QuickCheck. A. Yes B. No C. Maybe. It would depend on other factors Pearson Education, Inc. Slide 20-1

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1 QuickCheck A large 20ºC ice cube is dropped into a super-insulated container holding a small amount of 5ºC water, then the container is sealed. Ten minutes later, is it possible that the temperature of the ice cube will be colder than 20ºC? A. Yes B. No C. Maybe. It would depend on other factors Pearson Education, Inc. Slide 20-1

2 Irreversible Processes and the Second Law of Thermodynamics When two gases are brought into thermal contact, heat energy is transferred from the warm gas to the cold gas until they reach a common final temperature. Energy could still be conserved if heat was transferred in the opposite direction, but this never happens. The transfer of heat energy from hot to cold is an example of an irreversible process, a process that can happen only in one direction Pearson Education, Inc. Slide 20-2

3 Q20.1 Which statement about these two thermodynamic processes is correct? A. Both are reversible. B. Both are irreversible. C. The upper one is reversible and the lower one is irreversible. D. The upper one is irreversible and the lower one is reversible Pearson Education, Inc. Metal box at 0 C Ice at 0 C Metal box at 70 C Ice at 0 C Metal box at 0 C Liquid water at 0 C Metal box at 40 C Liquid water at 40 C

4 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. A block of ice melts irreversibly when we place it in a hot metal box Pearson Education Inc.

5 Directions of thermodynamic processes A block of ice at 0 C can be melted reversibly if we put it in a 0 C metal box Pearson Education Inc.

6 Which Way to Equilibrium? The figure shows two boxes containing identical balls. Once every second, one ball is chosen at random and moved to the other box. What do you expect to see if you return several hours later? Although each transfer is reversible, it is more likely that the system will evolve toward a state in which N 1 N 2 than toward a state in which N 1 >> N 2. The macroscopic drift toward equilibrium is irreversible Pearson Education, Inc. Slide 20-6

7 Molecular Collisions Are Reversible 2017 Pearson Education, Inc. Slide 20-7

8 A Car Crash Is Irreversible 2017 Pearson Education, Inc. Slide 20-8

9 Q20.2 An ideal gas is taken around the cycle shown in this p-v diagram, from a to b to c and back to a. Process b c is isothermal. Which of the processes in this cycle could be reversible? A. a b B. b c C. c a D. two or more of A, B, and C 2016 Pearson Education, Inc.

10 The second law of thermodynamics The second law of thermodynamics can be stated in several ways: It is impossible for any system to undergo a process in which it absorbs heat from a reservoir at a single temperature and converts the heat completely into mechanical work, with the system ending in the same state in which it began. We will call this the engine statement of the second law. It is impossible for any process to have as its sole result the transfer of heat from a cooler to a hotter body. We ll call this the refrigerator statement of the second law Pearson Education Inc.

11 Energy Reservoirs An energy reservoir is an object or a part of the environment so large that its temperature does not change when heat is transferred between the system and the reservoir. A reservoir at a higher temperature than the system is called a hot reservoir. A reservoir at a lower temperature than the system is called a cold reservoir Pearson Education, Inc. Slide 20-11

12 Energy-Transfer Diagrams 2017 Pearson Education, Inc. Slide 20-12

13 Work into Heat Turning work into heat is easy just rub two objects together! Shown is the energy transfer diagram for this process. The conversion of work into heat is 100% efficient, in that all the energy supplied to the system as work is ultimately transferred to the environment as heat Pearson Education, Inc. Slide 20-13

14 Heat into Work Transforming heat into work is not easy. To be practical, a device that transforms heat into work must return to its initial state at the end of the process and be ready for continued use. It is impossible to invent a perfect engine that transforms heat into work with 100% efficiency and returns to its initial state so that it can continue to do work as long as there is fuel. The second law of thermodynamics forbids a perfect engine Pearson Education, Inc. Slide 20-14

15 Heat engines A heat engine is any device that partly transforms heat into work or mechanical energy. All motorized vehicles other than purely electric vehicles use heat engines for propulsion. (Hybrid vehicles use their internal-combustion engine to help charge the batteries for the electric motor.) 2016 Pearson Education Inc.

16 Heat Engines In a steam turbine of a modern power plant, expanding steam does work by spinning the turbine. The steam is then condensed to liquid water and pumped back to the boiler to start the process again. First heat is transferred to the water in the boiler to create steam, and later heat is transferred out of the water to an external cold reservoir, in the condenser Pearson Education, Inc. Slide 20-16

17 Heat engines Simple heat engines operate on a cyclic process during which they absorb heat Q H from a hot reservoir and discard some heat Q C to a cold reservoir. Shown is a schematic energy-flow diagram for a heat engine Pearson Education Inc.

