooler objects. In this context, "hot" means any temperature above zero degrees on the Kelvin scale, that is above absolute zero.


Section   5.5Phases of Matter

As we mentioned briefly in Section  5.3, matter can exist in three primary phases: solids, liquids, and gases. In solids, the atoms or molecules are constrained to lattice sites. This explains the basic properties of a solid, such as rigidity, constant shape, and constant volume. The atoms and molecules in a liquid, however, are free to move around each other, although the distance between the molecules remains about the same. This allows liquids to change shape but still requires that they maintain a constant volume. Gases, on the other hand, are made up of atoms or molecules that are essentially free to move about randomly, so gases can generally be expanded or compressed to any desired volume. This is true even in our atmosphere, where gravity forms a type of container that keeps the air constrained to a well-defined region around Earth. If this were not true, the rapidly moving air molecules in our solar-heated atmosphere would escape to fill the entire volume of space.

One additional phase of matter called a plasma is also possible. A plasma is formed when a gas is heated to such a high temperature that the atoms or molecules ionize, that is; they lose one or more of their outermost electrons. A plasma behaves much like a gas, but is made up of positive ions and negative electrons, both of which are essentially free to move around within the plasma. The motion of the ions and electrons in a plasma is, however, strongly affected by electrostatic forces that exist among the charged ions and electrons. These additional forces change the structure and behavior of a plasma so that it is somewhat more complex than a simple gas. Because of the extremely high temperatures normally required to cause ionization, we do not often encounter plasmas in our everyday lives, although they are present in and strongly influence the interactions that take place in our Sun and other stars.


Section   5.6The Kinetic Theory of Gases

The physical behavior of gases is described by the kinetic theory of gases, in which individual atoms or molecules are considered to travel at high speeds in all directions within the gas sample. The kinetic theory accounts for many physical properties of a gas, such as pressure, temperature, volume, diffusion rate, and so on. Pressure results from the frequent collisions of large numbers of gas molecules with the walls of the gas's container. The volume of a gas sample is variable because the spacing between the gas molecules can change to allow the gas to fill any size container in which it is placed.

Temperature is related to the average kinetic energy of the moving molecules of gas. The higher the temperature, the faster the molecules must be moving. In any equation dealing with the kinetic energy of gases, absolute temperature must be used as expressed using the Kelvin scale. The pressure of the gas is also important in such calculations. Other quantities that may be specified when describing a gas sample are the number of gas molecules present, the volume, and the average speed of the individual molecules. The relationships among pressure, temperature, volume, and number of gas molecules in a sample can be expressed using the gas law equations given in this Section of your textbook.


Section   5.7Thermodynamics

The study of heat leads to three fundamental laws called the first, second, and third laws of thermodynamics. The first law is simply a statement of the conservation of energy as applied to thermal processes; the second law defines the direction in which heat flows between materials that are at different temperatures; and the third law states the belief that the theoretically lowest temperature known, absolute zero, can never be physically attained. These laws are important in many applications in the fields of astronomy, nutrition, and nuclear physics to mention only a few. One interesting aspect of the second law of thermodynamics is that it provides the only working definition of the direction in which time progresses in real processes and is the basis for our belief that time can never run backwards.

Perhaps the laws of thermodynamics can best be understood by exploring the concept of a heat engine. These laws also explain the operation of refrigerators and heat pumps. Be sure you study carefully the discussions in the textbook that cover these thermodynamic laws, because they form the basis for understanding many important processes involved in our daily lives.

Return to Previous Page