physics solutions-3

8/13/2019 Physics solutions-3

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Chapter 12

The Laws of Thermodynamics

Quick Quizzes

1. (b). The work done on a gas during a thermodynamic process is thenegativeof the areaunder the curve on a PVdiagram. Processes in which the volume decreases do positivework on the gas, while processes in which the volume increases do negative work on thegas. The work done on the gas in each of the four processes shown is:

, W , W , and

Thus, the correct ranking (from most negative to most positive) is a,c,b,d.

54.00 10 JaW = 53.00 10 Jb = +

53.00 10 Jc= 54.00 10 JdW = +

2. Ais isovolumetric, Bis adiabatic, Cis isothermal, Dis isobaric.

3. (c). The highest theoretical efficiency of an engine is the Carnot efficiency given by1c ce T= hT . Hence, the theoretically possible efficiencies of the given engines are:

A

700 K1 0

1 000 Ke = = .300 ,

B

500 K1 0.375

800 Ke = = , and

C

300 K1 0

600 Ke = = .500

and the correct ranking (from highest to lowest) is C, B, A.

4. (b). 0rQ

ST

= = and Q in an adiabatic process. If the process was reversible, but not

adiabatic, the entropy of the system could undergo a non-zero change. However, in thatcase, the entropy of the systems surroundings would undergo a change of equalmagnitude but opposite sign, and the total change of entropy in the universe would bezero. If the process was irreversible, the total entropy of the universe would increase.

0=

5. The number 7 is the most probable outcome because there are six ways this could occur:1-6, 2-5, 3-4, 4-3, 5-2, and 6-1. The numbers 2 and 12 are the least probable because theycould only occur one way each: either 1-1, or 6-6. Thus, you are six times more likely tothrow a 7 than a 2 or 12.

431


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432 CHAPTER 12

Answers to Even Numbered Conceptual Questions

2. Either statement can be considered an instructive analogy. We choose to take the firstview. All processes require energy, either as energy content or as energy input. The kinetic

energy which it possessed at its formation continues to make Earth go around. Energyreleased by nuclear reactions in the core of the Sun drives weather on Earth andessentially all processes in the biosphere. The energy intensity of sunlight controls howlush a forest or jungle can be and how warm a planet is. Continuous energy input is notrequired for the motion of the planet. Continuous energy input is required for life becauseenergy tends to be continuously degraded, as energy is transferred by heat into lower-temperature sinks. The continuously increasing entropy of the Universe is the index toenergy-transfers completed.

4. Shaking opens up spaces between the jelly beans. The smaller ones have a chance offalling down into spaces below them. The accumulation of larger ones on top and smallerones on the bottom implies an increase in order and a decrease in one contribution to the

total entropy. However, the second law is not violated and the total entropy of the systemincreases. The increase in the internal energy of the system comes from the work requiredto shake the jar of beans (that is, work your muscles must do, with an increase in entropyaccompanying the biological process) and also from the small loss of gravitationalpotential energy as the beans settle together more compactly.

6. Temperature = A measure of molecular motion. Heat = the process through which energyis transferred between objects by means of random collisions of molecules. Internal energy= energy associated with random molecular motions plus chemical energy, strainpotential energy, and an objects other energy not associated with center of mass motionor location.

8. A higher steam temperature means that more energy can be extracted from the steam. For

a constant temperature heat sink at and steam at T , the maximum efficiency of the

power plant goes as

cT h

1h c

h h

T T T

T T

= c and is maximized for high .hT

10. max80 K

22%373 Kh

Te

T

= = (Assumes atmospheric temperature of 20C.)

12. An analogy due to Carnot is instructive: A waterfall continuously converts mechanicalenergy into internal energy. It continuously creates entropy as the organized motion of thefalling water turns into disorganized molecular motion. We humans put turbines into thewaterfall, diverting some of the energy stream to our use. Water flows spontaneouslyfrom high to low elevation and energy is transferred spontaneously from high to low

temperature by heat. Into the great flow of solar radiation from Sun to Earth, living thingsput themselves. They live on energy flow. A basking snake diverts high-temperatureenergy through itself temporarily, before it is inevitably lost as low-temperature energyradiated into outer space. A tree builds organized cellulose molecules and we buildlibraries and babies who look like their grandmothers, all out of a thin diverted stream inthe universal flow of energy crashing down to disorder. We do not violate the second law,for we build local reductions in the entropy of one thing within the inexorable increase in


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The Laws of Thermodynamics 433

the total entropy of the Universe. Your roommates exercise increases random molecularmotions within the room.

14. Even at essentially constant temperature, energy must be transferred by heat out of thesolidifying sugar into the surroundings. This action will increase the entropy of theenvironment. The water molecules become less ordered as they leave the liquid in the

container to mix with the entire atmosphere.16. A slice of hot pizza cools off. Road friction brings a skidding car to a stop. A cup falls to

the floor and shatters. Any process is irreversible if it looks funny or frightening whenshown in a videotape running backward. At fairly low speeds, air resistance is small andthe flight of a projectile is nearly reversible.


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434 CHAPTER 12

Answers to Even Numbered Problems

2. (a) 610 J (b) 0 (c) 410 J(d) 0 (e) 200 J

4. (a) 31 m s (b) 0.17

6. (c) More work is done in (a) because of higher pressure during the expansion.

