Question 1

Classify the following hypothetical perpetual motion machines as being perpetual motion machines of the first kind $(A)$ or the second kind (B).
-(e) A normal heat engine is used to drive an electric generator. Part of the power from this generator runs a refrigerator that absorbs the waste heat from the engine and pumps it back into the hot reservoir.

Accepted Answer

The answer of Classify the following hypothetical perpetual motion machines...

Question 2

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not.
-(a) Your body

Accepted Answer

The answer of Which of the following devices are heat...

Question 3

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not.
-(b) An automobile engine

Accepted Answer

An automobile engine is a heat engine because it converts thermal energy into mechanical work.

Question 4

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not.
-(c) A tornado

Accepted Answer

A tornado can be considered a heat engine because it converts thermal energy from warm, moist air into mechanical energy, driving its rotation and movement.

Question 5

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not.
-(d) The sun

Accepted Answer

The sun is not a heat engine; it is a massive nuclear fusion reactor that produces energy through nuclear reactions, not by converting heat into work.

Question 6

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not.
-(e) A rocket

Accepted Answer

The answer of Which of the following devices are heat...

Question 7

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not.
-A photovoltaic solar panel

Accepted Answer

A photovoltaic solar panel converts sunlight directly into electricity without using heat, so it is not a heat engine.

Question 8

In a maximally efficient heat engine, the amount of entropy that the hot reservoir loses as heat flows out of it is exactly balanced by the entropy that the cold reservoir gains as the waste heat flows into it.

Accepted Answer

In a maximally efficient (reversible) heat engine, the process is isentropic, meaning the total entropy change of the system (including both the hot and cold reservoirs) is zero. This implies that the entropy lost by the hot reservoir is exactly equal to the entropy gained by the cold reservoir.

Question 9

A heat engine produces $300 \mathrm{~W}$ of mechanical power while discarding $1200 \mathrm{~W}$ into the environment (its cold reservoir). What is this engine's efficiency?

Accepted Answer

The efficiency of a heat engine is given by the formula $\eta = \frac{W}{Q_{\text{in}}}$, where $W$ is the work output (mechanical power in this case) and $Q_{\text{in}}$ is the heat input. The heat input can be calculated as the sum of the work output and the heat discarded into the environment, which is $300 \mathrm{~W} + 1200 \mathrm{~W} = 1500 \mathrm{~W}$. Thus, the efficiency is $\eta = \frac{300}{1500} = 0.20$.

Question 10

Suppose that, in Iceland, scientists discover geothermal vents that produce pressurized steam at $300^{\circ} \mathrm{C}$. Engineers construct a heat engine that uses this steam as a hot reservoir and a nearby glacier as a cold reservoir. What is the maximum possible efficiency of this engine?

Accepted Answer

The maximum possible efficiency of a heat engine is given by the Carnot efficiency, which is $1 - \frac{T_c}{T_h}$, where $T_c$ and $T_h$ are the cold and hot reservoir temperatures in Kelvin, respectively. Converting $300^{\circ}C$ to Kelvin gives $573K$. Assuming the glacier is at $0^{\circ}C$ or $273K$, the maximum efficiency is $1 - \frac{273}{573} \approx 0.52$ or $52\%$.

Question 11

Suppose you are trying to design a personal fan that you wear on your head and operates between your body temperature $\left(37^{\circ} \mathrm{C}\right)$ and room temperature $\left(22^{\circ} \mathrm{C}\right)$. What is the maximum possible efficiency of this device?

Accepted Answer

The answer of Suppose you are trying to design a...

Question 12

You can keep your kitchen cool for hours by leaving the freezer door open.

Accepted Answer

Leaving the freezer door open will actually make the freezer work harder to cool down the increased area, using more energy and potentially increasing the room temperature due to the heat generated by the freezer's motor and coils working overtime.

Question 13

To function, a home air conditioning system must have access to the home's exterior.

Accepted Answer

A home air conditioning system needs access to the home's exterior to expel heat absorbed from the interior air, typically through a condenser unit located outside.

Question 14

As the temperature difference between its reservoirs increases, the COP of a refrigerator

Accepted Answer

The COP (Coefficient of Performance) of a refrigerator decreases as the temperature difference between the hot and cold reservoirs increases, because more work is required to transfer the same amount of heat.

Question 15

A refrigerator uses $100 \mathrm{~W}$ of electric power and discards $600 \mathrm{~W}$ of thermal power into the kitchen. What is its coefficient of performance?

Accepted Answer

The coefficient of performance (COP) for a refrigerator is given by the ratio of the heat removed from the refrigerated space (Q_c) to the work input (W), COP = Q_c/W. Here, Q_c is the thermal power discarded into the kitchen, which is 600 W, and W is the electric power used, which is 100 W. Therefore, COP = 600 W / 100 W = 6.

Question 16

Someone comes to your house selling a device that draws heat from groundwater and supplies that heat to your house. The salesperson claims that the amount of heat energy entering the house will far exceed the electrical energy supplied to the device. What the salesperson claims here is physically impossible.

Accepted Answer

The salesperson is describing a heat pump, which can indeed transfer more heat energy into a house than the electrical energy it consumes by using the refrigeration cycle to move heat from a cooler area (groundwater) to a warmer area (the house).

Question 17

One can convert work to heat (the reverse of what a heat engine does) with perfect efficiency.

Accepted Answer

This process is essentially what happens in a heat pump or refrigerator, where work is used to move heat from a cooler to a warmer area. According to the second law of thermodynamics, converting work to heat can indeed be done with perfect efficiency, as it does not decrease the entropy of the system.

