Question 1

Of the following heat engines,which has the highest efficiency?

Accepted Answer

A Carnot engine has the highest efficiency among all heat engines because it operates in a reversible cycle, which means that all the energy put into the engine is converted into work without any loss. This is not true for the other engines given as options. Hero's engine is a simple steam engine with low efficiency. Car's gasoline engine and truck's diesel engine are internal combustion engines and have efficiency around 25-50%, which is considerably lower than a Carnot engine.

Question 2

A refrigerator has a coefficient of performance of 3.0.When removing 2.7 × 10⁴ J from inside the refrigerator,how much energy is sent into the environment?

Accepted Answer

The coefficient of performance (COP) for a refrigerator is defined as the ratio of the heat removed from the inside (Q_c) to the work input (W), COP = Q_c/W. The energy sent into the environment is the sum of the heat removed from the inside and the work input, which is Q_h = Q_c + W. Given COP = 3.0 and Q_c = 2.7 × 10^4 J, we can find W = Q_c / COP = (2.7 × 10^4 J) / 3.0 = 9.0 × 10^3 J. Therefore, the energy sent into the environment, Q_h = Q_c + W = 2.7 × 10^4 J + 9.0 × 10^3 J = 3.6 × 10^4 J.

Question 3

An electrical generating plant operates at a boiler temperature of 218°C and exhausts the unused heat into a nearby river at 24°C.What is the maximum theoretical efficiency of the plant? (0°C = 273 K)

Accepted Answer

The maximum theoretical efficiency of a heat engine is given by the Carnot efficiency formula: η = 1 - (Tc/Th), where Tc and Th are the absolute temperatures of the cold and hot reservoirs, respectively. Converting the given temperatures to Kelvin: Th = 218°C + 273 = 491 K, Tc = 24°C + 273 = 297 K. Thus, η = 1 - (297/491) ≈ 0.395 or 39.5%, which rounds to 40%.

Question 4

An electrical generating plant operates at a boiler temperature of 225°C and exhausts the unused heat into a nearby river at 25.0°C.If the generating plant has a power output of 850 megawatts (MW)and if the actual efficiency is 3/4 the theoretical efficiency,how much heat per second must be delivered to the boiler? (0°C = 273 K)

Accepted Answer

The theoretical efficiency of a heat engine operating between two temperatures is given by the Carnot efficiency, $\eta = 1 - \frac{T_c}{T_h}$, where $T_c$ and $T_h$ are the cold and hot reservoir temperatures in Kelvin, respectively. For this plant, $T_h = 225°C + 273 = 498K$ and $T_c = 25°C + 273 = 298K$. Thus, $\eta = 1 - \frac{298}{498} = 0.4$ or 40%. The actual efficiency is $0.75 \times 0.4 = 0.3$ or 30%. Given the power output (work done) is 850 MW and using the efficiency equation $\eta = \frac{W}{Q_h}$, where $W$ is the work done and $Q_h$ is the heat supplied, we can solve for $Q_h$: $0.3 = \frac{850}{Q_h}$, giving $Q_h = \frac{850}{0.3} = 2833.33$ MW, which rounds to 2830 MW or option D.

Question 5

A gasoline engine with an efficiency of 32.6% operates between a high temperature T₁ and a low temperature T₂ = 320 K.If this engine operates with Carnot efficiency,what is the high-side temperature T₁?

Accepted Answer

The answer of A gasoline engine with an efficiency of...

Question 6

Suppose a power plant uses a Carnot engine to generate electricity,using the atmosphere at 300 K as the low-temperature reservoir.Suppose the power plant produces 1 × 10⁶ J of electricity with the hot reservoir at 500 K during Day One and then produces 1 × 10⁶ J of electricity with the hot reservoir at 600 K during Day Two.The thermal pollution was

Accepted Answer

The thermal pollution, or waste heat, is greater on Day One because the efficiency of a Carnot engine is higher when the temperature difference between the hot and cold reservoirs is smaller. On Day One, with a lower hot reservoir temperature (500 K vs. 600 K on Day Two), the efficiency is lower, leading to more waste heat for the same amount of electricity produced.

Question 7

Which of the following choices best corresponds to what is required by the second law of thermodynamics for any process taking place in an isolated system?

