© Houghton Mifflin Harcourt Publishing Company Preview Objectives Heat, Work, and Internal Energy...

© Houghton Mifflin Harcourt Publishing Company

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• Objectives

• Heat, Work, and Internal Energy

• Thermodynamic Processes

Chapter 10Section 1 Relationships

Between Heat and Work


Section 1 Relationships

Between Heat and WorkChapter 10

Objectives

• Recognize that a system can absorb or release energy as heat in order for work to be done on or by the system and that work done on or by a system can result in the transfer of energy as heat.

• Compute the amount of work done during a thermodynamic process.

• Distinguish between isovolumetric, isothermal, and adiabatic thermodynamic processes.


Chapter 10

Heat, Work, and Internal Energy

• Heat and work are energy transferred to or from a system. An object never has “heat” or “work” in it; it has only internal energy.

• A system is a set of particles or interacting components considered to be a distinct physical entity for the purpose of study.

• The environment the combination of conditions and influences outside a system that affect the behavior of the system.




Chapter 10

Heat, Work, and Internal Energy, continued

• In thermodynamic systems, work is defined in terms of pressure and volume change.



( )

work = pressure volume change

A FW Fd Fd Ad P V

A A

W P V

• This definition assumes that P is constant.


Chapter 10


• If the gas expands, as shown in the figure, V is positive, and the work done by the gas on the piston is positive.

• If the gas is compressed, V is negative, and the work done by the gas on the piston is negative. (In other words, the piston does work on the gas.)




Chapter 10


• When the gas volume remains constant, there is no displacement and no work is done on or by the system.

• Although the pressure can change during a process, work is done only if the volume changes.

• A situation in which pressure increases and volume remains constant is comparable to one in which a force does not displace a mass even as the force is increased. Work is not done in either situation.




Chapter 10

Thermodynamic Processes

• An isovolumetric process is a thermodynamic process that takes place at constant volume so that no work is done on or by the system.

• An isothermal process is a thermodynamic process that takes place at constant temperature.

• An adiabatic process is a thermodynamic process during which no energy is transferred to or from the system as heat.




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Visual Concept

Chapter 10Section 1 Relationships


Thermodynamic Processes


Preview

• Objectives

• Energy Conservation

• Sample Problem

• Cyclic Processes

Chapter 10Section 2 The First Law of Thermodynamics


Section 2 The First Law of ThermodynamicsChapter 10

Objectives

• Illustrate how the first law of thermodynamics is a statement of energy conservation.

• Calculate heat, work, and the change in internal energy by applying the first law of thermodynamics.

• Apply the first law of thermodynamics to describe cyclic processes.


Chapter 10

Energy Conservation

• If friction is taken into account, mechanical energy is not conserved.

• Consider the example of a roller coaster:– A steady decrease in the car’s total mechanical energy

occurs because of work being done against the friction between the car’s axles and its bearings and between the car’s wheels and the coaster track.

– If the internal energy for the roller coaster (the system) and the energy dissipated to the surrounding air (the environment) are taken into account, then the total energy will be constant.

Section 2 The First Law of Thermodynamics



Visual Concept


Energy Conservation


Chapter 10

Energy Conservation



Chapter 10

Energy Conservation, continued

• The principle of energy conservation that takes into account a system’s internal energy as well as work and heat is called the first law of thermodynamics.

• The first law of thermodynamics can be expressed mathematically as follows:

U = Q – W

Change in system’s internal energy = energy transferred to or from system as heat – energy

transferred to or from system as work



Chapter 10

Signs of Q and W for a system



Chapter 10

Sample Problem

The First Law of Thermodynamics

A total of 135 J of work is done on a gaseous refrigerant as it undergoes compression. If the internal energy of the gas increases by 114 J during the process, what is the total amount of energy transferred as heat? Has energy been added to or removed from the refrigerant as heat?



Chapter 10

Sample Problem, continued

1. Define

Given:

W = –135 J

U = 114 J


Tip: Work is done on the gas, so work (W) has a negative value. The internal energy increases during the process, so the change in internal energy (U) has a positive value.

Diagram:

Unknown:

Q = ?


Chapter 10


2. Plan

Choose an equation or situation:

Apply the first law of thermodynamics using the values for U and W in order to find the value for Q.

U = Q – W


Rearrange the equation to isolate the unknown:

Q = U + W


Chapter 10


3. Calculate

Substitute the values into the equation and solve:

Q = 114 J + (–135 J)

Q = –21 J


Tip: The sign for the value of Q is negative. This indicates that energy is transferred as heat from the refrigerant.


