Question 1

Which of the following statements is true of an isolated system consisting of 15 gas molecules?

Accepted Answer

A)   According to the second law, the entropy of the gas cannot decrease.
B)   According to the second law, the entropy of the gas cannot increase.
C)   According to the second law, the entropy of the gas is not likely to decrease.
D)   According to the second law, the entropy of the gas is not likely to increase.
E)   According to the second law, the entropy of the gas must stay the same. C

Question 2

You can vary the efficiency of a Carnot engine by varying the temperature of the cold reservoir while maintaining the hot reservoir at constant temperature. The curve that best represents the efficiency of such an engine as a function of the temperature of the cold reservoir is

Accepted Answer

A)   1
B)   2
C)   3
D)   4
E)   5 D

Question 3

A heat engine operating between the temperatures T1 and T2 takes in Q1 calories at temperature T1 and gives up Q2 calories at temperature T2. The efficiency of this heat engine is

Accepted Answer

A)   (Q1 - Q2) /Q2 
B)   (Q1 - Q2) /Q1 
C)   (T2 - T1) /T2 
D)   Q2/(Q1 - Q2) 
E)   T1/(T1 - T2)  B

Question 4

Use the following diagram to answer the next problem.  
An ideal heat engine uses 0.01 mol of gas and operates between a hot reservoir at TS1U1B1HS1U1B0 = 400 K and cold reservoir at TS1U1B1LS1U1B0 = 300 K, in a cycle from a$\to$b$\to$c$\to$d$\to$a. From a$\to$b the gas undergoes an isothermal expansion, changing its volume from VS1U1B1aS1U1B0 to 2.5VS1U1B1aS1U1B0. From b$\to$c, the pressure is reduced at a constant volume. From c$\to$d, the gas undergoes an isothermal compression, and from d$\to$a, the pressure is increased at a constant volume until the gas is back at the original condition at a.
-How much work is obtained from the engine in each cycle?

Accepted Answer

A)   22.9 J
B)   30.5 J
C)   7.62 J
D)   8.31 J
E)   0.917 J

Question 5

A refrigerator has a coefficient of performance 5.0. How much heat is exhausted to the hot reservoir when 200 kJ of heat are removed from the cold reservoir?

Accepted Answer

A)   50 kJ
B)   150 kJ
C)   200 kJ
D)   240 kJ
E)   Not enough information is given to answer this question.

Question 6

A refrigerator has a coefficient of performance 4.0. How much heat is exhausted to the hot reservoir when 400 kJ of heat are removed from the cold reservoir?

Accepted Answer

A)   500 kJ
B)   400 kJ
C)   100 kJ
D)   80 kJ
E)   Not enough information is given to answer this question.

Question 7

The efficiency of a Carnot engine operating between 300º C and 250ºC is about

Accepted Answer

A)   83%
B)   17%
C)   57%
D)   43%
E)   9%

Question 8

The Carnot cycle consists of

Accepted Answer

A)   a quasistatic isothermal absorption of heat from a hot reservoir.
B)   a quasistatic adiabatic expansion to a lower temperature.
C)   a quasistatic isothermal exhaustion of heat to a cold reservoir.
D)   a quasistatic adiabatic compression to the initial state of the system.
E)   All of these are correct.

Question 9

A heat engine absorbs 64 kcal of heat from a hot reservoir and exhausts 42 kcal to a cold reservoir each cycle. Its efficiency is

Accepted Answer

A)   30%
B)   34%
C)   38%
D)   40%
E)   42%

Question 10

A heat engine absorbs 70 kcal of heat from a hot reservoir and exhausts 50 kcal to a cold reservoir each cycle. Its efficiency is

Accepted Answer

A)   20%
B)   24%
C)   29%
D)   33%
E)   37%

Question 11

A heat engine absorbs 150 J of heat from a hot reservoir and rejects 90 J to a cold reservoir. What is the efficiency of this engine?

Accepted Answer

A)   20%
B)   40%
C)   60%
D)   67%
E)   90%

Question 12

The diagram below is a schematic of a heat engine. The three quantities, QH, QL, and W are represented, respectively, by

Accepted Answer

A)   1, 2, 3
B)   1, 3, 2
C)   2, 3, 1
D)   3, 1, 2
E)   3, 2, 1

Question 13

A refrigerator with a coefficient of performance 5.0 removes 25 kJ of heat from a cold reservoir. If this refrigerator is reversible and is run backward as a heat engine, what would be the efficiency of the heat engine?

