University Physics with Modern Physics

Hugh D. Young, Roger A. Freedman

Chapter 20 The Second Law of Thermodynamics - all with Video Answers

Educators

Chapter Questions

Problem 1

A diesel engine performs $2200 \mathrm{~J}$ of mechanical work and discards $4300 \mathrm{~J}$ of heat each cycle. (a) How much heat must be supplied to the engine in each cycle? (b) What is the thermal efficiency of the engine?

Eric Mockensturm

Numerade Educator

00:54

Problem 1

A pot is half-filled with water, and tis is placed on it, forming a tight seal so that no water vapor can escape. The pot is heated on a stove, forming water vapor inside the pot. The heat is then turned off and the water vapor condenses back to liquid. Is this cycle reversible or irreversible? Why?

Mayukh Banik

Numerade Educator

00:52

Problem 2

Give two examples of reversible processes and two examples of irreverible processes in purely mechanical systems, such as blocks sliding on planes, springs, pulleys, and strings. Explain what makes euch process reversible or imeversible.

Sanjeev Kumar

Numerade Educator

00:52

Problem 2

Give two examples of reversible processes and two examples of irreversible processes in purely mechanical systems, such as blocks sliding on planes, springs, pulleys, and strings. Explain what makes each process reversible or irreversible.

Sanjeev Kumar

Numerade Educator

01:12

Problem 3

What irreversitile processes occur in a gasoline engine? Why are they irreversible?

Aspen Fenzl

Numerade Educator

01:57

Problem 3

What irreversible processes occur in a gasoline engine? Why are they irreversible?

Narayan Hari

Numerade Educator

00:51

Problem 4

Suppose you try to cool the kitchen of your house by leaving the refrigerator door open. What happens? Why? Would the result be the same if you left open a picnic cooler full of ice? Explain the reason for any ?ifferences.

Sanjeev Kumar

Numerade Educator

00:51

Problem 4

Suppose yoa try to cool the kitchen of your bouse by leaving the refrigerator door open. What happens? Why? Would the result be the same if you left open a picnic cooler full of ice? Explain the reason for any differences.

Sanjeev Kumar

Numerade Educator

05:58

Problem 5

A member of the U.S. Congress proposed a scheme to produce cnersy as follows. Water molecules $\left(\mathrm{H}_{2} \mathrm{O}\right)$ are to be broken apart to produce hydrogen and oxygen. The hydrogen is then burned (that is, combined with oxygen), releasing energy in the process. The only product of this combustion is water, so there is no pollution. In light of the seoond law of thermodyzamics, what do you think of this energy-peoducing scheme?

Eric Mockensturm

Numerade Educator

01:29

Problem 5

A member of the U.S. Congress proposed a scheme to produce energy as follows. Water molecules $\left(\mathrm{H}_{2} \mathrm{O}\right)$ are to be broken apart to produce hydrogen and oxygen. The hydrogen is then burned (that is, combined with oxygen), releasing energy in the process. The only product of this combustion is water, so there is no pollution. In light of the second law of thermodynamics, what do you think of this energy-producing scheme?

Bhumika Jayee

Numerade Educator

01:18

Problem 6

Is it a violation of the second law of thermodynamics to convert mechanical energy completely into beat? To convert heat completely into work? Explain your answers,

Ajay Singhal

Numerade Educator

01:18

Problem 6

Is it a violation of the second law of thermodynamies to convert mechanical energy completely into beat? To convert heat completely into work? Explain your answers.

Ajay Singhal

Numerade Educator

02:56

Problem 7

Imagine a special air filter placed in a window of a house. The tiny holes in the filter allow only air molecules moving faster than a certain speed to exit the house, and allow only air molecules moving slower than that speed to enter the house from outside. Explain why such an air filter would cool the house, and why the second law of thermodynamics makes building such a filter an impossible task.

Ajay Singhal

Numerade Educator

02:56

Problem 7

Imagine a special air filter placed in a window of a house. The tiny holes in the filter allow only air molecules moving faster than s certain speod to exit the house, and allow only air molecules moving slower than that spoed to catiter the house from outside. Explain why such an air filter would cool the house, and why the second law of thermodynamics makes building such a filter an impossibie task.

Ajay Singhal

Numerade Educator

02:10

Problem 8

An electric motor has its shaft coupled to that of an electric generator. The motor drives the generator, and some current from the generator is used to run the motor. The excess current is used to light a home. What is wrong with this scheme?

Averell Hause

Carnegie Mellon University

02:10

Problem 8

An electrie motor has its shaft coupled to that of an electric geaerator. The motor drives the generator, and some current from the gencrator is used to run the motor. The cxcess current is used to Iight a homc. What is wrong with this scheme?

