Chapter20

Goals for Chapter 20
•  To learn what makes a process reversible or irreversible
Chapter 20
•  To understand heat engines and their efficiency
•  To see how internal combustion engines operate
The Second Law of
Thermodynamics
•  To learn the operation of refrigerators and heat engines
•  To see how the second law of thermodynamics limits the
operations of heat engines and refrigerators
•  To do calculations with Carnot engines and refrigerators
PowerPoint® Lectures for
University Physics, Thirteenth Edition
– Hugh D. Young and Roger A. Freedman
•  To understand entropy and to use it to analyze
thermodynamic processes
Lectures by Wayne Anderson
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Introduction
•  Why does heat flow from
the hot lava into the cooler
water? Could it flow the
other way?
•  It is easy to convert
mechanical energy
completely into heat, but
not the reverse. Why not?
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Directions of thermodynamic processes
•  The direction of a reversible process can be reversed by an
infinitesimal change in its conditions. The system is always in
or very close to thermal equilibrium. All other thermodynamic
processes are irreversible.
•  Figure 20.1 illustrates an irreversible and a reversible process.
•  We need to use the second
law of thermodynamics
and the concept of entropy
to answer the above
questions.
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1
Heat engines
The efficiency of a heat engine
•  A heat engine is any device
that partly transforms heat into
work or mechanical energy.
•  The thermal efficiency e of a heat
engine is the fraction of QH that is
converted to work.
•  Simple heat engines operate
on a cyclic process during
which they absorb heat QH
from a hot reservoir and
discard some heat QC to a cold
reservoir.
•  e = W/QH
•  Figure 20.3 at the right shows
a schematic energy-flow
diagram for a heat engine.
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•  Read Problem-Solving Strategy
20.1.
•  Follow Example 20.1 to analyze a
heat engine using Figure 20.4 at the
right.
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Internal-combustion engines
The Otto cycle and the Diesel cycle
•  Figure 20.5 below illustrates a four-stroke internal-combustion
engine. The compression ratio r is the ratio of the maximum
volume to the minimum volume during the cycle.
•  Figures 20.6 and 20.7 below show pV-diagrams for idealized Otto
cycle and Diesel cycle engines. In both cases, the efficiency
depends on the compression ratio r.
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Copyright © 2012 Pearson Education Inc.
2
Refrigerators
•  A refrigerator takes heat from a cold
place (inside the refrigerator) and
gives it off to a warmer place (the
room). An input of mechanical work is
required to do this.
Practical refrigerators
•  Figure 20.9 below shows the principle of the
mechanical refrigeration cycle and how the key
elements are arranged in a practical refrigerator.
•  A refrigerator is essentially a heat
engine operating in reverse.
•  Figure 20.8 at the right shows an
energy-flow diagram of a refrigerator.
•  The coefficient of performance K of a
refrigerator is K = |QC|/|W|.
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Air conditioner
The second law of thermodynamics
•  An air conditioner works on the same principle as a refrigerator.
(See Figure 20.10 below.)
•  A heat pump operates in a similar way.
•  The second law of
thermodynamics can be
stated in several ways:
!  No cyclic process can
convert heat completely
into work.
!  No cyclic process can
transfer heat from a colder
place to a hotter place
without the input of
mechanical work.
•  Figure 20.11 at the right
illustrates both statements.
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3
The Carnot cycle
Analyzing a Carnot cycle
•  A Carnot cycle has two adiabatic segments and two isothermal
segments.
•  Follow the derivation
of the efficiency of a
Carnot engine.
•  The pV-diagram in Figure 20.13 below shows the complete cycle.
e=
Weng
Qh
= 1−
Qc
Qh
= 1−
TC
TH
•  Follow Example 20.2
using Figure 20.14 at
the right.
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The Carnot refrigerator
The Carnot cycle and the second law
•  A Carnot engine run in reverse is a Carnot refrigerator.
•  No engine can be more efficient than a Carnot engine operating
between the same two temperatures. Follow the proof of this in
the text, using Figure 20.15 below.
•  The coefficient of performance of a Carnot refrigerator
is Kcarnot = TC/(TH – TC).
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4
Entropy and disorder
•  Entropy, S, is a state variable,
which provides a quantitative
measure of disorder. The
explosion of the firecracker in
Figure 20.17 increases its
disorder and entropy.
Microscopic interpretation of entropy
•  Follow the discussion of the
microscopic interpretation of
entropy, using Figure 20.21
at the right.
•  The entropy of a macrostate
having w microstates is
S = k ln w.
•  A main result is that isolated
systems tend toward disorder
and entropy is a natural
measure of this disorder.
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Entropy change in some adiabatic processes
•  Follow Example 20.9.
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Entropy in cyclic processes
•  The entropy change during any reversible cycle is zero. Figure
20.19 below helps to explain why.
•  For an irreversible process the entropy of an isolated system
always increases. Entropy is not a conserved quantity.
•  Follow Example 20.10.
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5
Entropy and the second law
Entropy and Heat
•  The second law of thermodynamics can be stated in terms of
entropy: No process is possible in which the total entropy of an
isolated system decreases.
•  The original formulation of entropy dealt with
the transfer of energy by heat in a reversible
process
•  In Figure 20.20 below, the entropy (disorder) of the ink-water
system increases as the ink mixes with the water. Spontaneous
unmixing of the ink and water is never observed.
•  Let dQr be the amount of energy transferred by
heat when a system follows a reversible path
•  The change in entropy, dS is
dS =
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dQr
T
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More About Change in Entropy
ΔS for a Reversible Cycle
•  dQr is measured along a reversible path, even
if the system may have followed an irreversible
path
•  ΔS = 0 for any reversible cycle
•  The meaningful quantity is the change in
entropy and not the entropy itself
•  For a finite process,
f
ΔS = ∫ dS = ∫
i
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i
f
•  In general,
!∫
dQr
=0
T
•  This integral symbol indicates the integral is
over a closed path
dQr
T
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6
Heat Death of the Universe
•  Ultimately, the entropy of the Universe should
reach a maximum value
•  At this value, the Universe will be in a state of
uniform temperature and density
•  All physical, chemical, and biological
processes will cease
•  The state of perfect disorder implies that no
energy is available for doing work
•  This state is called the heat death of the
Universe
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7

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