Chapter 20 The Second Law of Thermodynamics Power

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Chapter 20 The Second Law of Thermodynamics Power. Point® Lectures for University Physics, Thirteenth

Chapter 20 The Second Law of Thermodynamics Power. Point® Lectures for University Physics, Thirteenth Edition – Hugh D. Young and Roger A. Freedman Lectures by Wayne Anderson Copyright © 2012 Pearson Education Inc.

Goals for Chapter 20 • To learn what makes a process reversible or irreversible

Goals for Chapter 20 • To learn what makes a process reversible or irreversible • To understand heat engines and their efficiency • To see how internal combustion engines operate • To learn the operation of refrigerators and heat engines • To see how the second law of thermodynamics limits the operations of heat engines and refrigerators • To do calculations with Carnot engines and refrigerators • To understand entropy and to use it to analyze thermodynamic processes Copyright © 2012 Pearson Education Inc.

Introduction • Why does heat flow from the hot lava into the cooler water?

Introduction • Why does heat flow from the hot lava into the cooler water? Could it flow the other way? • It is easy to convert mechanical energy completely into heat, but not the reverse. Why not? • We need to use the second law of thermodynamics and the concept of entropy to answer the above questions. Copyright © 2012 Pearson Education Inc.

Directions of thermodynamic processes • The direction of a reversible process can be reversed

Directions of thermodynamic processes • The direction of a reversible process can be reversed by an infinitesimal change in its conditions. The system is always in or very close to thermal equilibrium. All othermodynamic processes are irreversible. • Figure 20. 1 illustrates an irreversible and a reversible process. Copyright © 2012 Pearson Education Inc.

Heat engines • A heat engine is any device that partly transforms heat into

Heat engines • A heat engine is any device that partly transforms heat into work or mechanical energy. • Simple heat engines operate on a cyclic process during which they absorb heat QH from a hot reservoir and discard some heat QC to a cold reservoir. • Figure 20. 3 at the right shows a schematic energy-flow diagram for a heat engine. Copyright © 2012 Pearson Education Inc.

The efficiency of a heat engine • The thermal efficiency e of a heat

The efficiency of a heat engine • The thermal efficiency e of a heat engine is the fraction of QH that is converted to work. • e = W/QH • Read Problem-Solving Strategy 20. 1. • Follow Example 20. 1 to analyze a heat engine using Figure 20. 4 at the right. Copyright © 2012 Pearson Education Inc.

Internal-combustion engines • Figure 20. 5 below illustrates a four-stroke internal-combustion engine. The compression

Internal-combustion engines • Figure 20. 5 below illustrates a four-stroke internal-combustion engine. The compression ratio r is the ratio of the maximum volume to the minimum volume during the cycle. Copyright © 2012 Pearson Education Inc.

The Otto cycle and the Diesel cycle • Figures 20. 6 and 20. 7

The Otto cycle and the Diesel cycle • Figures 20. 6 and 20. 7 below show p. V-diagrams for idealized Otto cycle and Diesel cycle engines. In both cases, the efficiency depends on the compression ratio r. Copyright © 2012 Pearson Education Inc.

Refrigerators • A refrigerator takes heat from a cold place (inside the refrigerator) and

Refrigerators • A refrigerator takes heat from a cold place (inside the refrigerator) and gives it off to a warmer place (the room). An input of mechanical work is required to do this. • A refrigerator is essentially a heat engine operating in reverse. • Figure 20. 8 at the right shows an energy-flow diagram of a refrigerator. • The coefficient of performance K of a refrigerator is K = |QC|/|W|. Copyright © 2012 Pearson Education Inc.

Practical refrigerators • Figure 20. 9 below shows the principle of the mechanical refrigeration

Practical refrigerators • Figure 20. 9 below shows the principle of the mechanical refrigeration cycle and how the key elements are arranged in a practical refrigerator. Copyright © 2012 Pearson Education Inc.

Air conditioner • An air conditioner works on the same principle as a refrigerator.