Averell Hause

Carnegie Mellon University

03:13

Problem 9

When a wet cloth is hung up in a hot wind in the desert, it is cooled by evaporation to a temperature that may be $20 \mathrm{C}^{\circ}$ or so below that of the air, Discuss this process in light of the second law of thermodynamics,

Megan Doucette

Numerade Educator

00:51

Problem 9

When a wet cloth is hung up in a hot wind in the desert, it is cooled by evaporatioa to a temperature that may be $20 \mathrm{C}^{\circ}$ or 50 below that of the air. Discuss this process in light of the socond law of thermodynemici.

Sanjeev Kumar

Numerade Educator

02:22

Problem 10

Compare the $p V$ -diagram for the Otto cycle in Fig $20.6$ with the diagram for the Carnot heat engine in Fig. 20.13. Explain some of the important differences between the two cycles,

Nathan Silvano

Numerade Educator

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Problem 10

Compare the $p V$ -diagmm for the Oto cycle in Fig. $20.6$ with the diagram for the Carnot heat engine in Fig. 20.13. Bxplain some of the important differences between the two cycles,

Victor Salazar

Numerade Educator

02:16

Problem 11

If no real eagine can be as cfficient as a Carnot engine operating between the same two temperatures, what is the point of developing and using Bq. (20.14)?

Ma Ednelyn Lim

Numerade Educator

01:06

Problem 12

The efficicncy of heat cngines is high when the temperature difference between the bot and cold reservoirs is large. Refrigerators, on the other hand, work better when the temperature difference is small. Thinking of the mechunical refrigerutor cycle shown in Fig. 20.9, explain in physical terms why it tnkes less work to remove heat from the worlaing substance if the two reservoirs (the iaside of the refrigerator and the outside air) aro at nearly the same temperature, than if the outside air is much warmer than the interior of the refrigerator.

Mayukh Banik

Numerade Educator

00:57

Problem 13

What would be the efficieacy of $\mathrm{z}$ Camot engine operating with $T_{\mathrm{H}}=T_{\mathrm{C}}$ ? What would be the efficiency if $T_{\mathrm{c}}=0 \mathrm{~K}$ and $T_{\mathrm{H}}$ were any ternperuture above $0 \mathrm{~K}$ ? Interpret your answers.

Mirza Aslam Beig

Numerade Educator

02:01

Problem 14

Real heat engines, like the gasoline engine in a car, always heve sorne friction between thelr moving parts, slthough lutricants keep the friction to a minimum. Would a beat engine with completely frictionless parts be $100 \%$ efficient? Why or why not? Does the answer depend on whether or not the engine runs on the Carnot cycle? Again, why or why not?

Vipender Yadav

Numerade Educator

03:42

Problem 15

Does a refrigerutor full of food cossume more power if the room temperature is $20^{\circ} \mathrm{C}$ than if it is $15^{\circ} \mathrm{C}$ ? $\mathrm{Or}$ is the power consumption the same? Explain your reasoning.

Shital Rijal

Numerade Educator

01:44

Problem 16

In Example $20.4$, u Curnot refrigerator requires a work input of only $230 \mathrm{~J}$ to extract $346 \mathrm{~J}$ of heat from the cold reservoir .Doesn't this discrepancy imply a violation of the law of conservation of cnergy? Explain why or wly not.

Km Neeraj

Numerade Educator

12:06

Problem 17

Explain why each of the following processes is an example of increasing disorder of randomness: mixing hot and cold water, free expansion of a gas; imeversible heat Bow; developing heat by mechanicul friction. Are entropy increases involved in ill of these? Why or why not?

Linda Winkler

Numerade Educator

00:45

Problem 18

The free cxpansion of a gas is an adiabatic process and so no heat is transferred. No work is done, so the internal cnergy does not change. Thus, $Q / T=0$, yet the disorder of the system and thos its entropy have increased after the expansion. Why does Eq. $20.19$ not apply to this situation?

Mayukh Banik

Numerade Educator

01:05

Problem 19

Are the earth and sun in thermal equilibrium? Are there cntropy changes associated with fhe transmission of energy from the sun to the earth? Does radiation differ from other modes of heat transfer with respect to entropy changes? Explain your reasoning.

Mayukh Banik

Numerade Educator

00:22

Problem 20

Discuss the entropy changes involved in the preparation and consumption of a hot fodge sundae.

Sanjeev Kumar

Numerade Educator

01:39

Problem 21

If you run a movie film backwards, it is as if the ?iroction of time were reversed. In the time-reversed movic, would you sce processes that vlolate coaservation of energy? Conservation of linear momentum? Would you see processes thal violate the second law of thermodynamics? In each case, if law-breaking processes could oceur, give some examples.

