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• Objectives

• Heat, Work, and Internal Energy

• Thermodynamic Processes

Chapter 10 Section 1 Relationships

Between Heat and Work

Section 1 Relationships

Between Heat and Work Chapter 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.

Section 1 Relationships

Between Heat and Work

Chapter 10

Heat, Work, and Internal Energy, continued

• In thermodynamic systems, work is defined in terms

of pressure and volume change.

Section 1 Relationships

Between Heat and Work

( )

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

Heat, Work, and Internal Energy, continued

• 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.)

Section 1 Relationships

Between Heat and Work

Chapter 10

Heat, Work, and Internal Energy, continued

• 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.

Section 1 Relationships

Between Heat and Work

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.

Section 1 Relationships

Between Heat and Work

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Chapter 10 Section 1 Relationships

Between Heat and Work

Thermodynamic Processes

Preview

• Objectives

• Energy Conservation

• Sample Problem

• Cyclic Processes

Chapter 10 Section 2 The First Law of

Thermodynamics

Section 2 The First Law of

Thermodynamics Chapter 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

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Chapter 10 Section 2 The First Law of

Thermodynamics

Energy Conservation

Chapter 10

Energy Conservation

Section 2 The First Law of

Thermodynamics

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

Section 2 The First Law of

Thermodynamics

Chapter 10

Signs of Q and W for a system

Section 2 The First Law of

Thermodynamics

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?

Section 2 The First Law of

Thermodynamics

Chapter 10

Sample Problem, continued

1. Define

Given:

W = –135 J

U = 114 J

Section 2 The First Law of

Thermodynamics

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

Sample Problem, continued

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

Section 2 The First Law of

Thermodynamics

Rearrange the equation to isolate the unknown:

Q = U + W

Chapter 10

Sample Problem, continued

3. Calculate

Substitute the values into the equation and solve:

Q = 114 J + (–135 J)

Q = –21 J

Section 2 The First Law of

Thermodynamics

Tip: The sign for the value of Q is negative. This

indicates that energy is transferred as heat from

the refrigerant.

Chapter 10

Sample Problem, continued

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.

Section 2 The First Law of

Thermodynamics

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Chapter 10 Section 2 The First Law of

Thermodynamics

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

Section 2 The First Law of

Thermodynamics

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).

Section 2 The First Law of

Thermodynamics

• The internal-combustion engine found in most

vehicles is an example of a heat engine.

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Chapter 10 Section 2 The First Law of

Thermodynamics

Combustion Engines

Chapter 10

The Steps of a Gasoline Engine Cycle

Section 2 The First Law of

Thermodynamics

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Chapter 10 Section 2 The First Law of

Thermodynamics

Refrigeration

Chapter 10

The Steps of a Refrigeration Cycle

Section 2 The First Law of

Thermodynamics

Chapter 10

Thermodynamics of a Refrigerator

Section 2 The First Law of

Thermodynamics

Preview

• Objectives

• Efficiency of Heat Engines

• Sample Problem

• Entropy

Chapter 10 Section 3 The Second Law of

Thermodynamics

Section 3 The Second Law of

Thermodynamics Chapter 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.

Section 3 The Second Law of

Thermodynamics

• 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 –Qc

Qh 1

Qc

Qh

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.

Section 3 The Second Law of

Thermodynamics

1. Define

Given: Diagram:

Qh = 204 J

Qc = 153 J

Unknown

eff = ?

Chapter 10

Sample Problem, continued

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.

Section 3 The Second Law of

Thermodynamics

eff Wnet

Qh 1

Qc

Qh

Chapter 10

Sample Problem, continued

3. Calculate

Substitute the values into the equation and

solve:

Section 3 The Second Law of

Thermodynamics

eff 1Qc

Qh 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.

Section 3 The Second Law of

Thermodynamics

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.

Section 3 The Second Law of

Thermodynamics

• 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.

Section 3 The Second Law of

Thermodynamics

Chapter 10

Energy Changes Produced by a Refrigerator

Freezing Water

Section 3 The Second Law of

Thermodynamics

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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Chapter 10 Section 3 The Second Law of

Thermodynamics

Entropy of the Universe