Air conditioner • An air conditioner works on the same principle as a refrigerator. (See Figure 20. 10 below. ) • A heat pump operates in a similar way. Copyright © 2012 Pearson Education Inc.

The second law of thermodynamics • The second law of thermodynamics can be stated

The second law of thermodynamics • The second law of thermodynamics can be stated in several ways: ü No cyclic process can convert heat completely into work. ü No cyclic process can transfer heat from a colder place to a hotter place without the input of mechanical work. • Figure 20. 11 at the right illustrates both statements. Copyright © 2012 Pearson Education Inc.

The Carnot cycle • A Carnot cycle has two adiabatic segments and two isothermal

The Carnot cycle • A Carnot cycle has two adiabatic segments and two isothermal segments. • The p. V-diagram in Figure 20. 13 below shows the complete cycle. Copyright © 2012 Pearson Education Inc.

Analyzing a Carnot cycle • Follow the derivation of the efficiency of a Carnot

Analyzing a Carnot cycle • Follow the derivation of the efficiency of a Carnot engine. • e. Carnot = 1 – TC/TH • Follow Example 20. 2 using Figure 20. 14 at the right. • Follow Example 20. 3. Copyright © 2012 Pearson Education Inc.

The Carnot refrigerator • A Carnot engine run in reverse is a Carnot refrigerator.

The Carnot refrigerator • A Carnot engine run in reverse is a Carnot refrigerator. • The coefficient of performance of a Carnot refrigerator is Kcarnot = TC/(TH – TC). • Follow Example 20. 4. Copyright © 2012 Pearson Education Inc.

The Carnot cycle and the second law • No engine can be more efficient

The Carnot cycle and the second law • No engine can be more efficient than a Carnot engine operating between the same two temperatures. Follow the proof of this in the text, using Figure 20. 15 below. Copyright © 2012 Pearson Education Inc.

Entropy and disorder • Entropy provides a quantitative measure of disorder. The explosion of

Entropy and disorder • Entropy provides a quantitative measure of disorder. The explosion of the firecracker in Figure 20. 17 increases its disorder and entropy. • Follow the discussion in the text of the entropy for reversible processes. • Follow Example 20. 5 for melting ice. • Follow Example 20. 6 for heating water. Copyright © 2012 Pearson Education Inc.

Entropy change in some adiabatic processes • Follow Conceptual Example 20. 7. • Follow

Entropy change in some adiabatic processes • Follow Conceptual Example 20. 7. • Follow Example 20. 8 using Figure 20. 18 below. • Follow Example 20. 9. Copyright © 2012 Pearson Education Inc.

Entropy in cyclic processes • The entropy change during any reversible cycle is zero.

Entropy in cyclic processes • The entropy change during any reversible cycle is zero. Figure 20. 19 below helps to explain why. • For an irreversible process the entropy of an isolated system always increases. Entropy is not a conserved quantity. • Follow Example 20. 10. Copyright © 2012 Pearson Education Inc.

Entropy and the second law • The second law of thermodynamics can be stated

Entropy and the second law • The second law of thermodynamics can be stated in terms of entropy: No process is possible in which the total entropy of an isolated system decreases. • In Figure 20. 20 below, the entropy (disorder) of the ink-water system increases as the ink mixes with the water. Spontaneous unmixing of the ink and water is never observed. Copyright © 2012 Pearson Education Inc.

Microscopic interpretation of entropy • Follow the discussion of the microscopic interpretation of entropy,

Microscopic interpretation of entropy • Follow the discussion of the microscopic interpretation of entropy, using Figure 20. 21 at the right. • The entropy of a macrostate having w microstates is S = k ln w. Copyright © 2012 Pearson Education Inc.

A microscopic calculation of entropy change • Follow Example 20. 11 for a free

A microscopic calculation of entropy change • Follow Example 20. 11 for a free expansion using Figure 20. 22 below. Copyright © 2012 Pearson Education Inc.