Accepted Answer

A)   50%
B)   80%
C)   83%
D)   17%
E)   20%

Question 14

Use the following diagram to answer the next problem.  
An ideal heat engine uses 0.01 mol of gas and operates between a hot reservoir at TS1U1B1HS1U1B0 = 400 K and cold reservoir at TS1U1B1LS1U1B0 = 300 K, in a cycle from a$\to$b$\to$c$\to$d$\to$a. From a$\to$b the gas undergoes an isothermal expansion, changing its volume from VS1U1B1aS1U1B0 to 2.5VS1U1B1aS1U1B0. From b$\to$c, the pressure is reduced at a constant volume. From c$\to$d, the gas undergoes an isothermal compression, and from d$\to$a, the pressure is increased at a constant volume until the gas is back at the original condition at a.
-If the engine operates at 50 cycles per second, the power output is

Accepted Answer

A)   381 W
B)   45.8 W
C)   1145 W
D)   415 W
E)   1525 W

Question 15

We wish to increase the efficiency of an ideal heat engine from 25% to 35%. If the initial temperature of the hot reservoir is 650S1U1P1$\circ$S1S1P0C, to what temperature would this need to be increased assuming the exhaust temperature remains the same?

Accepted Answer

A)   1065 ∘\circ∘ C
B)   910 ∘\circ∘ C
C)   1019 ∘\circ∘ C
D)   973 ∘\circ∘ C
E)   792 ∘\circ∘ C

Question 16

If you run a refrigerator in a closed room with the door to the refrigerator open, the temperature of the room

Accepted Answer

A)   increases.
B)   remains the same.
C)   decreases.
D)   Any of these can happen depending on how efficient the refrigerator is.
E)   Any of these can happen depending on the relative sizes of the room and the refrigerator.

Question 17

A steam power plant with an efficiency of 65% of the maximum thermodynamic efficiency operates between 250º C and 40ºC. How much heat is rejected to the cold reservoir in doing 1.0 kJ of work?

Accepted Answer

A)   3.8 kJ
B)   0.83 kJ
C)   2.8 kJ
D)   1.8 kJ
E)   0.67 kJ

Question 18

Two refrigerators, one with a COP of 4.0 and another with a COP of 5.0, both extract 400 kJ of heat from the cold reservoir (food). Calculate the difference in energy they exhaust to the hot reservoir and hence the room.

Accepted Answer

A)   320 kJ
B)   80 kJ
C)   125 kJ
D)   100 kJ
E)   20 kJ

Question 19

A heat pump needs to supply 20 kW of power to maintain the inside temperature of a house at 25S1U1P1$\circ$S1S1P0C while the outside temperature is 0S1U1P1$\circ$S1S1P0C. What is the power needed for the heat pump?

Accepted Answer

A)   3.36 kW
B)   2.42 kW
C)   2.78 kW
D)   4.12 kW
E)   1.68 kW

Question 20

A steam power plant with an efficiency of 65% of the maximum thermodynamic efficiency operates between 250º C and 40ºC. What is the change in the entropy of the universe when this plant does 1.0 kJ of work?

Accepted Answer

A)   16 J/K
B)   1.7 J/K
C)   55 J/K
D)   1.7 mJ/K
E)   0.85 J/K

Exam 19: The Second Law of Thermodynamics

Which of the following statements is true of an isolated system consisting of 15 gas molecules?

You can vary the efficiency of a Carnot engine by varying the temperature of the cold reservoir while maintaining the hot reservoir at constant temperature. The curve that best represents the efficiency of such an engine as a function of the temperature of the cold reservoir is

A heat engine operating between the temperatures T₁ and T₂ takes in Q₁ calories at temperature T₁ and gives up Q₂ calories at temperature T₂. The efficiency of this heat engine is

A refrigerator has a coefficient of performance 5.0. How much heat is exhausted to the hot reservoir when 200 kJ of heat are removed from the cold reservoir?

A refrigerator has a coefficient of performance 4.0. How much heat is exhausted to the hot reservoir when 400 kJ of heat are removed from the cold reservoir?

The efficiency of a Carnot engine operating between 300º C and 250ºC is about

The Carnot cycle consists of

A heat engine absorbs 64 kcal of heat from a hot reservoir and exhausts 42 kcal to a cold reservoir each cycle. Its efficiency is

A heat engine absorbs 70 kcal of heat from a hot reservoir and exhausts 50 kcal to a cold reservoir each cycle. Its efficiency is

A heat engine absorbs 150 J of heat from a hot reservoir and rejects 90 J to a cold reservoir. What is the efficiency of this engine?

The diagram below is a schematic of a heat engine. The three quantities, Q_H, Q_L_, and W are represented, respectively, by

A refrigerator with a coefficient of performance 5.0 removes 25 kJ of heat from a cold reservoir. If this refrigerator is reversible and is run backward as a heat engine, what would be the efficiency of the heat engine?