18 Heat Engines We can measure the performance of a heat engine in terms of its thermal efficiency e defined as Actual car engines and steam generators have e Pearson Education, Inc. Slide 20-18

19 QuickCheck The efficiency of this heat engine is A B C D Pearson Education, Inc. Slide 20-19

20 Q-RT20.1 Rank the following heat engines in order from highest to lowest thermal efficiency. A. An engine that in one cycle absorbs 2500 J of heat and rejects 2250 J of heat B. An engine that in one cycle absorbs 50,000 J of heat and does 4000 J of work C. An engine that in one cycle does 800 J of work and rejects 5600 J of heat 2016 Pearson Education, Inc.

21 A Heat-Engine Example: Slide 1 of Pearson Education, Inc. Slide 20-21

22 A Heat-Engine Example: Slide 2 of Pearson Education, Inc. Slide 20-22

23 A Heat-Engine Example: Slide 3 of 3 Shown is the heatengine process on a pv diagram. No work is done during the isochoric step 2 3. The net work per cycle is W net = W lift W ext = (W s ) (W s ) Pearson Education, Inc. Slide 20-23

24 Ideal-Gas Heat Engines Some heat engines use an ideal gas as the working substance. A gas heat engine can be represented by a closed-cycle trajectory on a pv diagram. The net work done during a full cycle is 2017 Pearson Education, Inc. Slide 20-24

25 QuickCheck How much work is done in one cycle? A J B J C J D J 2017 Pearson Education, Inc. Slide 20-25

26 QuickCheck How much heat is exhausted to the cold reservoir? A J B J C J D J 2017 Pearson Education, Inc. Slide 20-26

27 QuickCheck Which heat engine has the larger efficiency? A. Engine 1 B. Engine 2 C. They have the same efficiency. D. Can t tell without knowing the number of moles of gas Pearson Education, Inc. Slide 20-27

28 Example 1 - The pv diagram in the figure shows a cycle of a heat engine that uses mole of an ideal gas having γ=1.40. The curved part ab of the cycle is adiabatic. (a) Find the pressure of the gas at point a. (b) How much heat enters this gas per cycle? When? (c) How much heat leaves this gas in a cycle? When? (d) How much work does this engine do in a cycle? (e) What is the thermal efficiency of the engine?

29 In-class Activity #1 A gasoline truck engine takes in 10,000 J and delivers 2000 J of mechanical work per cycle. The heat is obtained by burning gasoline with heat of combustion L c = 5.0 x 10 4 J/g. (a) What is the thermal efficiency of this engine? (b) How much heat is discarded in each cycle? (c) If the engine goes through 25 cycles per second, what is its power output in watts? (d) How much gasoline is burned in each cycle? 2016 Pearson Education, Inc.

30 Internal-combustion engines 2016 Pearson Education Inc.

31 Internal-combustion engines 2016 Pearson Education Inc.

32 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. Shown is an energy-flow diagram of a refrigerator. Q H = Q C +W in 2016 Pearson Education Inc.

33 Refrigerators: Coefficient of performance From an economic point of view, the best refrigeration cycle is one that removes the greatest amount of heat from the inside of the refrigerator for the least expenditure of mechanical work. The relevant ratio is therefore Q C / W ; the larger this ratio, the better the refrigerator. We call this ratio the coefficient of performance, K: 2016 Pearson Education Inc.

34 Principle of the mechanical refrigeration cycle 2016 Pearson Education Inc.

35 QuickCheck The coefficient of performance of this refrigerator is A B C D Pearson Education, Inc. Slide 20-35

36 Example 2 - A freezer has a coefficient of performance of The freezer is to convert 1.80 kg of water at 25.0ºC to 1.80 kg of ice at -5.0ºC in one hour. (a) What amount of heat must be removed from the water at 25.0ºC to convert it to ice at - 5.0ºC? (b) How much electrical energy is consumed by the freezer during this hour? (c) How much wasted heat is rejected to the room in which the freezer sits? 2016 Pearson Education, Inc.

37 The second law of thermodynamics The second law of thermodynamics can be stated in several ways: It is impossible for any system to undergo a process in which it absorbs heat from a reservoir at a single temperature and converts the heat completely into mechanical work, with the system ending in the same state in which it began. We will call this the engine statement of the second law. It is impossible for any process to have as its sole result the transfer of heat from a cooler to a hotter body. We ll call this the refrigerator statement of the second law Pearson Education Inc.

38 The second law of thermodynamics If a workless refrigerator were possible, it could be used in conjunction with an ordinary heat engine to form a 100%-efficient engine, converting heat Q H Q C completely to work Pearson Education Inc.

39 The second law of thermodynamics If a 100%-efficient engine were possible, it could be used in conjunction with an ordinary refrigerator to form a workless refrigerator, transferring heat Q C from the cold to the hot reservoir with no input of work Pearson Education Inc.

40 In-class Activity #2 - A refrigerator has a coefficient of performance of Each cycle it absorbs J of heat from the cold reservoir. (a) How much mechanical energy is required each cycle to operate the refrigerator? (b) During each cycle, how much heat is discarded to the high-temperature reservoir?