8. -465 J

10. (a) 12.0 MJ (b) 12.0 MJ

12. 0 06P V

14. 0, 0U Q W =

16. (a)12 kJ (b)12 kJ

18. >0,


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40. (a)

End Result Possible Draws

Total Number of

Same Result

All R RRR 1

1G, 2R RRG, RGR, GRR 3

2G, 1R GGR, GRG, RGG 3

All G GGG 1

(b)


Total Number

of Same Result

All R RRRRR 1

1G, 4R RRRRG, RRRGR, RRGRR, RGRRR, GRRRR 5

2G, 3R RRRGG, RRGRG, RGRRG, GRRRG, RRGGR,

RGRGR, GRRGR, RGGRR, GRGRR, GGRRR

10

3G, 2R GGGRR, GGRGR, GRGGR, RGGGR, GGRRG,

GRGRG, RGGRG, GRRGG, RGRGG, RRGGG

10

4G, 1R GGGGR, GGGRG, GGRGG, GRGGG, RGGGG 5

All G GGGGG 1

42. (a) 1 52 (b) 1 1 3 (c) 1 4

44. 3.0

46. 16.0 J K, 26.7 J K, 10.7 J Kh c UniverseS S S = = =

48. 0.55 kg

50. (a) 251 J (b) 314 J (c) 104 J by the gas(d) 104 J on the gas (e) zero in both cases

52. 45.97 10 kg s

54. (a) (b) (c) 9.07 kJ0 04P V 0 04P V

56. (a) (b) 16 kJ (c) 16 kJ24.9 10 J

58. (a) 021

2 nRT (b)0

17

2 nRT

(c) 0.190 (or 19.0%) (d) 0.833 (or 83.3%)

60. (a) (b)335 J+ 32.09 10 J

62. (a) (b) (c) 2.862.4 10 J 61.6 10 J 210 J


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436 CHAPTER 12

Problem Solutions

12.1 From kinetic theory, the average kinetic energy per molecule is

B

A

3 32 2

molecule RKE k T T N

= =

For a monatomic ideal gas containing Nmolecules, the total energy associated withrandom molecular motions is

A

3 3

2 2molecule

NU N KE RT nRT

N

= = =

PV nRT=

Since for an ideal gas, the internal energy of a monatomic ideal gas is found

to be given by 32

U PV= .

12.2 (a) ( )

( ) ( )3 3

5 10 m3 1.013 10 Pa 3.0 L 1.0 L 610 J1 L

ab a b aV V

=

= =

W P

(b) ( ) 0bc c bW P V V = =

(c) ( )

( ) ( )3 3

5 10 m2 1.013 10 Pa 1.0 L 3.0 L 410 J1 L

cd c d cV V

=

= =

W P

(d) ( ) 0da a dV V= =W P

(e) 610 J 0 410 J 0 200 Jnet ab bc cd daW W W= + + + = + + + = +W W


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12.3 The number of molecules in the gas is AN nN= and the total internal energy is

( )

( ) ( )

A B

4

3 3

2 2

3 J3.0 mol 8.31 1.1 10 J2 m

U N KE nN k T

= = =

303 K

ol K

nRT

= =

Alternatively, use the result of Problem 12.1,

3 3

2 2U PV nRT = = , just as found above.

12.4 (a) The work done bythe gas on the projectile is given by the area under the curve inthe PVdiagram. This is

( ) ( )

( )( ) ( ) ( )( )

( ) ( )

by gas

0 0 0 0

35 3

6 3

triangular area rectangular area

1 1

2 2

1 111 1.0 10 Pa 40.0 8.0 cm 19 J

2 1

f f f f f f

W

P P V V P V V P P V V

= +

= + = +

= + =

0

m

0 cm

From the work-energy theorem, 21

02

E mvW K= = where Wis the work done

on the projectile by the gas. Thus, the speed of the emerging projectile is

( )3

2 2 19 J31 m s

40.0 10 kg

Wv

m = = =

(b) The air in front of the projectile would exert a retarding force of

( ) ( )( )5 2 2 4 21.0 10 Pa 1.0 cm 1 m 10 cm 10 Nr airF P A = = =

(

on the projectile as it moves down the launch tube. The energy spent overcoming

this retarding force would be

)( )10 N 0.32 m 3.2 Jspent rW F s= = =

and the needed fraction is3.2 J

0.1719 J

spentW

W= =


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438 CHAPTER 12

12.5 In each case, the work done onthe gas is given by the negative of the area under thepath on the PVdiagram. Along those parts of the path where volume is constant, no

work is done. Note that 1 .5 3atm 1.013 10 Pa and 1 Liter 10 m= = 3

(a) ( )

( ) ( )5 3 3

0

4.00 1.013 10 Pa 4.00 2.00 10 m 810 J

IAF IA AF I A I W P V V

= + = +

= =

W W

(b) ( ) ( )

( )( ) ( ) ( )( )

( )( ) ( )5 3

triangular area rectangular area

1 1

2 2

14.00 1.00 1.013 10 Pa 4.00 2.00 10 m

2

507 J

IF

I B F B B F B I B F BP P V V P V V P P V V

= = +

= +

=

3

W =

(c) ( )

( ) ( )5 3 3

0

1.00 1.013 10 Pa 4.00 2.00 10 m 203 J

IBF IB BF B F I W P V V

= + =

= =

W W

12.6 The sketches for (a) and (b) are shown below:

(c) As seen from the areas under the paths in the PVdiagrams above, the higherpressure during the expansion phase of the process results in more work done bythe gas in (a) than in (b).

