Chapter 11 Using Energy PowerPoint Lectures for College Physics: A Strategic Approach, Second Edition
11 Using Energy Slide 11-2
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Reading Quiz 1. 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? A. 100% B. 85% C. 70% D. 35% E. 30% Slide 11-6
Answer 1. 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? A. 100% B. 85% C. 70% D. 35% E. 30% Slide 11-7
Reading Quiz 2. 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. A. 1, 2, and 3 B. 1 and 2 C. 4 and 5 D. 2 and 3 E. All of 1 5 Slide 11-8
Answer 2. 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. A. 1, 2, and 3 B. 1 and 2 C. 4 and 5 D. 2 and 3 E. All of 1 5 Slide 11-9
Reading Quiz 3. A refrigerator is an example of a A. reversible process. B. heat pump. C. cold reservoir. D. heat engine. E. hot reservoir. Slide 11-10
Answer 3. A refrigerator is an example of a A. reversible process. B. heat pump. C. cold reservoir. D. heat engine. E. hot reservoir. Slide 11-11
Efficiency e = E out E in e = P out P in Slide 11-13
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
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? This is a typical efficiency for the human body Slide 11-14a
Example Problems 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 11-14b
Example Problem How far could a 68 kg person cycle at 15 km/hr on the energy in one slice of pizza? Slide 11-17a
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? Slide 11-17
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? Slide 11-17
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? Slide 11-17
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? Heat When you heat something up, it gets hotter This means its temperature goes up But what exactly is temperature a measure of? Slide 11-17
T C = 5 [T F 9 32 ] T F = 9 T C 5 + 32 T K = T C + 273
Checking Understanding:Temperature Scales Rank the following temperatures, from highest to lowest. A. 300 C > 300 K > 300 F B. 300 K > 300 C > 300 F C. 300 F > 300 C > 300 K D. 300 C > 300 F > 300 K Slide 11-19
Answer Rank the following temperatures, from highest to lowest. A. 300 C > 300 K > 300 F B. 300 K > 300 C > 300 F C. 300 F > 300 C > 300 K D. 300 C > 300 F > 300 K Slide 11-20
The Ideal Gas Model An ideal gas is one in which: The atoms only have kinetic energy They interact only through elastic collisions Slide 11-18
The Ideal Gas Model An ideal gas is one in which: The atoms only have kinetic energy They interact only through elastic collisions It exists at some temperature Slide 11-18
The Ideal Gas Model Add heat, and the temperature goes up Slide 11-18
The Ideal Gas Model Add heat, and the temperature goes up But so does the kinetic energy of the atoms There is a relationship between the Kelvin temperature and the average kinetic energy Slide 11-18
The Ideal Gas Model T = 2 3 K avg (per atom) k B k B = Boltzmann s constant = 1.38 10 23 J/K K avg per atom = 3 2 k BT ΔE th = 3 2 Nk BΔT Slide 11-18
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? A. P, Q B. P, P C. Q, P D. Q, Q Slide 11-21
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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 10 26 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 10 22 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
Heat Transfer The energy transferred from hot to cold is heat Heat transfer continues until the two sides have the same temperature the same average kinetic energy At that point they have reached thermal equilibrium, and there will be no more net transfer of energy Spontaneous heat transfer is always from higher to lower temperature This is the basis of heat engines
Operation of a Heat Engine Slide 11-26
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
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. A. All of the above B. 1 and 4 C. 2 and 3 Slide 11-31
Entropy Higher entropy states are more likely. Systems naturally evolve to states of higher entropy. Slide 11-33
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
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, A. Q > 0. B. Q = 0. C. Q < 0. Slide 11-39
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? A. 300 C 30 C B. 250 C 30 C C. 200 C 20 C D. 100 C 10 C E. 90 C 0 C Slide 11-41