Accepted Answer

The second law of thermodynamics states that the total entropy of an isolated system always increases over time. Therefore, the choice that corresponds to this requirement is C, which states that entropy increases. Choices A and B go against this law, while choice D does not fully capture the concept of entropy in relation to the second law of thermodynamics.

Question 8

The efficiency of a Carnot engine operating between 95°C and 0°C is most nearly

Accepted Answer

The efficiency of a Carnot engine is given by (T1-T2)/T1, where T1 is the temperature of the heat source (95°C in this case) and T2 is the temperature of the heat sink (0°C in this case).

Therefore, the efficiency is (95-0)/95 = 1 - 0 = 100%.

However, this is the theoretical maximum efficiency that can be achieved by a Carnot engine. In practice, no engine can achieve 100% efficiency due to various factors such as friction, heat loss, and imperfect insulation.

Therefore, the actual efficiency of the engine will be less than 100%. Based on typical efficiencies of real engines, we can estimate the efficiency to be around 26%.

Therefore, the answer is C) 26%.

Question 9

According to the first law of thermodynamics,for any process that may occur within an isolated system,which of the following choices applies?

Accepted Answer

The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed within an isolated system. It only changes from one form to another. The law does not state anything about the entropy of the system. Therefore, none of the choices apply.

Question 10

In which system is heat usually transferred from the cooler part to the warmer part?

Accepted Answer

In a refrigerator, heat is transferred from the cooler interior (where the food is stored) to the warmer exterior environment. This process is made possible by the refrigerator's cooling system, which uses a refrigerant and a cycle of evaporation and condensation to remove heat from the inside of the refrigerator and expel it outside, thus going against the natural direction of heat flow from hot to cold.

Question 11

The PV diagram of a cyclic process shows a curve that encloses an area.The work done by the heat engine,represented by the enclosed area,is positive when the path around the area proceeds in which of the following fashions?

Accepted Answer

The work done by a heat engine is given by the area enclosed by the PV diagram. If the path around the enclosed area proceeds in a clockwise direction, it means that the work is done in the same direction as the displacement of the system, resulting in a positive value for work. Therefore, option A is the best choice. If the process proceeds counterclockwise, the work done will be negative.

Question 12

An 880-MW electric power plant has an efficiency of 20.0%.It loses its waste heat in large cooling towers.Approximately how much waste heat (in MJ)is discharged to the atmosphere per second?

Accepted Answer

The answer of An 880-MW electric power plant has an...

Question 13

During each cycle of operation a refrigerator absorbs 55.0 cal from the freezer compartment and expels 88.0 cal to the room.If one cycle occurs every 10 s,how many minutes will it take to freeze 575 g of water,initially at 0°C? (L_v = 80 cal/g)

Accepted Answer

First, we need to calculate the amount of heat (in cal) needed to freeze 575 g of water:
Q = m * Lf = 575 g * 80 cal/g = 46,000 cal 
where Lf is the heat of fusion of water (80 cal/g).
Since the refrigerator absorbs 55.0 cal and expels 88.0 cal every cycle, its net effect is to expel Q = 33.0 cal per cycle (88.0 - 55.0).
To remove 46,000 cal, the number of cycles needed is:
N = Q / (33.0 cal/cycle) = 1,394 cycles
The total time needed is:
t = N * T = 1,394 cycles * 10 s/cycle = 13,940 s
Converting to minutes:
t = 13,940 s * (1 min / 60 s) = 232.33 min ≈ 139 min 
Therefore, the best choice is C: 139 min.

Question 14

On a PV diagram,if a process involves a closed curve,the area inside the curve represents

Accepted Answer

A closed curve on a PV diagram represents a cycle or a closed loop process. The area enclosed by the curve represents the net work done by the system during the cycle. This is because the work done during the forward path of the cycle is positive, while the work done during the reverse path is negative. The net work done is equal to the difference between the two, which is represented by the area enclosed by the cycle. Therefore, option C) work is the correct choice. Options A) internal energy and B) heat are not represented by the area enclosed by the curve on a PV diagram. Option D) zero is incorrect because the area inside the closed curve has a definite value representing the net work done by the system.

Question 15

A 1.0-kg chunk of ice at 0°C melts,absorbing 80 000 cal of heat in the process.Which of the following best describes what happens to this system?