Chapter 10


4. Evaluate

Although the internal energy of the refrigerant increases under compression, more energy is added as work than can be accounted for by the increase in the internal energy. This energy is removed from the gas as heat, as indicated by the minus sign preceding the value for Q.




Visual Concept


First Law of Thermodynamics for Special Processes


Chapter 10

Cyclic Processes

• A cyclic process is a thermodynamic process in which a system returns to the same conditions under which it started.

• Examples include heat engines and refrigerators.

• In a cyclic process, the final and initial values of internal energy are the same, and the change in internal energy is zero.

Unet = 0 and Qnet = Wnet



Chapter 10

Cyclic Processes, continued

• A heat engine uses heat to do mechanical work.

• A heat engine is able to do work (b) by transferring energy from a high-temperature substance (the boiler) at Th (a) to a substance at a lower temperature (the air around the engine) at Tc (c).


• The internal-combustion engine found in most vehicles is an example of a heat engine.



Visual Concept


Combustion Engines


Chapter 10

The Steps of a Gasoline Engine Cycle




Visual Concept


Refrigeration


Chapter 10

The Steps of a Refrigeration Cycle



Chapter 10

Thermodynamics of a Refrigerator



Preview

• Objectives

• Efficiency of Heat Engines

• Sample Problem

• Entropy

Chapter 10Section 3 The Second Law of Thermodynamics


Section 3 The Second Law of ThermodynamicsChapter 10

Objectives

• Recognize why the second law of thermodynamics requires two bodies at different temperatures for work to be done.

• Calculate the efficiency of a heat engine.

• Relate the disorder of a system to its ability to do work or transfer energy as heat.


Chapter 10

Efficiency of Heat Engines

• The second law of thermodynamics can be stated as follows:

No cyclic process that converts heat entirely into work is possible.

• As seen in the last section, Wnet = Qnet = Qh – Qc.

– According to the second law of thermodynamics, W can never be equal to Qh in a cyclic process.

– In other words, some energy must always be transferred as heat to the system’s surroundings (Qc > 0).

Section 3 The Second Law of Thermodynamics


Chapter 10

Efficiency of Heat Engines, continued

• A measure of how well an engine operates is given by the engine’s efficiency (eff ).

• In general, efficiency is a measure of the useful energy taken out of a process relative to the total energy that is put into the process.


• Note that efficiency is a unitless quantity. • Because of the second law of thermodynamics, the

efficiency of a real engine is always less than 1.

eff Wnet

QhQh –QcQh

1 QcQh


Chapter 10

Sample Problem

Heat-Engine Efficiency

Find the efficiency of a gasoline engine that, during one cycle, receives 204 J of energy from combustion and loses 153 J as heat to the exhaust.


1. Define

Given: Diagram:

Qh = 204 J

Qc = 153 J

Unknown

eff = ?


Chapter 10


2. Plan

Choose an equation or situation: The efficiency of a heat engine is the ratio of the work done by the engine to the energy transferred to it as heat.


eff Wnet

Qh1

QcQh


Chapter 10


3. Calculate

Substitute the values into the equation and solve:


eff 1 QcQh

1 153 J

204 J

eff 0.250

4. Evaluate

Only 25 percent of the energy added as heat is used by the engine to do work. As expected, the efficiency is less than 1.0.


Chapter 10

Entropy

• In thermodynamics, a system left to itself tends to go from a state with a very ordered set of energies to one in which there is less order.

• The measure of a system’s disorder or randomness is called the entropy of the system. The greater the entropy of a system is, the greater the system’s disorder.

• The greater probability of a disordered arrangement indicates that an ordered system is likely to become disordered. Put another way, the entropy of a system tends to increase.



Chapter 10

Entropy, continued

• If all gas particles moved toward the piston, all of the internal energy could be used to do work. This extremely well ordered system is highly improbable.


• Greater disorder means there is less energy to do work.


Chapter 10

Entropy, continued

• Because of the connection between a system’s entropy, its ability to do work, and the direction of energy transfer, the second law of thermodynamics can also be expressed in terms of entropy change:

The entropy of the universe increases in all natural processes.

• Entropy can decrease for parts of systems, provided this decrease is offset by a greater increase in entropy elsewhere in the universe.



Chapter 10

Energy Changes Produced by a Refrigerator Freezing Water


Because of the refrigerator’s less-than-perfect efficiency, the entropy of the outside air molecules increases more than the entropy of the freezing water decreases.

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