12.7 The constant pressure is ( )( )5 51.5 atm 1.013 10 Pa atm 1.52 10 PaP= = (

and the work

done on the gas is )W P .V=

(a) = and34.0 mV +

( ) ( )( )5 31.52 10 Pa 4.0 m 6.1 10 J + = 5W P V= =


9/34


(b) = , so33.0 mV

( ) ( )( )5 31.52 10 Pa 3.0 m 4.6 10 J = + 5W P V= =

12.8 As the temperature increases, while pressure is held constant, the volume increases by

( )f if i

nRT nR TnRTV V V

P P P

= = =

where the change in absolute temperature is C 280 KT T = = . The work done on the

gas is

( ) ( ) ( ) ( ) ( )0.200 mol 8.31 J mol K 280 K 465 JW P V nR T = = = =

12.9 (a) From the ideal gas law, f f inR PV T PV T i= = . With pressure constant this gives

( ) ( ) 3273 K 4 1.09 10 Kf

f i

i

VT T

V

= = =

(b) The work done on the gas is

( ) ( ) ( )

( )( )( )3

1.00 mol 8.31 J mol K 1 092 J 273 K

6.81 10 J 6.81 kJ

f i f iW P V PV PV nR T T = = =

=

= =

12.10 (a) The work done on the fluid is the negative of the area under the curve on the PVdiagram. Thus,

( )( ){

( ) ( )

( )( ) }

6 3

6 3

6 3

6.00 10 Pa 2.00 1.00 m

1 6.00 2.00 10 Pa 2.00 1.00 m

2

2.00 10 Pa 4.00 2.00 m

ifW =

+

+

71.20 10 J 12.0 MJifW = =


10/34

440 CHAPTER 12

(b) When the system follows the process curve in the reverse direction, the work doneon the fluid is the negative of that computed in (a), or

12.0 MJfi ifW W= = +

12.11 (a) Because the volume is held constant, 0W= . Energy is transferred by heatfromthe

burning mixture, so 0Q< . The first law then gives U Q W Q = + = , so 0U < .

(b) Again, since volume is constant, 0W= . Energy is transferred by heat from the

burning mixture to the water, so 0Q> . Then, U Q W Q = + = gives 0U > .

12.12 The work done on the gas is the negative of the area under the curve on the PVdiagram,or

( ) ( )( )0 0 0 0 0 0 0 01 3

2 2 22 2

W P V V P P V V P V

= + = 0

From the result of Problem 1,

( )( )0 0 0 0 0 03 3 3 3 9

2 22 2 2 2 2

f f i iU P V PV P V P V P V = = =

Thus, from the first law, 0 0 0 09 32 2W P V P V

= = 0 06P V=Q U

12.13 (a) ( ) ( )( )5 3 31.013 10 Pa 10 m

0.800 atm 7.00 L 567 J1 atm 1 L

V

= = = W P

(b) 400 J+567 J 167 JU Q W+ = = =


11/34


12.14 The work done on the gas is the negative of the area under the curve on the PVdiagram,

so ( ) ( )( )0 0 0 0 0 0 0 0 01 3

2 2 22 2

W P V V P P V V P

= + = + V , or 0W>

From the result of Problem 1,

( )( ) ( )( )0 0 0 03 3 3 3

2 22 2 2 2

f f i iU P V PV P V P V = = = 0

Then, from the first law, 0 0 0 03 3

02 2

W P V P V = = = Q U , or 0Q<

12.15 (a) Along the direct path IF, the work done on the gas is

( )

( ) ( ) ( )( )

( )5 3 3

11.00 atm 4.00 L 2.00 L 4.00 atm 1.00 atm 4.00 L 2.00 L

2

1.013 10 Pa 10 m5.00 atm L 506.5 J

1 atm 1 L

W area under curve

W

=

= +

= =

Thus, 418 J 506.5 J= 88.5 JU Q W = + =

(b) Along path IAF, the work done on the gas is

( )( )5 3 31.013 10 Pa 10 m

4.00 atm 4.00 L 2.00 L 810 J1 atm 1 L

W

= =

From the first law, ( )88.5 J 810 J 722 JW= = =Q U


12/34

442 CHAPTER 12

12.16 (a) In a cyclic process, and the first law gives

, or Q W

0U =

0U Q W = + = =

The total work done on the gas is ABC AB BC CAW WW W= + + , and on each step the

work is the negative of the area under the curve on the PVdiagram, or

( )( )

( )

3 3m

2.0 kPa 10 m 6.0 m+ ( )3 3

.0

kPa 20 kJ

=

2.0 kP

ABW =

0BCW =

a 10 m

18.00

2

(

6

, and )( )3 3.0 m 10 m 8.0 kJ =

12 kJ

2.0cA

0 8+ +

kPa

.0 kJ

6 +

=

W

20 kJABC =

Thus, W , and 12= = kJQ W

(b) If the cycle is reversed,

( )12 kJ 12 kJCBA ABCW W= = = and 12 kJ= = Q W

12.17 (a) The change in the volume occupied by the gas is

( ) ( )( )2 20.150 m 0.200 m 3.00 10 mf i f iV V V A L L = = = =

(

3

and the work done by the gas is

) ( ) ( )2 3by gas 6 000 Pa 3.00 10 m 180 JW P V = + = =

(b) The first law of thermodynamics is input on gas output by gasU Q W Q W = + = . Thus, if

, the energy transferred out of the system by heat is8.00 JU =

( ) ( )output= by gas 8.00 JQ U W = 180 J 188 J = +

12.18 Volume is constant in process BC, so 0BCW = . Given that 0BCQ < , the first law shows

that . Thus,0BC BC BC BCU Q W Q = + = + 0BCU < .

For process CA, , so0 . Then, given

that 0CAU < , the first law gives Q UCA WCA CA= and 0CAQ < .


13/34


In processAB, the work done on the system is ( )area under curveW AB= where

( ) ( ) ( )( ) 0B A B AP P V V 1

area under curve2

A B AAB P V V= + >

Hence, 0ABW < . For the cyclic process, 0AB BC CAU U U U =

( + + = , so,

)AB BC CAU U U + = . This gives 0ABU > , since both andBC CAU U are negative.

