THERMODYNAMICS Objectives • Describe what happens to the 4 THERMODYNAMICS temperature of a substance when the thermal motion of the atoms approaches zero. THE BIG (24.1) Heat normally flows from • State the first law of IDEA ........ hot to cold. thermodynamics. (24.2) • Describe the effect of adiabatic compression on a gas. (24.3) he study of heat and its transformation into mechanical • State the second law of energy is called thermodynamics. The word thermody- thermodynamics. (24.4) namics stems from Greek words meaning “movement • Explain how the second law T of heat.” The science of thermodynamics was developed in of thermodynamics applies to heat engines. (24.5) the mid-1800s, before the atomic and molecular nature of • Describe what happens to the matter was understood. So far, our study of heat has been orderly state of any natural concerned with the microscopic behavior of jiggling atoms system. (24.6) and molecules. Now we will see that thermodynamics • Describe what happens to the bypasses the molecular details of systems and focuses entropy of any system. (24.7) on the macroscopic level—mechanical work, pres- sure, temperature, and their roles in energy trans- This chapter focuses on the formation. The foundation of thermodynamics environment and the atmosphere, and provides some physics is the conservation of energy and the fact insights into processes that that heat flows from hot to cold, and not underlie weather. This chapter the other way around. It provides the basic is not a prerequisite to the theory of heat engines, from steam tur- chapters that follow. It may be skipped if a brief discussion of bines to fusion reactors, and the basic theory of heat is sufficient. refrigerators and heat pumps. We begin our study of thermodynamics with a look at one of its early con- cepts—a lowest limit of temperature. discover! MATERIALS rubber band discover! EXPECTED OUTCOME When stretched, the rubber band Can Temperature Change Without Analyze and Conclude felt slightly warmer; when Heat Transfer? 1. Observing Describe what you felt when the contracted, it felt slightly 1. Place a rubber band, loosely looped over rubber band was stretched and then allowed cooler. your index fingers, in contact with your to contract rapidly. ANALYZE AND CONCLUDE upper lip. 2. Predicting What do you think you would 1. See Expected Outcome. 2. Quickly stretch the rubber band. feel if the rubber band were stretched and 2. No temperature change 3. Now let the rubber band contract quickly. allowed to contract more slowly? 3. The faster a process, the less Do not snap the rubber band. 3. Making Generalizations Why do you think the time for other changes to rate of performing a process may affect the occur. outcome of the process? 468 468 0468_CP09_SE_CH24.indd 468 11/13/07 11:05:55 AM 0468_CP09_SE_CH24.indd 469 00468_cp09te_CH24.indd468_cp09te_CH24.indd 446868 11/29/08/29/08 99:20:08:20:08 AAMM 24.1 Absolute Zero 24.1 Absolute Zero Key Terms As thermal motion of atoms increases, temperature increases. There thermodynamics, absolute zero seems to be no upper limit of temperature. In contrast, there is a defi- Teaching Tip Use the nite limit at the other end of the temperature scale. If we continually following analogy: Electrons still move in the lowest energy state decrease the thermal motion of atoms in a substance, the tempera- of an atom, just as atoms still ture will drop. As the thermal motion of atoms in a substance vibrate in the lowest temperature approaches zero, the kinetic energy of the atoms approaches zero, state of a solid. and the temperature of the substance approaches a lower limit. Teaching Tip Review This limit is the absolute zero of temperature. Absolute zero is the the Celsius and Fahrenheit temperature at which no more energy can be extracted from a sub- temperature scales. Discuss the stance and no further lowering of its temperature is possible. This idea of a lowest temperature— absolute zero and the Kelvin (K) limiting temperature is 273 degrees below zero on the Celsius scale. scale. State that the Kelvin scale This value was found in the 1800s by experimenters who discovered is “nature’s scale” and its zero that all gases contract by the same proportion when temperature is point is the coldest possible value. decreased.24.1 (Note the degree symbol (°) is not used with K. This should reinforce Absolute zero corresponds to zero degrees on the Kelvin, or ther- the concept that the Kelvin scale’s modynamic, scale and is written 0 K (short for “zero kelvin”). Unlike zero point was not chosen by the Celsius scale, there are no negative numbers on the thermody- human