PPT: Introduction to Second Law of Thermodynamics | Thermodynamics - Mechanical Engineering PDF Download

 Page 1


The Second Law of
Thermodynamics – Introduction
The absence of the process illustrated above indicates
that conservation of energy is not the whole story. If it
were, movies run backwards would look perfectly normal
to us!
© 2014 Pearson Education, Inc.
Page 2


The Second Law of
Thermodynamics – Introduction
The absence of the process illustrated above indicates
that conservation of energy is not the whole story. If it
were, movies run backwards would look perfectly normal
to us!
© 2014 Pearson Education, Inc.
The Second Law of
Thermodynamics – Introduction
The second law of thermodynamics is a statement about
which processes occur and which do not. There are many
ways to state the second law; here is one:
Heat can flow spontaneously from a hot object to a cold
object; it will not flow spontaneously from a cold object
to a hot object.
© 2014 Pearson Education, Inc.
Page 3


The Second Law of
Thermodynamics – Introduction
The absence of the process illustrated above indicates
that conservation of energy is not the whole story. If it
were, movies run backwards would look perfectly normal
to us!
© 2014 Pearson Education, Inc.
The Second Law of
Thermodynamics – Introduction
The second law of thermodynamics is a statement about
which processes occur and which do not. There are many
ways to state the second law; here is one:
Heat can flow spontaneously from a hot object to a cold
object; it will not flow spontaneously from a cold object
to a hot object.
© 2014 Pearson Education, Inc.
15-5 Heat Engines
It is easy to produce thermal energy using work, but how
does one produce work using thermal energy?
This is a heat engine;
mechanical energy can
be obtained from thermal
energy only when heat
can flow from a higher
temperature to a lower
temperature.
© 2014 Pearson Education, Inc.
Page 4


The Second Law of
Thermodynamics – Introduction
The absence of the process illustrated above indicates
that conservation of energy is not the whole story. If it
were, movies run backwards would look perfectly normal
to us!
© 2014 Pearson Education, Inc.
The Second Law of
Thermodynamics – Introduction
The second law of thermodynamics is a statement about
which processes occur and which do not. There are many
ways to state the second law; here is one:
Heat can flow spontaneously from a hot object to a cold
object; it will not flow spontaneously from a cold object
to a hot object.
© 2014 Pearson Education, Inc.
15-5 Heat Engines
It is easy to produce thermal energy using work, but how
does one produce work using thermal energy?
This is a heat engine;
mechanical energy can
be obtained from thermal
energy only when heat
can flow from a higher
temperature to a lower
temperature.
© 2014 Pearson Education, Inc.
Heat Engines
We will discuss only engines that run in a repeating
cycle; the change in internal energy over a cycle is zero,
as the system returns to its initial state.
The high temperature reservoir transfers an amount of
heatQ
H
to the engine, where part of it is transformed
into workW and the rest,Q
L
, is exhausted to the lower
temperature reservoir. Note that all three of these
quantities are positive.
© 2014 Pearson Education, Inc.
Page 5


The Second Law of
Thermodynamics – Introduction
The absence of the process illustrated above indicates
that conservation of energy is not the whole story. If it
were, movies run backwards would look perfectly normal
to us!
© 2014 Pearson Education, Inc.
The Second Law of
Thermodynamics – Introduction
The second law of thermodynamics is a statement about
which processes occur and which do not. There are many
ways to state the second law; here is one:
Heat can flow spontaneously from a hot object to a cold
object; it will not flow spontaneously from a cold object
to a hot object.
© 2014 Pearson Education, Inc.
15-5 Heat Engines
It is easy to produce thermal energy using work, but how
does one produce work using thermal energy?
This is a heat engine;
mechanical energy can
be obtained from thermal
energy only when heat
can flow from a higher
temperature to a lower
temperature.
© 2014 Pearson Education, Inc.
Heat Engines
We will discuss only engines that run in a repeating
cycle; the change in internal energy over a cycle is zero,
as the system returns to its initial state.
The high temperature reservoir transfers an amount of
heatQ
H
to the engine, where part of it is transformed
into workW and the rest,Q
L
, is exhausted to the lower
temperature reservoir. Note that all three of these
quantities are positive.
© 2014 Pearson Education, Inc.
Heat Engines
A steam engine is one
type of heat engine.
© 2014 Pearson Education, Inc.