Finally, from the first law, Q U W= shows that 0ABQ > since both

and WAB ABU are positive.

12.19 (a) ( )

( ) ( )3

5 3 3

6 3

1 m1.013 10 Pa 1.09 cm 1.00 cm 9.12 10 J

10 cm

V

=

= =

3

W P

(b) To freeze the water, the required energy transfer by heat is

( )( )3 51.00 10 kg 3.33 10 J kg 333 JfQ mL = = =

The first law then gives

3333 J 9.12 10 J 333 JU Q W = + = =

12.20 Treating the air as an ideal gas at constant pressure, the final volume is

( )f i f iV V T T = , or the change in volume is

( )3 3

5 310 m 310 K 273 K0.600 L 8.13 10 m1 L 273 K

f i

f i i

i

T TV V V V

T

= =

= =

(a) The work done on the lungs bythe air is

( ) ( )( )5 5 3by gas 1.013 10 Pa 8.13 10 m 8.24 JW P V = + = =

(b) Using the result of Problem 1, the change in the internal energy of the air is

( ) ( )( )5 5 33 3

1.013 10 Pa 8.13 10 m 12.4 J2 2

U P V = = =


14/34

444 CHAPTER 12

(c) The energy added to the air by heat is

( ) ( ) ( )3 5

2 2Q U W P V P V P V = = =

or, ( )( )5 5 35 1.013 10 Pa 8.13 10 m 20.6 J2= =Q

12.21 (a) The original volume of the aluminum is

3 30 3 3

5.0 kg1.85 10 m

2.70 10 kg m

mV

= = =

and the change in volume is ( ) ( ) ( )0 03 V TV V T =

( )

= , or

( )( )1

6 33 24 10 C 1.85 10 70 CV

=

(

6 3

9.3 10= m

The work done bythe aluminum is then

) ( )( )5 63 10 Pa 9.3 10 m 0 =3by system 1.01 .95W P V= + = J

(b) The energy transferred by heat to the aluminum is

( ) ( ) ( ) ( ) 5Al 5.0 kg 900 J kg C 70 C 3.2 10 JQ mc T = = =

(c) The work done on the aluminum is W Wby system 0.95 J= = , so the first law gives

5 5= 3.2 10 J3.2 10 J 0.95 JU Q W = + =

12.22 (a) The work done on the gas in each process is the negative of the area under theprocess curve on the PVdiagram.

For path IAF, W W , or0IAF IA AF AFW W= + = +

( ) ( )

3 3

5 Pa 10 m1.50 atm 1.013 10 0.500 Latm 1 L

76.0 J

IAFW

=

=


15/34


For path IBF, W W , or0IBF IB BF IBW W= + = +

( ) ( )3 3Pa 10 m

0.500 Latm 1 L

IBFW

52.00 atm 1.013 10

101 J

=

=

For path IF, ( )IF AFW W triangular= area , or

( ) ( )3 3

51 Pa76. 0 0.500 L2 atm

IFW10 m

1 L0 J 0.500 atm 1.013 1

88.7 J

=

=

(b) Using the first law, with ( )180 91.0 J 89.0 JF AU U U = = = , for each processgives

( )89IAF IAFQ U W= = .0 J 76.0 J 165 J =

( )89IBF IBFQ U W= = .0 J 101 J 190 J =

( )89.0IF IFQ U W= = J 88.7 J 178 J =

12.23 The maximum efficiency possible is that of a Carnot engine operating between the givenreservoirs.

( )293 K

1 1 0.489 or 48.9%573 K

h c cc

h h

T T Te

T T

= = = =

12.24 The maximum possible efficiency of an engine is the Carnot efficiency,

1h cch h

T T Te

T T

= =

300 F 149 C 422 = =

ccT, where are the absolute temperatures of the reservoirs the

engine operates between. For the given engine, the temperatures of the reservoirs are

and 15 so the maximum efficiency is

andhT

0 F 65. =K 6 C 339 K =

339 K1 0.

422 Kce == = 197 or 19.7%


16/34

446 CHAPTER 12

12.25 (a)eng eng

eng

10.333 or 33.3%

3 3h

W W

Q W = = =e

(b)eng

1 c

h h

W Q

Q Qe = , so

1 21 1

3 3c

h

Qe

Q= = =

12.26 (a) Fromeng

1 c

h h

W Q

Q Qe = , the energy intake each cycle is

8 000 J10 667 J 10.7 kJ

1 1 0.250c

h

QQ

e= = = =

(b) Fromeng c

W e Q

t t= = , the time for one cycle is

( ) ( )3

0.250 10 667 J0.533 s

5.00 10 W

ce Qt= = =

12.27 (a) The maximum efficiency possible is that of a Carnot engine operating between thespecified reservoirs.