convention.) namic scale. Degrees on the Kelvin scale are the same size as those on the Celsius scale. Thus, ice melts at 0°C, or 273 K, and water boils at When William Thompson was 100°C, or 373 K. The Kelvin scale was named after the British physi- made a Baron, he took the title cist Lord Kelvin, who coined the word thermodynamics and first sug- Lord Kelvin from the Kelvin River gested such a scale. that ran through his estate. Figure 24.1 shows the temperature of various objects and phe- nomena with respect to absolute zero. At very high temperatures, the measurements of temperature on the Kelvin and Celsius scales are close to identical. CONCEPT What happens to a substance’s temperature as the CHECK ...... motion of its atoms approaches zero? CONCEPT As the thermal CHECK ...... motion of atoms in a think! substance approaches zero, the kinetic energy of the atoms A sample of hydrogen gas has a temperature of 0°C. If the gas is heated approaches zero, and the until its molecules have doubled their average kinetic energy (the gas temperature of the substance has twice the absolute temperature), what will be its temperature in approaches a lower limit. degrees Celsius? Answer: 24.1 Teaching Resources FIGURE 24.1 ᭤ • Reading and Study The figure shows the abso- Workbook lute temperatures of various • Laboratory Manual 67 objects and phenomena. • Transparency 47 • PresentationEXPRESS • Interactive Textbook 469 CHAPTER 24 THERMODYNAMICS 469 11/13/07 11:05:55 AM 0468_CP09_SE_CH24.indd 469 11/13/07 11:06:02 AM 00468_cp09te_CH24.indd468_cp09te_CH24.indd 446969 11/29/08/29/08 99:20:12:20:12 AAMM 24.2 First Law of Thermodynamics Looking for a diet 24.2 First Law of Thermodynamics plan? Burn more Key Term In the eighteenth century, heat was thought to be an invisible fluid first law of thermodynamics calories than you consume. This is the called caloric, which flowed like water from hot objects to cold Teaching Tip Introduce the only diet plan firmly objects. Caloric was conserved in its interactions, a discovery that led first law of thermodynamics based on the first to the law of conservation of energy. In the 1840s, using the appa- by citing the findings of Count law of thermodynam- Rumford: that when cannon ratus shown in Figure 24.2, scientist James Joule demonstrated that ics—and guaranteed the flow of heat was nothing more than the flow of energy itself. The barrels were being drilled to work! and became very hot, it was caloric theory of heat was gradually abandoned.24.2.1 Today we view the friction of the drills that heat as a form of energy. Energy can neither be created nor destroyed. produced the heating. Recall The first law of thermodynamics is the law of conservation of the definition of work, force 3 distance, and explain that the energy applied to thermal systems. The first law of thermody- metal is heated by the frictional namics states that whenever heat is added to a system, it transforms force 3 distance that the various to an equal amount of some other form of energy. parts of the drill bit move. Have your students rub their hands Heat By system, we mean any group of atoms, molecules, particles, together and feel them warm up. or objects we wish to deal with. The system may be the steam in a Teaching Tip Discuss the steam engine, the whole Earth’s atmosphere, or even the body of a account of James Joule with living creature. It is important to define what is contained within his paddle-wheel apparatus the system as well as what is outside of it. If we add heat energy for measuring the mechanical equivalent of heat. Joule to the steam in a steam engine, to Earth’s atmosphere, or to the attempted to extend this body of a living creature, these systems will be able to do work on experiment to a larger scale external things. This added energy does one or both of two things: while on his honeymoon in (1) increases the internal energy of the system if it remains in the Switzerland. Joule and his bride honeymooned near the system and (2) does external work if it leaves the system. So, the first Chamonix waterfall. According law of thermodynamics states to Joule’s conception of heat, increase in external work done the gravitational PE of the Heat added water at the top should go internal energy by the system into increasing the internal Let’s say you put an air-filled, rigid, airtight can on a hotplate and energy of the water at the bottom. Joule made a rough add a certain amount of energy to the can. Caution: Do not actually estimate of the increase in water do this. Since the can has a fixed volume, the walls of the can don’t temperature at the bottom of move, so no work is done.