Chapter 11 Using Energy

11 Using Energy Slide 11-2

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Reading Quiz A machine uses 1000 J of electric energy to raise a heavy mass, increasing its potential energy by 300 J. What is the efficiency of this process? 100% 85% 70% 35% 30% Answer: E Slide 11-6

Answer A machine uses 1000 J of electric energy to raise a heavy mass, increasing its potential energy by 300 J. What is the efficiency of this process? 100% 85% 70% 35% 30% Answer: E Slide 11-7

Reading Quiz When the temperature of an ideal gas is increased, which of the following also increases? (1) The thermal energy of the gas; (2) the average kinetic energy of the gas; (3) the average potential energy of the gas; (4) the mass of the gas atoms; (5) the number of gas atoms. 1, 2, and 3 1 and 2 4 and 5 2 and 3 All of 1–5 Answer: B Slide 11-8

Answer When the temperature of an ideal gas is increased, which of the following also increases? (1) The thermal energy of the gas; (2) the average kinetic energy of the gas; (3) the average potential energy of the gas; (4) the mass of the gas atoms; (5) the number of gas atoms. 1, 2, and 3 1 and 2 4 and 5 2 and 3 All of 1–5 Answer: B Slide 11-9

Reading Quiz A refrigerator is an example of a reversible process. heat pump. cold reservoir. heat engine. hot reservoir. Answer: B Slide 11-10

Answer A refrigerator is an example of a reversible process. heat pump. cold reservoir. heat engine. hot reservoir. Answer: B Slide 11-11

Example Problem Light bulbs are rated by the power that they consume, not the light that they emit. A 100 W incandescent bulb emits approximately 4 W of visible light. What is the efficiency of the bulb? Slide 11-12

Efficiency Slide 11-13

Example Problems A person lifts a 20 kg box from the ground to a height of 1.0 m. A metabolic measurement shows that in doing this work her body uses 780 J of energy. What is her efficiency? A 75 kg person climbs the 248 steps to the top of the Cape Hatteras lighthouse, a total climb of 59 m. How many Calories does he burn? Slide 11-14

Checking Understanding When you walk at a constant speed on level ground, what energy transformation is taking place? A. Echem  Ug B. Ug  Eth C. Echem  K D. Echem  Eth E. K  Eth Answer: D Slide 11-15

Answer When you walk at a constant speed on level ground, what energy transformation is taking place? A. Echem  Ug B. Ug  Eth C. Echem  K D. Echem  Eth E. K  Eth Answer: D Slide 11-16

Example Problem How far could a 68 kg person cycle at 15 km/hr on the energy in one slice of pizza? How far could she walk, at 5 km/hr? How far could she run, at 15 km/hr? Do you notice any trends in the distance values that you’ve calculated? Chemical energy from food is used for each of these activities. What happens to this energy—that is, in what form does it end up? Slide 11-17

The Ideal Gas Model Slide 11-18

Checking Understanding:Temperature Scales Rank the following temperatures, from highest to lowest. 300 °C > 300 K > 300 °F 300 K > 300 °C > 300 °F 300 °F > 300 °C > 300 K 300 °C > 300 °F > 300 K Answer: D Slide 11-19

Answer Rank the following temperatures, from highest to lowest. 300 °C > 300 K > 300 °F 300 K > 300 °C > 300 °F 300 °F > 300 °C > 300 K 300 °C > 300 °F > 300 K Answer: D Slide 11-20

Checking Understanding Two containers of the same gas (ideal) have these masses and temperatures: Which gas has atoms with the largest average thermal energy? Which container of gas has the largest thermal energy? P, Q P, P Q, P Q, Q Answer: C Slide 11-21

Answer Two containers of the same gas (ideal) have these masses and temperatures: Which gas has atoms with the largest average thermal energy? Which container of gas has the largest thermal energy? P, Q P, P Q, P Q, Q Answer: C Slide 11-22

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Example Problem Using a fan to move air in a room will make you feel cooler, but it will actually warm up the room air. A small desk fan uses 50 W of electricity; all of this energy ends up as thermal energy in the air of the room in which it operates. The air in a typical bedroom consists of about 8.0 x 1026 atoms. Suppose a small fan is running, using 50 W. And suppose that there is no other transfer of energy, as work or heat, into or out of, the air in the room. By how much does the temperature of the room increase during 10 minutes of running the fan? Slide 11-24