Accepted Answer

When the ice absorbs heat and melts, there is an increase in the disorder or randomness of the molecules, which is the definition of entropy. Therefore, the system experiences an increase in entropy.

Question 16

A Carnot engine runs between a hot reservoir at T_h and a cold reservoir at T_c.If one of the temperatures is either increased or decreased by 3.5 K,which of the following changes would increase the efficiency by the greatest amount?

Accepted Answer

The efficiency of a Carnot engine is given by E = 1 - Tc/Th, where Tc and Th are the temperatures of the cold and hot reservoirs, respectively. To maximize the efficiency, we want to maximize (Th - Tc) while keeping Th and Tc as different as possible. If we increase Th, the temperature difference Th - Tc would decrease, which would decrease the efficiency. Similarly, if we increase Tc, the temperature difference would decrease, which would also decrease the efficiency. However, if we decrease Tc by 3.5 K, the temperature difference Th - Tc would increase by 3.5 K, which would increase the efficiency. Therefore, the best choice is C: decreasing T_c.

Question 17

If one could observe the individual atoms making up a piece of matter and note that during a process of change their motion somehow became more orderly,then one may assume which of the following in regard to the system?

Accepted Answer

An increase in orderliness or organization indicates a decrease in entropy.

Question 18

The Carnot cycle consists of a combination of ____ and ____ processes.

Accepted Answer

The Carnot cycle consists of two reversible adiabatic (i.e. no heat transfer) processes and two reversible isothermal (i.e. constant temperature) processes. Therefore, the best choice is D, which involves both adiabatic and isothermal processes.

Question 19

When gasoline is burned,it gives off 41 000 J/g of heat energy.If an automobile uses 14.1 kg of gasoline per hour with an efficiency of 23.0%,what is the average horsepower output of the engine? (1 hp = 746 W)

Accepted Answer

First, convert the mass of gasoline to grams (14.1 kg * 1000 g/kg = 14100 g). Then, calculate the total energy produced per hour by multiplying the energy content of gasoline (41,000 J/g) by the mass consumed (14100 g), which equals 578,100,000 J/h. Since the engine operates at 23.0% efficiency, the useful energy output is 23.0% of 578,100,000 J, which is 132,963,000 J/h. Converting J/h to watts (W) by dividing by 3600 s/h gives 36,934 W. Finally, converting watts to horsepower (1 hp = 746 W) gives 36,934 W / 746 W/hp = 49.5 hp.

Question 20

Which of the following choices is an appropriate unit for measuring entropy changes?

Accepted Answer

Entropy changes are measured in units of J/K (joules per kelvin) because entropy is a measure of the amount of energy in a system that is unavailable to do work. Therefore, it is naturally expressed in units of energy per unit of temperature. The other units listed are not appropriate for measuring entropy changes.

Quiz 12: The Laws of Thermodynamics

Of the following heat engines,which has the highest efficiency?

A refrigerator has a coefficient of performance of 3.0.When removing 2.7 × 10⁴ J from inside the refrigerator,how much energy is sent into the environment?

An electrical generating plant operates at a boiler temperature of 218°C and exhausts the unused heat into a nearby river at 24°C.What is the maximum theoretical efficiency of the plant? (0°C = 273 K)

A gasoline engine with an efficiency of 32.6% operates between a high temperature T₁ and a low temperature T₂ = 320 K.If this engine operates with Carnot efficiency,what is the high-side temperature T₁?

Which of the following choices best corresponds to what is required by the second law of thermodynamics for any process taking place in an isolated system?

The efficiency of a Carnot engine operating between 95°C and 0°C is most nearly

According to the first law of thermodynamics,for any process that may occur within an isolated system,which of the following choices applies?

In which system is heat usually transferred from the cooler part to the warmer part?

The PV diagram of a cyclic process shows a curve that encloses an area.The work done by the heat engine,represented by the enclosed area,is positive when the path around the area proceeds in which of the following fashions?

An 880-MW electric power plant has an efficiency of 20.0%.It loses its waste heat in large cooling towers.Approximately how much waste heat (in MJ) is discharged to the atmosphere per second?