( )703 K

1 1 0.672 or 67.2%2 143 K

h c cc

h h

T T Te

T T

= = = =

(b) Fromeng

h

WQ

e= , we find ( )5 4eng 0.420 1.40 10 J 5.88 10 JhQ= = = W e

so4

eng 45.88 10 J 5.88 10 W 58.8 kW1.00 s

W

t

= = = =


17/34


12.28 The work done by the engine equals the change in the kinetic energy of the bullet, or

( )( )22 3

eng

1 10 2.40 10 kg 320 m s 123 J

2 2b fW m v

= = =

Since the efficiency of an engine may be written as eng engengh c

W WeQ W Q= = +

where cQ is

the exhaust energy from the engine, we find that

( ) 4eng1 1

1 123 J 1 1.10 10 J0.0110

cQ We

= = =

This exhaust energy is absorbed by the 1.80-kg iron body of the gun, so the rise intemperature is

( ) ( )

41.10 10 J

13.7 C1.80 kg 448 J kg C

c

gun iron

Q

T m c

= = =

12.29 (a) ( )eng 1 200 J

1 1 0.294 or 29.4%1700 J

c

h h

W Q

Q Q = = =e

(b) eng 1 700 J 1 200 J 500 Jh cQ= = =W Q

(c)eng 3500 J 1.67 10 W 1.67 kW

0.300 s

W

t= = = =

12.30 (a) The Carnot efficiency represents the maximum possible efficiency. With, this efficiency is given by20.0 C 293 K and 5.00 C 278 Kh cT T= = = =

( )278 K

1 1 0.0512 or 5.12%293 K

cc

h

Te

T= = =

(b) The efficiency of an engine is eng hQ=e W , so the minimum energy input by heat

each hour is

( )( )6

eng 12

minmax max

75.0 10 J s 3 600 s 5.27 10 J0.0512

hW tQe e

= = = = P

(c) As fossil-fuel prices rise, this could be an attractive way to use solar energy.However, the potential environmental impact of such an engine would require

serious study. The energy output, ( )eng 1c h hQ Q W Q e= = , to the low

temperature reservoir (cool water deep in the ocean) could raise the temperature ofover a million cubic meters of water by 1 C every hour.


18/34

448 CHAPTER 12

12.31 The actual efficiency of the engine is300 J

1 1 0.500 J

c

h

Q

Q= = = 400e

If this is 60.0% of the Carnot efficiency, then0.400 2

0.600 0.600 3c

ee = = =

Thus, from 1 cch

Te

T= , we find

2 11 1

3 3ce = =

c

h

T

T=

12.32 (a) The Carnot efficiency is353 K

1 1 0.433623 K

cc

h

Te

T= = = , so the maximum power

output is

( ) ( )eng maxmax

0.433 21.0 kJ

9.10 kW1.00 s

c hW e Q

t t= = = =

(b) From 1 c

h

Q

Qe= , the energy expelled by heat each cycle is

( ) ( ) ( )1 21.0 kJ 1 0.433 11.9 kJc hQ Q e= = =

12.33 Fromeng h

W e Q

t t= = , the energy input by heat in time tis h

tQ

e

=

Thus, from h

h

Q Qe

Q

= c , the energy expelled in time tis

( ) ( )1

1 1c ht

Q Q e e te e

= = =

1


19/34


In time t, the mass of cooling water used is ( )61.0 10 kg sm t= , and its rise intemperature is

( )

( )( )( )

6

6

6

11

1.0 10 kg s

1 000 10 J s 11

0.331.0 10 kg s 4 186 J kg C

cQ tTmc et c

= =

=

or 0.49 CT =

12.34 The actual efficiency of the engine is

( ) 21eng 2actual train actualactualh h

W m ve

Q Q= =

(

while the theoretically possible efficiency (the Carnot efficiency) is

) 21eng2theoretical train theoretical

c

h h

W m ve

Q Q= =

The energy input from the high temperature reservoir is the same in the two cases sinceit is specified that the same amount of fuel is consumed in both cases. Thus, we find

( )( )

2eng

eng

c theoretical theoretical

actual actualactual

We v

e vW

= =

or ( )

226.50 m s

0.200 0.3385.00 m s

theoreticalc actual

actual

ve e

v

= =

=

But, we also know that 1 cch

Te

T= , giving

300 K453 K

1 1 0.338c

h

c

TT

e= = =

12.35 The energy transferred from the water by heat, and absorbed by the freezer, is

( ) ( )( )3 3 3 3 5J

10 kg m 1.0 10 m 3.33 10 3.3 10 Jkg

f fQ mL V L = = = =

5

Thus, the change in entropy of the water is


20/34

450 CHAPTER 12

(a)( ) 5 33.3 10 J J1.2 10 1.2 kJ K

273 K K

r waterwater

QS

T

= = = =

and that of the freezer is

(b) ( )53.3 10 J 1.2 kJ K

273 K

r freezer

freezerQS

T + = = + =

12.36 The energy added to the water by heat is

( ) ( )6 61.00 kg 2.26 10 J kg 2.26 10 Jr vQ mL = = =

so the change in entropy is

632.26 10 J J

6.06 10 6.06 kJ K373 K KrQ

S T

= = = =

12.37 The potential energy lost by the log is transferred away by heat, so

( ) ( )( )2 470 kg 9.80 m s 25 m 1.7 10 JQ mgh= = =

and the change in entropy is41.7 10 J

57 J K300 K

rQST

= = =

12.38 The total momentum before collision is zero, so the combined mass must be at rest afterthe collision. The energy dissipated by heat equals the total initial kinetic energy, of

( )( )22 512 2 000 kg 20 m s 8.0 10 J 800 kJ

2Q mv

= = = =

The change in entropy is then800 kJ

2.7 kJ K296 K

rQST

= = =


21/34


12.39 A quantity of energy, of magnitude Q, is transferred from the Sun and added to Earth.

Thus, SunSun

QS

T

= and Earth

Earth

QS

T

+ = , so the total change in entropy is

( )1 1

1 000 J 3.27 J K290 K 5 700 K

total Earth Sun

Earth Sun

Q QS S S

T T = + =

= =

12.40 (a)


Total Number

of Same Result

All R RRR 1

1G, 2R RRG, RGR, GRR 3

2G, 1R GGR, GRG, RGG 3

All G GGG 1

(b)


Total Number

of Same Result

All R RRRRR 1

1G, 4R RRRRG, RRRGR, RRGRR, RGRRR, GRRRR 5

2G, 3R RRRGG, RRGRG, RGRRG, GRRRG, RRGGR,

RGRGR, GRRGR, RGGRR, GRGRR, GGRRR

10

3G, 2R GGGRR, GGRGR, GRGGR, RGGGR, GGRRG,

GRGRG, RGGRG, GRRGG, RGRGG, RRGGG

10

4G, 1R GGGGR, GGGRG, GGRGG, GRGGG, RGGGG 5

All G GGGGG 1


22/34

452 CHAPTER 12

12.41 (a) The table is shown below. On the basis of the table, the most probable result of a

toss is 2 H and 2 T .

