KFUPM-Physics102-032

Dr.Aljalal

Sec07

Sec08

Sec09

Recitation ch 21

Answers at the end

Question 1

A heat engine operates between 600 K and 300 K. In each cycle

it takes 100 J from the hot reservoir, loses 25 J to the cold

reservoir, and does 75 J of work. This heat engine violates:

a The second law but not the first law of thermodynamics.

b The first law but not the second law of the thermodynamics.

c Both, the first law and the second law of thermodynamics.

d Conservation of energy.

e Neither the first law nor the second law.

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Question 2

Five moles of an ideal monatomic gas are taken though the cycle

shown in the Figure (1). Calculate the efficiency of the cycle.

a 0.17.

b 0.06.

c 0.28.

d 0.45.

e 0.83.

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Question 3

A 4.0-kg piece of iron at 800 K is dropped into a lake

whose temperature is 280 K. Assume that the lake is so

large that its temperature rise is negligible.

Find the change in the entropy of the lake.

[specific heat of iron = 0.11 kcal/kg.K].

a zero

b + 0.20 kcal/K

c + 0.82 kcal/K

d - 0.82 kcal/K

e - 0.20 kcal/K

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Question 4

50.0 g of water at 15.0 degrees-C are converted slowly

into ice at - 15.0 degrees-C.

What is the change of entropy of water ?

specific heat of water = 4190 J/kg.K.

specific heat of ice = 2220 J/kg.K.

Lf = 333kJ/kg, Lv = 2256kJ/kg

a + 78.5 J/K

b - 78.5 J/K

c + 83.4 J/K

d - 17.5 J/K

e - 83.4 J/K

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Question 5

A Carnot refrigerator has a coefficient of performance

equal to 5. The refrigerator absorbs 120 J of heat from a

cold reservoir in each cycle.

How much heat is expelled to the hot reservoir ?

a 600 J

b 480 J

c 144 J

d 720 J

e 125 J

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Question 6

A Carnot heat engine operates between two reservoirs

whose temperatures are 27 degrees-C and 127 degrees-C.

If we want to double the efficiency of the heat engine,

what should be the temperature of the hot reservoir?

Assume the temperature of the cold reservoir is kept

constant.

a 800 K

b 600 K

c 1500 K

d 1200 K

e 900 K

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Question 7

A sample of an ideal monatomic gas undergoes

the reversible process A to B displayed in the

T-S diagram shown in figure 6. The process is :

a an isothermal compression.

b a change of phase.

c a free expansion.

d a constant-volume process.

e an isothermal expansion.

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Question 8

Five moles of an ideal gas undergo a reversible isothermal

compression from volume V to volume V/2 at temperature

30 degrees C. What is the change in the entropy of the gas?

a -18 J/K.

b 18 J/K.

c -29 J/K.

d 29 J/K.

e -81 J/K.

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Question 9

An automobile engine operates with an overall efficiency

of 20%. How many gallons of gasoline is wasted for each

10 gallons burned?

a 6.

b 12.

c 10.

d 2.

e 8.

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Question 10

A Carnot heat engine absorbs 70.0 kJ as heat and expels

55.0 kJ as heat in each cycle. If the low-temperature

reservoir is at 120 degrees-C, find the temperature of

the high-temperature reservoir.

a 35.8 degrees-C

b 153 degrees-C

c 227 degrees-C

d 450 degrees-C

e 393 degrees-C

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Question 11

One mole of an ideal gas is taken through the reversible

cycle shown in figure 1, with Q1 = 6.0 kJ, Q2 = 30 kJ,

Q3 = 18 kJ and Q4 = 10 kJ.

What is the efficiency of this cycle ?

a 0.54

b 0.29

c 0.33

d 0.22

e 0.44

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Question 12

240 grams of water at 8 degrees-C are cooled to ice at

at - 5 degrees-C. Calculate the change in entropy of the

water.

c(ice)=2090 J/kg*K, c(water)=4186 J/kg*K, Lf=3.33*10**(5) J/Kg.

a 254 J/K.

b -172 J/K.

c -254 J/K.

d -331 J/K.

e 331 J/K.

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Question 13

Which one of the following statements is WRONG?

a A refrigerator works like a heat engine in reverse.

b No heat engine has higher efficiency than Carnot efficiency.

c Thermal energy cannot be transferred spontaneously from a

cold object to a hot object.

d After a system has gone through a reversible cyclic process,

its total entropy does not change.

e The total entropy of a system increases only if it absorbs

heat.

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Answers

1 a

2 a

3 c

4 b

5 c

6 b

7 e

8 c

9 e

10 c

11 d

12 d

13 e