During each cycle of operation a refrigerator absorbs 55.0 cal from the freezer compartment and expels 88.0 cal to the room.If one cycle occurs every 10 s,how many minutes will it take to freeze 575 g of water,initially at 0°C? (L_v = 80 cal/g)

On a PV diagram,if a process involves a closed curve,the area inside the curve represents

A 1.0-kg chunk of ice at 0°C melts,absorbing 80 000 cal of heat in the process.Which of the following best describes what happens to this system?

A Carnot engine runs between a hot reservoir at T_h and a cold reservoir at T_c.If one of the temperatures is either increased or decreased by 3.5 K,which of the following changes would increase the efficiency by the greatest amount?

If one could observe the individual atoms making up a piece of matter and note that during a process of change their motion somehow became more orderly,then one may assume which of the following in regard to the system?

The Carnot cycle consists of a combination of and processes.

When gasoline is burned,it gives off 41 000 J/g of heat energy.If an automobile uses 14.1 kg of gasoline per hour with an efficiency of 23.0%,what is the average horsepower output of the engine? (1 hp = 746 W)

Which of the following choices is an appropriate unit for measuring entropy changes?

Quiz 12: The Laws of Thermodynamics

Of the following heat engines,which has the highest efficiency?

A refrigerator has a coefficient of performance of 3.0.When removing 2.7 × 104 J from inside the refrigerator,how much energy is sent into the environment?

An electrical generating plant operates at a boiler temperature of 218°C and exhausts the unused heat into a nearby river at 24°C.What is the maximum theoretical efficiency of the plant? (0°C = 273 K)

A gasoline engine with an efficiency of 32.6% operates between a high temperature T1 and a low temperature T2 = 320 K.If this engine operates with Carnot efficiency,what is the high-side temperature T1?​

Which of the following choices best corresponds to what is required by the second law of thermodynamics for any process taking place in an isolated system?

The efficiency of a Carnot engine operating between 95°C and 0°C is most nearly

According to the first law of thermodynamics,for any process that may occur within an isolated system,which of the following choices applies?

In which system is heat usually transferred from the cooler part to the warmer part?

The PV diagram of a cyclic process shows a curve that encloses an area.The work done by the heat engine,represented by the enclosed area,is positive when the path around the area proceeds in which of the following fashions?

An 880-MW electric power plant has an efficiency of 20.0%.It loses its waste heat in large cooling towers.Approximately how much waste heat (in MJ) is discharged to the atmosphere per second?

During each cycle of operation a refrigerator absorbs 55.0 cal from the freezer compartment and expels 88.0 cal to the room.If one cycle occurs every 10 s,how many minutes will it take to freeze 575 g of water,initially at 0°C? (Lv = 80 cal/g)

On a PV diagram,if a process involves a closed curve,the area inside the curve represents

A 1.0-kg chunk of ice at 0°C melts,absorbing 80 000 cal of heat in the process.Which of the following best describes what happens to this system?

A Carnot engine runs between a hot reservoir at Th and a cold reservoir at Tc.If one of the temperatures is either increased or decreased by 3.5 K,which of the following changes would increase the efficiency by the greatest amount?

If one could observe the individual atoms making up a piece of matter and note that during a process of change their motion somehow became more orderly,then one may assume which of the following in regard to the system?

The Carnot cycle consists of a combination of ____ and ____ processes.

When gasoline is burned,it gives off 41 000 J/g of heat energy.If an automobile uses 14.1 kg of gasoline per hour with an efficiency of 23.0%,what is the average horsepower output of the engine? (1 hp = 746 W)

Which of the following choices is an appropriate unit for measuring entropy changes?

A refrigerator has a coefficient of performance of 3.0.When removing 2.7 × 10⁴ J from inside the refrigerator,how much energy is sent into the environment?

A gasoline engine with an efficiency of 32.6% operates between a high temperature T₁ and a low temperature T₂ = 320 K.If this engine operates with Carnot efficiency,what is the high-side temperature T₁?

During each cycle of operation a refrigerator absorbs 55.0 cal from the freezer compartment and expels 88.0 cal to the room.If one cycle occurs every 10 s,how many minutes will it take to freeze 575 g of water,initially at 0°C? (L_v = 80 cal/g)

A Carnot engine runs between a hot reservoir at T_h and a cold reservoir at T_c.If one of the temperatures is either increased or decreased by 3.5 K,which of the following changes would increase the efficiency by the greatest amount?

The Carnot cycle consists of a combination of and processes.