End Result Possible Tosses

Total Number

of Same Result

All H HHHH 1

1T, 3H HHHT, HHTH, HTHH, THHH 4

2T, 2H HHTT, HTHT, THHT, HTTH, THTH, TTHH 6

3T, 1H TTTH, TTHT, THTT, HTTT 4

All T TTTT 1

(b) The most ordered state is the least likely. This is seen to be all H or all T .

(c) The least ordered state is the most likely. This is seen to be 2H and 2T .

12.42 (a) There is only one ace of spades out of 52 cards, so the probability is1

52

(b) There are four aces out of 52 cards, so the probability is4 1

52 13=

(c) There are 13 spades out of 52 cards, so the probability is13 1

52 4=

12.43 The maximum efficiency is that of a Carnot engine and is given by

100 K1 1 0.

200 Kc

c

h

Te

T= = = 50 , or max 50%e = . The claim is invalid


23/34


12.44 Operating between reservoirs having temperatures of 100 C 373 KhT = = and

, the theoretical efficiency of a Carnot engine is20 C 293 KcT = =

293 K1 0.21

373 K= =1 cc

h

Te

T=

If the temperature of the hotter reservoir is changed to 550 C 823 KhT = = , the

theoretical efficiency of the Carnot engine increases to

293 K1 0.64

823 K= =1 cc

h

Te

T=

The factor by which the efficiency has increased is

0.643.0

0.21c

c

e

e

= =

12.45 (a) The entropy change of the hot reservoir, with an energy output of magnitude hQ ,

is

hrh

h h

QQS

T T

= =

(b) For the cold reservoir, with an energy input of magnitude cQ , the change in

entropy is

crc

c c

QQS

T T

+ = =

(c) The engine has an energy input of magnitude hQ from a reservoir at temperature

and an energy output of magnitudehT cQ to a reservoir at temperature T . The

net change in entropy for the engine isc

eng

h cr

h c

Q QQS

T T T

= =

(d) For the isolated system consisting of the engine and the two reservoirs, the changein entropy is

eng 0h h c c

isolated h csystem h h c c

Q Q Q QS S S S

T T T T

= + + = + + =


24/34

454 CHAPTER 12

12.46 In this case, 8 000 Jh cQ Q= = . The change in entropy of the hot reservoir is

8 000 J16.0 J K

500 Kh

h

h

QS

T

= = =

For the cold reservoir, 8 000 J 26.7 J K300 Kcc

c

QS T

+ = = =

The net entropy change for this irreversible process is

( )16.0+26.7 J K 10.7 J K 0universe h cS S S = + = = >

12.47 The energy output to the river each minute has magnitude

( ) ( ) ( ) ( )

8 1J

1 1 1 0.30 25 10 60 s 1.05 10 Js

h

c h

Q

Q e Q e tt

= = = =

69.0 10 kg

1

so the rise in temperature of the of cooling water used in one minute is

( )( )

11

6

1.05 10 J= 2.8 C

9.0 10 kg 4 186 J kg C

cQTmc

= =

12.48 The energy exhausted from a heat engine is

eng

eng eng eng1 1c h

WQ Q W W W e e

= = =

h

where Q is the energy input from the high temperature reservoir, engW is the useful

work done, and eng he W Q= is the efficiency of the engine.

For a Carnot engine, the efficiency is ( )1c c h h ce T T T T = = hT

so we now have eng eng1h c

h c h c

T TW

T T T T

= =

cQ W


25/34


Thus, if T , the energy exhausted when the engine

has done of work is

100 C 373 K and 20 C=293 Kh cT= = = 45.0 10 J

( )4 510 J293 K

5.0 10 J 1.83373 K 293 K

cQ

= =

The mass of ice (at 0C) this exhaust energy could melt is

5

5, water

1.83 10 J0.55 kg

3.33 10 J kgc

f

Qm

L

= = =

12.49 The work output from the engine in an interval of one second is W . Since

the efficiency of an engine may be expressed as

eng 1500 kJ=

eng eng

engh c

W We Q W Q= = +

the exhaust energy each second is ( ) 3eng 1 1 500 kJ 10 kcQ We

= =

1 11 4.5

0.25

=

J

The mass of water flowing through the cooling coils each second is

( )( )( )3 3 3 310 kg m 60 L 10 m 1 L 60 kgm V = = =

so the rise in the temperature of the water is

( ) ( )

64.5 10 J18 C

60 kg 4 186 J kg C

c

water

QT

mc

= = =

12.50 (a) From the first law, ( )1 3 123 123 418 J 167 J 251 JU Q W = + = + + =

(b) The difference in internal energy between states 1 and 3 is independent of the pathused to get from state 1 to state 3.

Thus, ,

and

1 3 143 143 251 JU Q W = + =

( )143 143251 J 251 JQ W= = 63.0 J 314 J =


26/34

456 CHAPTER 12

(c) ( ) ( )12341 123 341 123 143 167 J 63.0 J 104 JW W W= + = + = = W W

or 104 J of work is done the gasby in the cyclic process 12341.

