© 2010 Pearson Education, Inc. PowerPoint ® Lectures for College Physics: A Strategic Approach,...

© 2010 Pearson Education, Inc.

PowerPoint® Lectures forCollege Physics: A Strategic Approach, Second Edition

Chapter 11

Using Energy

© 2010 Pearson Education, Inc. Slide 11-2

11 Using Energy


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%

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Answer1. 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%

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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 3B. 1 and 2C. 4 and 5D. 2 and 3E. All of 1–5

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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 3B. 1 and 2C. 4 and 5D. 2 and 3E. All of 1–5

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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.

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Answer3. A refrigerator is an example of a

A. reversible process.B. heat pump.C. cold reservoir.D. heat engine.E. hot reservoir.

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Example ProblemLight 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?

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Efficiency

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Example ProblemsA 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?

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Checking UnderstandingWhen you walk at a constant speed on level ground, what energy transformation is taking place?

A. Echem Ug

B. Ug Eth

C. Echem KD. Echem Eth

E. K Eth

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AnswerWhen you walk at a constant speed on level ground, what energy transformation is taking place?

A. Echem Ug

B. Ug Eth

C. Echem KD. Echem Eth

E. K Eth

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Example ProblemHow 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?

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The Ideal Gas Model

avg

B

2

3

KT

k

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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

Checking Understanding:Temperature Scales

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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

Answer

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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, QB. P, PC. Q, PD. Q, Q

Checking Understanding

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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, QB. P, PC. Q, PD. Q, Q

Answer

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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?

Example Problem

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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?

Example Problem: Work and Heat in an Ideal Gas

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Operation of a Heat Engine

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The Theoretical Maximum Efficiency of aHeat Engine

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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?

Example Problem: Geothermal Efficiency

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Operation of a Heat Pump

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Coefficient of Performance of a Heat Pump

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Checking Understanding: Increasing Efficiency of a Heat PumpWhich 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 aboveB. 1 and 4C. 2 and 3

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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.

A. All of the aboveB. 1 and 4C. 2 and 3

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EntropyHigher entropy states are more likely. Systems naturally evolve to states of higher entropy.

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Second Law of Thermodynamics

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Example Problem: Coming to a StopA 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.

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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.

A. 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?

B. 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?

C. 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?

D. Compare the energy output of the heat engine and the energy input to the heat pump. Comment on the feasibility of the proposed scheme.

Example Problem: A Second-Law Workaround?

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Summary

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Summary

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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.

Additional Questions

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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,

A. Q > 0.

B. Q = 0.

C. Q < 0.

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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

Additional Questions

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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?

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

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