Example Problem: Work and Heat in an Ideal Gas A container holds 4.0 x 1022 molecules of an ideal gas at 0 °C. A piston compresses the gas, doing 30 J of work. At the end of the compression, the gas temperature has increased to 10 °C. During this process, how much heat is transferred to or from the environment? Slide 11-25

Operation of a Heat Engine Slide 11-26

The Theoretical Maximum Efficiency of a Heat Engine Slide 11-27

Example Problem: Geothermal Efficiency At The Geysers geothermal power plant in northern California, electricity is generated by using the temperature difference between the 15 °C surface and 240 °C rock deep underground. What is the maximum possible efficiency? What happens to the energy that is extracted from the steam that is not converted to electricity? Slide 11-28

Operation of a Heat Pump Slide 11-29

Coefficient of Performance of a Heat Pump Slide 11-30

Checking Understanding: Increasing Efficiency of a Heat Pump Which of the following changes would allow your refrigerator to use less energy to run? (1) Increasing the temperature inside the refrigerator; (2) increasing the temperature of the kitchen; (3) decreasing the temperature inside the refrigerator; (4) decreasing the temperature of the kitchen. All of the above 1 and 4 2 and 3 Answer: B Slide 11-31

Answer Which of the following changes would allow your refrigerator to use less energy to run? (1) Increasing the temperature inside the refrigerator; (2) increasing the temperature of the kitchen; (3) decreasing the temperature inside the refrigerator; (4) decreasing the temperature of the kitchen. All of the above 1 and 4 2 and 3 Answer: B Slide 11-32

Entropy Higher entropy states are more likely. Systems naturally evolve to states of higher entropy. Slide 11-33

Second Law of Thermodynamics Slide 11-34

Example Problem: Coming to a Stop A typical gasoline-powered car stops by braking. Friction in the brakes brings the car to rest by transforming kinetic energy to thermal energy. Electric vehicles often stop by using regenerative braking, with the engine used as a generator, transforming the kinetic energy of the vehicle into electric energy that recharges the battery. The energy is thus ultimately transformed to chemical energy in the battery. Which type of stopping involves a larger change in entropy? Which vehicle is apt to be more efficient? Explain, using energy and entropy concepts. Slide 11-35

Example Problem: A Second-Law Workaround? When you run a heat engine, some (or most) of the energy is “wasted” as heat transferred to the cold reservoir. Suppose someone suggests making a 100% efficient heat engine by using some of the output of the heat engine to run a heat pump to transfer this heat back to the hot reservoir. Let’s do a calculation to see if this is a workable solution. If you have a heat engine that runs between a hot reservoir at 100°C and a cold reservoir at a temperature of 0°C, what is the maximum efficiency? If the engine draws 100 J from the hot reservoir, what is the maximum possible energy output? How much heat is deposited in the cold reservoir? How much energy would it take to run a heat pump between the cold and the hot reservoirs to pump this heat back to the hot reservoir? Compare the energy output of the heat engine and the energy input to the heat pump. Comment on the feasibility of the proposed scheme. Slide 11-36

Summary Slide 11-37

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Additional Questions Consider your body as a system. Your body is “burning” energy in food, but staying at a constant temperature. This means that, for your body, Q > 0. Q = 0. Q < 0. Answer: C Slide 11-39

Answer Consider your body as a system. Your body is “burning” energy in food, but staying at a constant temperature. This means that, for your body, Q > 0. Q = 0. Q < 0. Answer: C Slide 11-40

Additional Questions The following pairs of temperatures represent the temperatures of hot and cold reservoirs for heat engines. Which heat engine has the highest possible efficiency? 300°C 30°C 250°C 30°C 200°C 20°C 100°C 10°C 90°C 0°C Answer: A Slide 11-41

Answer The following pairs of temperatures represent the temperatures of hot and cold reservoirs for heat engines. Which heat engine has the highest possible efficiency? 300°C 30°C 250°C 30°C 200°C 20°C 100°C 10°C 90°C 0°C Answer: A Slide 11-42