We wish to increase the efficiency of an ideal heat engine from 25% to 35%. If the initial temperature of the hot reservoir is 650^$\circ$C, to what temperature would this need to be increased assuming the exhaust temperature remains the same?

If you run a refrigerator in a closed room with the door to the refrigerator open, the temperature of the room

A steam power plant with an efficiency of 65% of the maximum thermodynamic efficiency operates between 250º C and 40ºC. How much heat is rejected to the cold reservoir in doing 1.0 kJ of work?

Two refrigerators, one with a COP of 4.0 and another with a COP of 5.0, both extract 400 kJ of heat from the cold reservoir (food) . Calculate the difference in energy they exhaust to the hot reservoir and hence the room.

A heat pump needs to supply 20 kW of power to maintain the inside temperature of a house at 25^$\circ$C while the outside temperature is 0^$\circ$C. What is the power needed for the heat pump?

A steam power plant with an efficiency of 65% of the maximum thermodynamic efficiency operates between 250º C and 40ºC. What is the change in the entropy of the universe when this plant does 1.0 kJ of work?

Exam 19: The Second Law of Thermodynamics

Which of the following statements is true of an isolated system consisting of 15 gas molecules?

You can vary the efficiency of a Carnot engine by varying the temperature of the cold reservoir while maintaining the hot reservoir at constant temperature. The curve that best represents the efficiency of such an engine as a function of the temperature of the cold reservoir is

A heat engine operating between the temperatures T1 and T2 takes in Q1 calories at temperature T1 and gives up Q2 calories at temperature T2. The efficiency of this heat engine is

A refrigerator has a coefficient of performance 5.0. How much heat is exhausted to the hot reservoir when 200 kJ of heat are removed from the cold reservoir?

A refrigerator has a coefficient of performance 4.0. How much heat is exhausted to the hot reservoir when 400 kJ of heat are removed from the cold reservoir?

The efficiency of a Carnot engine operating between 300º C and 250ºC is about

The Carnot cycle consists of

A heat engine absorbs 64 kcal of heat from a hot reservoir and exhausts 42 kcal to a cold reservoir each cycle. Its efficiency is

A heat engine absorbs 70 kcal of heat from a hot reservoir and exhausts 50 kcal to a cold reservoir each cycle. Its efficiency is

A heat engine absorbs 150 J of heat from a hot reservoir and rejects 90 J to a cold reservoir. What is the efficiency of this engine?

The diagram below is a schematic of a heat engine. The three quantities, QH, QL, and W are represented, respectively, by

A refrigerator with a coefficient of performance 5.0 removes 25 kJ of heat from a cold reservoir. If this refrigerator is reversible and is run backward as a heat engine, what would be the efficiency of the heat engine?

We wish to increase the efficiency of an ideal heat engine from 25% to 35%. If the initial temperature of the hot reservoir is 650 ∘\circ∘ C, to what temperature would this need to be increased assuming the exhaust temperature remains the same?

If you run a refrigerator in a closed room with the door to the refrigerator open, the temperature of the room

A steam power plant with an efficiency of 65% of the maximum thermodynamic efficiency operates between 250º C and 40ºC. How much heat is rejected to the cold reservoir in doing 1.0 kJ of work?

Two refrigerators, one with a COP of 4.0 and another with a COP of 5.0, both extract 400 kJ of heat from the cold reservoir (food) . Calculate the difference in energy they exhaust to the hot reservoir and hence the room.

A heat pump needs to supply 20 kW of power to maintain the inside temperature of a house at 25 ∘\circ∘ C while the outside temperature is 0 ∘\circ∘ C. What is the power needed for the heat pump?

A steam power plant with an efficiency of 65% of the maximum thermodynamic efficiency operates between 250º C and 40ºC. What is the change in the entropy of the universe when this plant does 1.0 kJ of work?

A heat engine operating between the temperatures T₁ and T₂ takes in Q₁ calories at temperature T₁ and gives up Q₂ calories at temperature T₂. The efficiency of this heat engine is

The diagram below is a schematic of a heat engine. The three quantities, Q_H, Q_L_, and W are represented, respectively, by

We wish to increase the efficiency of an ideal heat engine from 25% to 35%. If the initial temperature of the hot reservoir is 650^$\circ$C, to what temperature would this need to be increased assuming the exhaust temperature remains the same?

A heat pump needs to supply 20 kW of power to maintain the inside temperature of a house at 25^$\circ$C while the outside temperature is 0^$\circ$C. What is the power needed for the heat pump?