Chapter_6_lecture.pdf

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Thermodynamics: An Engineering Approach, 6 th Edition
Yunus A. Cengel, Michael A. Boles
McGraw-Hill, 2008
Chapter 6
THE SECOND LAW OF
THERMODYNAMICS
Created by Mehmet Kanoglu
Modified by Kai Ming Kiang
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Objectives
•
Introduce the second law of thermodynamics.
•
Identify valid processes as those that satisfy both the first and second
laws of thermodynamics.
•
Discuss thermal energy reservoirs, reversible and irreversible
processes, heat engines, refrigerators, and heat pumps.
•
Describe the Kelvin–Planck and Clausius statements of the second law
of thermodynamics.
•
Discuss the concepts of perpetual-motion machines.
•
Apply the second law of thermodynamics to cycles and cyclic devices.
•
Apply the second law to develop the absolute thermodynamic
temperature scale.
•
Describe the Carnot cycle.
•
Examine the Carnot principles, idealized Carnot heat engines,
refrigerators, and heat pumps.
•
Determine the expressions for the thermal efficiencies and coefficients
of performance for reversible heat engines, heat pumps, and
refrigerators.
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INTRODUCTION TO THE SECOND LAW
A cup of hot coffee
does not get hotter in
a cooler room.
Transferring
heat to a
paddle wheel
will not cause
it to rotate.
These processes
cannot occur
even though they
are not in violation
of the first law.
Transferring
heat to a wire
will not
generate
electricity.
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Processes occur in a
certain direction, and not
in the reverse direction.
A process must satisfy both
the first and second laws of
thermodynamics to proceed.
MAJOR USES OF THE SECOND LAW
1. The second law may be used to identify the direction of processes.
2. The second law also asserts that energy has quality as well as quantity.
The first law is concerned with the quantity of energy and the
transformations of energy from one form to another with no regard to its
quality. The second law provides the necessary means to determine the
quality as well as the degree of degradation of energy during a process.
3. The second law of thermodynamics is also used in determining the
theoretical limits for the performance of commonly used engineering
systems, such as heat engines and refrigerators, as well as predicting
the degree of completion of chemical reactions.
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THERMAL ENERGY RESERVOIRS
A source
supplies
energy in the
form of heat,
and a sink
absorbs it.
Bodies with relatively large thermal
masses can be modeled as thermal
energy reservoirs.
•
A hypothetical body with a relatively large thermal energy capacity (mass x
specific heat) that can supply or absorb finite amounts of heat without
undergoing any change in temperature is called a thermal energy reservoir ,
or just a reservoir.
•
In practice, large bodies of water such as oceans, lakes, and rivers as well as
the atmospheric air can be modeled accurately as thermal energy reservoirs
because of their large thermal energy storage capabilities or thermal masses.
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