Question 18

The Carnot engine is important because

Accepted Answer

The Carnot engine is significant because it demonstrates the theoretical limit of efficiency that any heat engine can achieve using ideal gas processes, making it the benchmark for maximal efficiency rather than perfect efficiency.

Question 19

A maximally efficient Carnot engine would produce zero power.

Accepted Answer

A Carnot engine is a theoretical heat engine that operates on the Carnot cycle. The Carnot cycle is a reversible cycle that consists of two isothermal processes and two adiabatic processes. The efficiency of a Carnot engine is given by the following equation:```η = 1 - T_C / T_H```where:* η is the efficiency of the engine* T_C is the temperature of the cold reservoir* T_H is the temperature of the hot reservoirThe efficiency of a Carnot engine is always less than 100%. This is because some of the heat that is absorbed by the engine is lost to the environment during the adiabatic processes.If the efficiency of a Carnot engine is 100%, then the temperature of the cold reservoir would be equal to the temperature of the hot reservoir. This is impossible, because the Carnot cycle is a reversible cycle. Therefore, a maximally efficient Carnot engine would produce zero power.

Question 20

According to the argument in the text, the Moon's average surface temperature (since it has no atmosphere and is the same distance from the sun as the earth) should also be $255 \mathrm{~K}$, or it would be if all points on its surface had the same temperature (and the Moon has the same average reflectance as the earth). But the Moon rotates very slowly relative to the sun (once a month) and has no atmosphere or ocean to redistribute energy differences. How do you think that these facts might affect the Moon's average temperature? (Hint: Suppose that half the moon was $100 \mathrm{~K}$ hotter and the other was $100 \mathrm{~K}$ colder than the average. Would it still radiate as much energy as it would if its surface area were all at the same temperature?)

Accepted Answer

The slow rotation and lack of atmosphere mean that the temperature differences across the Moon's surface are extreme, leading to a lower average temperature than if the surface temperature were uniform. This is because the energy radiated by a body increases with the fourth power of its temperature (according to the Stefan-Boltzmann law), so a surface with varying temperatures will radiate less total energy than if the whole surface were at the higher average temperature.

Quiz 6: Some Processes Are Irreversible Thermal Physics

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not. -(a) Your body

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not. -(b) An automobile engine

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not. -(c) A tornado

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not. -(d) The sun

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not. -(e) A rocket

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not. -A photovoltaic solar panel

In a maximally efficient heat engine, the amount of entropy that the hot reservoir loses as heat flows out of it is exactly balanced by the entropy that the cold reservoir gains as the waste heat flows into it.

A heat engine produces $300 \mathrm{~W}$ of mechanical power while discarding $1200 \mathrm{~W}$ into the environment (its cold reservoir) . What is this engine's efficiency?

Suppose that, in Iceland, scientists discover geothermal vents that produce pressurized steam at $300^{\circ} \mathrm{C}$ . Engineers construct a heat engine that uses this steam as a hot reservoir and a nearby glacier as a cold reservoir. What is the maximum possible efficiency of this engine?

Suppose you are trying to design a personal fan that you wear on your head and operates between your body temperature $\left(37^{\circ} \mathrm{C}\right)$ and room temperature $\left(22^{\circ} \mathrm{C}\right)$ . What is the maximum possible efficiency of this device?

You can keep your kitchen cool for hours by leaving the freezer door open.

To function, a home air conditioning system must have access to the home's exterior.

As the temperature difference between its reservoirs increases, the COP of a refrigerator

A refrigerator uses $100 \mathrm{~W}$ of electric power and discards $600 \mathrm{~W}$ of thermal power into the kitchen. What is its coefficient of performance?

One can convert work to heat (the reverse of what a heat engine does) with perfect efficiency.

The Carnot engine is important because

A maximally efficient Carnot engine would produce zero power.

Quiz 6: Some Processes Are Irreversible Thermal Physics

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not. -(a) Your body

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not. -(b) An automobile engine

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not. -(c) A tornado

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not. -(d) The sun

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not. -(e) A rocket

Which of the following devices are heat engines? In each case, answer T (True) if it is a heat engine, and F (False) if it is not. -A photovoltaic solar panel

In a maximally efficient heat engine, the amount of entropy that the hot reservoir loses as heat flows out of it is exactly balanced by the entropy that the cold reservoir gains as the waste heat flows into it.

A heat engine produces 300 W300 \mathrm{~W}300 W of mechanical power while discarding 1200 W1200 \mathrm{~W}1200 W into the environment (its cold reservoir) . What is this engine's efficiency?

You can keep your kitchen cool for hours by leaving the freezer door open.

To function, a home air conditioning system must have access to the home's exterior.

As the temperature difference between its reservoirs increases, the COP of a refrigerator

A refrigerator uses 100 W100 \mathrm{~W}100 W of electric power and discards 600 W600 \mathrm{~W}600 W of thermal power into the kitchen. What is its coefficient of performance?

One can convert work to heat (the reverse of what a heat engine does) with perfect efficiency.

The Carnot engine is important because

A maximally efficient Carnot engine would produce zero power.

A heat engine produces $300 \mathrm{~W}$ of mechanical power while discarding $1200 \mathrm{~W}$ into the environment (its cold reservoir) . What is this engine's efficiency?

A refrigerator uses $100 \mathrm{~W}$ of electric power and discards $600 \mathrm{~W}$ of thermal power into the kitchen. What is its coefficient of performance?