(d) ( ) ( )14321 143 321 143 123 63.0 J 167 J 104 JW W W= + = + = = +W W

or 104 J of work is done the gason in the cyclic process 14321.

(e) The change in internal energy is zero for both parts (c) and (d) since both are

cyclic processes.

12.51 (a) The work done bythe system in processABequals the area under this curve on thePVdiagram. Thus,

( ) ( )by system triangular area rectangular areaW = + , or

( ) ( )

( ) ( )

bysystem

3 35

4

14.00 atm 40.0 L

2

Pa 10 m 1.00 atm 40.0 L 1.013 10

atm L

1.22 10 J 12.2 kJ

W

=

+

= =

bysystem

12.2 kJAB

Note that the work done on the system is W W= = for this process.

(b) The work done on the system (that is, the work input) for process BCis the negativeof the area under the curve on the PVdiagram, or

( ) ( )3 3

5 Pa 10 m1.00 atm 10.0 L 50.0 L 1.013 10atm 1 L

4.05 kJ

BCW

=

=

(c) The change in internal energy is zero for any full cycle, so the first law gives

( )

( )

0

0 12.2 kJ 4.05 kJ 0 8.15 kJ

cycle cycle cycle AB BC CAQ U W W W W = = + +

= + + =


27/34


12.52 The efficiency of the plant is300 K

1 1 0.4500 K

cc

h

Te e

T= = = = 00

Also, 1 c

h

Qe

Q= , so

1 0.600c c

h

Q QQ

e= =

Fromeng 0.400

0.600h c

W e Q Q

t t

= = =

t,

the rate of energy transfer to the river by heat is

( ) 91.50 1.50 1 000 MW 1.50 10 J scQ t= = =

The flow rate in the river is then

( )( ) ( )( )

941.50 10 J s 5.97 10 kg s

4 186 J kg C 6.00 C

c

water river

Q tm

t c T= = =

12.53 (a) The change in length, due to linear expansion, of the rod is

( ) ( ) ( )( )16 4

0 11 10 C 2.0 m 40 C 20 C 4.4 10 mL L T = = =

The load exerts a force ( )( )2 46 000 kg 9.80 m s 5.88 10 NF mg= = =

(

on the end of

the rod in the direction of movement of that end. Thus, the work done on the rod is

)( )4 45.88 10 N 4.4 10 m 26 JW F L = = =

(b) The energy added by heat is

( ) ( ) ( ) 5J

100 kg 448 20 C 9.0 10 Jkg C

Q mc T

= = =

(c) From the first law, 5 59.0 10 J+26 J 9.0 10 JU Q W = + = =

12.54 (a) The work done bythe gas during each full cycle equals the area enclosed by thecycle on the PVdiagram. Thus

( )( )by gas 0 0 0 0 0 03 3 4W P P V V P= = V


28/34

458 CHAPTER 12

(b) Since the work done on the gas is by gas 0 04P VW W= = and 0U = for any cyclic

process, the first law gives

( )0 00 4Q U W P V = = = 0 04P V

(c) From the ideal gas law, P V0 0 0nRT= , so the work done by the gas each cycle is

( ) ( )by gas 0

3

J4 4 1 1 273 K

mol K

9.07 10 J

W nRT

= =

=

.00 mol 8.3

9.07 kJ=

12.55 (a) The energy transferred to the gas by heat is

( ) ( ) ( )

3

J1.00 mol 20.79 120 Kmol K

2.49 10 J 2.49 kJ

Q mc T

= =

= =

(b) Treating the neon as an ideal gas, the result of Problem 1 gives the change ininternal energy as

( ) ( ) (3 3 3

2 2 2f f i i f iU P V PV nRT nRT nR T = = = ) , or

( ) ( ) 33 J1.00 mol 8.31 120 K 1.50 10 J 1.50 kJ2 mol K

U = = =

(c) From the first law, the work done onthe gas is

3 31.50 10 J 2.49 10 J 990 JW U Q= = =


29/34


12.56 (a) The change in volume of the aluminum is

( ) ( ) ( ) ( )0 3V V T m T = =

( )

, or

( )16 7 3

3 3

1.0 kg

3 24 10 C 18 C 4.8 10 m2.70 10 kg mV

= =

( ) (

so the work done on the aluminum is

)( )5 7 3 21.013 10 Pa 4.8 10 m 4.9 10 JW P V = = =

(b) The energy added by heat is

( ) ( ) ( ) ( ) 41.0 kg 900 J kg C 18 C 1.6 10 J 16 kJQ mc T = = = =

(c) The first law gives the change in internal energy as

4 2 41.6 10 J 4.9 10 J=1.6 10 J 16 kJU Q W = + = =

12.57 (a) The energy input by heat from the molten aluminum is

( )( )3 5Al 1.00 10 kg 3.97 10 J kg 397 Jh fQ m L = = =

and the energy output to the frozen mercury is

( )( )3 4

Hg 15.0 10 kg 1.18 10 J kg 177 Jc fQ m L

= = =

The efficiency of the heat engine is given by

eng 177 J1 1 0.554 or 55

397 Jh c c

h h h

W Q Q Qe

Q Q Q

= = = = = .4%

(b) T = and T660 C 933 Kh = 38.9 C 234 Kc= = . The Carnot efficiency for a heat

engine operating between these two reservoirs is

231 1

cc

h

Te T= =

4 K

933 K 0.749 or 74.9%=


30/34

460 CHAPTER 12

12.58 (a) From the result of Problem 1,

( ) ( )( )0 0 0 0 0 0 03 3 15 15

3 22 2 2

A C C C A AU P V P V P V P V P V n = = = =

(

2RT

The work done on the gas in processABCequals the negative of the area under theprocess curve on the PVdiagram, or