Lewis Rose

Numerade Educator

02:23

Problem 22

Some critics of biological evolution claim that it violates the socond law of thermodynamics, since evolution involves simple Hife forms developing into morv complex and more highly ordered organisms, Explain why this is not a valld argument against evolution.

Josee Pacheco

Numerade Educator

04:28

Problem 23

You design an engine that takes in $1.50 \times 10^{4} \mathrm{~J}$ of heat a $650 \mathrm{~K}$ in each cycle and rejects heat at s temperuture of $350 \mathrm{~K}$. The cngine completes 240 cycles in 1 minute. What is the theorctical maximum power output of yoat engine, in borsepower?

Ravindra Yadav

Numerade Educator

03:18

Problem 24

(a) Sbow that the efficiency e of a Carnot cngine and the coefficient of performance $K$ of a Camot refrigerator are related by $K=(1-e) / e$. The engine and refrigerntor operute between the same bot and cold reservoirs. (b) What is $K$ for the limiting values $e \rightarrow 1$ and $e \rightarrow 0 ?$ Rxplain.

Khoobchandra Agrawal

Numerade Educator

02:24

Problem 25

A sophomore with nothing bctter to do adds heat to $0350 \mathrm{~kg}$ of ice at $0.0^{\circ} \mathrm{C}$ until it is all melted. (a) What is the change in entropy of the water? (b) The source of heat is a very massive body at a temperature of $25.0^{\circ} \mathrm{C}$. What is the change in entropy of this body? (c) What is the total change in entropy of the water and the heat source?

Hubert Agamasu

Numerade Educator

08:26

Problem 26

You decide to take a nice bot bath but discover thut your. thoughtless rocmimate has used up most of the hot water. You fill the tub with $270 \mathrm{~kg}$ of $30.0^{\circ} \mathrm{C}$ watet and attempt to warm it further by pouring in $5.00 \mathrm{~kg}$ of boiling water from the stove. (a) Is this u reversible or an irreversible frocess? Use physical reasoning to explain.
(b) Calculate the final temperafure of the bath water. (c) Calculate the net change in entropy of the system (bath water + boiling water), nssuming no heat exchange with the air or the tub itself.

Eric Mockensturm

Numerade Educator

06:29

Problem 27

A $15.0 \mathrm{~kg}$ block of ice at $0.0^{\circ} \mathrm{C}$ melts to liquid water at $0.0^{\circ} \mathrm{C}$ inside a large mom that has a temperature of $20.0^{\circ} \mathrm{C}$. Treat the ice and the room as an isolated system, and assume that the room is large enough for its temperuture change to be ignored. (a) Is the melting of the ice reversible ot irreversible? Explain, using simple plysical reasoning witbout resorting to any equations. (b) Calculete the net entropy change of the system during this process. Explain whether or not this result is consistent with yoar answer to part (n).

Shital Rijal

Numerade Educator

03:59

Problem 28

You make ten with $0.250 \mathrm{~kg}$ of $85.0^{\circ} \mathrm{C}$ water and let it cool to room temperature $\left(20.0^{\circ} \mathrm{C}\right)$ before drinking it. (s) Calculate the cntropy change of tho water while it cools. (b) The cooling process is essentially isothermal for the sir in your kitchen. Calculate the change in entropy of the air while the tea cools, assuming that all the heat lost by the water goes into the alir. What is the total entropy change of the system tea $+$ air?

Katie Mcalpine

Numerade Educator

02:41

Problem 29

Three moles of an ideal gas undergo a reversible isothermsi compression at $20.0^{\circ} \mathrm{C}$. During this compression, $1850 \mathrm{~J}$ of work is done on the gas. What is the change of entropy of the gas?

Shital Rijal

Numerade Educator

01:27

Problem 30

What is the change in entropy of $0.130 \mathrm{~kg}$ of helium gas at the normal boiling point of helium when it all condenses isothermally to $1.00 \mathrm{~L}$ of hiquid helium? (Hint: See Table $17.4$ in Section 17.6.)

Katie Mcalpine

Numerade Educator

01:24

Problem 31

(a) Calculate the change in entropy when $1.00 \mathrm{~kg}$ of water at $1009 \mathrm{C}$ is vaporized and converted to steam et $100^{-} \mathrm{C}$ (see Table $17.4$ ). (b) Compare yoar answer to the change in entropy when $1.00 \mathrm{~kg}$ of ice is melted at $0{ }^{\circ} \mathrm{C}$, calculated in Example $20.5$ (Section 20.7). Is the change in entropy greiter for melting or for vaporization? Interpret your answer using the idea that entropy is s measure of the randomness of a systern.

Manne Andergronde

Numerade Educator