)( )0 0 0 0 0 03 2 3 3ABCW P V V P V nRT = = =

1 2ABCQ Q Q

The total energy input by heat, = + , is found from the first law as

( )0 0 015 21

32 2

ABC A C ABCQ U W nRT nRT nRT = = =

(b) For process CDA, the work done on the gas is the negative of the area under curve

CDA, or ( )( )0 0 0 0 02CDAW P V V P V n = = + = + 0RT . The change in internal energy is

0

15

2A CU U nRT = C A = . Thus, the energy inputby heat for this process is

0 0

15 17

2 2C A CDAU W nRT nRT = = = 0CDAQ nRT

The total energy outputby heat for the cycle is

3 4 0 0

17 17

2 2CDAQ nRT nRT

+ = = =

Q Q

(c) The efficiency of a heat engine using this cycle is

( )( )

0

0

17 2 171 1 1 0.190 or 19.0%

2121 2

output

input

Q nRTe

nRTQ= = = =

(d)( )( ) ( )( )

0 0

0 0

51 1 1 0.833 or 83.3%

3 2 6

A AA

C C C

P V nR P VT

T P VP V nR = = = =ce =

12.59 The mass of coal consumed in time tis given by h coalM Q Q = where hQ

is the

required energy input and Q is the heat of combustion of coal. Thus, if is the

power output and eis the efficiency of the plant,coal

( )enghcoal coal coal

W eQ tM

Q Q e Q

= =

=


31/34


(a) The coal used each day is

( )( )

( )

6

33

6 3

150 10 J s 86 400 s d

10 gcal 4.186 J0.15 7.8 10

g 1 kg 1 cal

2.6 10 kg d 2.6 10 metric ton d

coal

tM

e Q

= =

= =

(b) The annual fuel cost is: ( ) ( )cost coal used yearly rate= , or

( )

( )( )

( )

6 7

33

ton

J s 3.156 10 s yr 1 t

1010 gcal 4.186 J0

g 1 kg 1 cal

6

3

$8.0

150 10 on $8.0$7.7 10 yr

kg ton0.15 7.8 1

coal

tcost

e Q

=

= =

(c) The rate of energy transfer to the river by heat is

( )eng engeng 11

hc W e WQ WQ

t t t e

= = =

Thus, the flow required is

( ) ( )

( )( )

64

1 1

150 10 J s 11 4.1 10 kg

4186 J kg C 5.0 C 0.15

c

water water

Q tmt c T c T e

= =

= =

s

12.60 (a) The energy transfer by heat required to raise the temperature of the water to theboiling point is

( ) ( )( )( )31.00 10 kg 4 186 J kg C 100C 20.0C 335 JQ mc T = = =

We neglect the very small volume expansion (and associated work done) by thewater while in the liquid state. The first law of thermodynamics then gives thechange in internal energy as

335 J 0 335 JU Q W = + = + =


32/34

462 CHAPTER 12

(b) To completely evaporate the water, the required energy input by heat is

( )( )3 61.00 10 kg 2.26 10 J kg 2.26 10 JvQ mL = = =

( ) ( )(

3

The work done on the water in this process is

)3

5 3 3

6 3

1 m1.013 10 Pa 1 671 cm 1.00 cm 169 J

10 cmf iW P V V

= = =

so the change in internal energy is

3 32.26 10 J 169 J 2.09 10 JU Q W = + = =

12.61. (a) The work done on the gas during this isobaric expansion process is

( ) ( ) ( ) ( )( )2 28 000 Pa 0.10 m 4.0 10 m 32 JW P V P A x = = = + =

The first law of thermodynamics then gives the change in the internal energy of thesystem as

42 J 32 J 10 JU Q W = + = =

(b) If the piston is clamped in a fixed position, then 0V = and

the work done on the gas is zero

U Q

In this case, the first law gives 42W J 0 = + = + = 42 J

12.62. (a) The energy transferred from the water by heat as it cools is

( )

( ) ( )3 3

3

6

g 10 cm cal 4.186 J1.0 1.0 L 1.0 570 C 4.0 C

cm 1 L g C 1 cal

or 2.4 10 J

h

h

Q mc T V c T

Q

= =

=

=


33/34


(b) The maximum efficiency of a heat engine is the Carnot efficiency. Thus,

( )

( )

4.0 273 K 277 K1 1 1 0.

570+273 K 843 Kc

c

h

Te

T

+= = = =

(

67

The maximum useful work output is then

) ( )( )6 6engmax

0.67 2.4 10 J 1.6 10 Jc hW e Q= = =

(c) The energy available from oxidation of the hydrogen sulfide in 1.0 L of this water is

( ) ( )3 3mol J

310 kJ mol 0.90 10 1.0 L 310 10 2.8 10 JL mol

U n

= = =

2

12.63. The work that you have done is

( ) ( ) ( )2

eng

step4.448N in 2.54 10 m150 lb 90.0 30.0 min 8.00

1 lb min step 1 inW mg h

= =

5eng 3.66 10 JW =

or

If the energy input by heat was ( ) 64 186 J

600 kcal 2.51 10 J1 kcal

hQ

= = , your efficiency

has been

5

6

3.66 10 J0.146 or 14.6%

2.51 10 J

eng

h

We

Q

= = =

0.180 or 18.0%e

If the actual efficiency was = , the actual energy input was

( )5

63.66 10 J 1 kcal2.03 10 J 486 kcal0.180 4 186 J

eng

h actualactual

WQ

e

= = = =


34/34

464 CHAPTER 12

physics solutions-3

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