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Thermodynamics Notes 1
Srikanth Vedantam

1

Introduction

Thermodynamics is the study of energy
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We require energy for every kind of useful application
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We have
used animals to get from place to place
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We have used energy from flowing water to grind wheat
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A few simple rules (and several clear definitions) can allow us to precisely predict when
and how much energy we are likely to be able to obtain
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We will learn these definitions
and rules going forward
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Note that we actually use forms of these statements
in our own experience quite often
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500 in our wallet when we left
home on one morning
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120 when we returned home
later that day, we try to think about what happened to the difference in the amount of
money
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500 to Rs
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We will recall that we spent Rs
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100 that she had borrowed
from us a previous day
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280 and
only then feel satisfied that everything is accounted for
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Our net worth may consist of various components such as properties we own,
stocks and mutual funds
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While the concept of
bookkeeping is straightforward, the actual process should be done in detail and carefully
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The concepts are simple
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There are other kinds of statements which is widely used in Thermodynamics which
are also based on our practical experience
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In the grocery store we had put the oranges in a random
pile in the box
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We would wonder if someone put them in that manner
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This asymmetry in our reactions
is also an important practical experience which is taken into account in formulating the
second law of Thermodynamics
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Of course, unlike the example of counting currency notes or oranges,
we need to clearly define the quantities involved so that two different engineers can come
to the same conclusions
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In this sense,
Thermodynamics, Fluid Mechanics and Heat Transfer are intimately related
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We know that there
are many forms of energy
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The process
of transformation of energy from fluids to other forms is the subject of fluid mechanics
whereas if thermal energy is one of the forms of energy in the process, the subject is
heat transfer
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It only looks at the final result
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Thermodynamics does
not worry about the details of the transformation processes and only looks at the final
forms
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Macroscopic viewpoints in Thermodynamics

We all know that materials are composed of atoms and molecules
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For example, a liter of gas at room temperature and pressure contains about
1023 molecules
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For this reason, thermodynamics principles have
been developed on the basis of assumption that the material is a continuum
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There have been alternative viewpoints of thermodynamics by considering averages
of a large number of molecules
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In this course, we will use continuum thermodynamics
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3

Systems and Control Volumes

A system is region with a fixed quantity of mass
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The boundary of the system should be clearly defined
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An example of a
2

system could be a heated steel billet which is formed into different shapes
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Then at that temperature, it may
be rolled into a sheet
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Finally it may be deformed further at the lower temperature
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On the other hand, a control volume is a region of space with a clearly defined
boundary
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The boundary may be fixed or
changing with time in a known manner
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An example of a control volume could be a lake with streams of
water feeding it
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Other streams may be taking water out of the lake
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Or there could also be water added
to the lake due to rain
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So, why do we have these two definitions? Why not choose only one and stick with it?
The first definition — the system — allows us to easily keep track of how much energy
is being moved around
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The control volume, on the
other hand, is slightly more difficult to analyze
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The basic principles of analysis for both systems and control volumes is the same
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4

Properties, State and Equilibrium

Since our eventual goal is to extract energy from various kinds of systems, we need to
keep track of what is happening to it
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We know that any system can have properties such as colour, smell, etc
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However, we are generally interested only in the properties which describe the system
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We would not be interested in its colour
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Two properties are said to be independent if one can be varied without changing
the other
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This will become clear with
examples which we will look at later
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It is assumed that the state of a system
can be completely described by a finite number of properties
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Many of the systems we will consider in
thermodynamics will consist of simple compressible systems
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For example, if the system consists of an ideal gas, the pressure
and volume could be the two properties which are sufficient to describe the state of the
system
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Such a system is static (it may have a velocity but
does not have an acceleration)
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A system is said to be in equilibrium
if there are no unbalanced driving forces causing it to move from its current state
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Similarly,
the system is in mechanical equilibrium if there are no pressure differences from point to
point within the system such that the system is static
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5

Processes and Cycles

The change of a system from one equilibrium state to another is called a process
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It
is possible to go from an initial state to a final state by any one of many paths
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As
seen in Figure ??, this can usually be represented on a graph
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Now, in reality,
such a thing cannot happen exactly
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For
example, consider a cube of copper which is in thermal equilibrium at 10◦ C
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The copper block may eventually reach a uniform
final temperature of 0◦ C
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This happens till the entire block
cools down
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And, in fact, the exact nature and time of these changes is
the subject of Heat Transfer which we will learn later in the course
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And this change also happens in small
increments of temperature such that each intermediate state is an equilibrium state
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However, this estimate is still useful for us to understand what is the best
we can do
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A process for which the final state is the same as the initial state is called a cycle
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This is because
we need to extract energy continuously in most applications
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6

Forms of Energy

While we are aware of the many forms of energy from high school physics, in thermodynamics we classify energy broadly as stored energy or energy transferred across
boundaries (of a system or a control volume)
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1

Stored energy

Stored energy can be understood to consist of two components: microscopically stored
energy or macroscopic energy
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Examples would be the kinetic energy of translation or rotation of the entire
system
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Microscopically stored energy would be the internal energy due to the kinetic energy
and potential energy of atomic interactions and chemical energy due to chemical reactivity
(bond formation and bond breaking)
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2

Transferred energy

Energy can be transferred across the boundaries of systems only through two means: heat
transfer and work
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Work transfer is all other forms of energy transfer
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The main qualitative difference between the two forms of energy transfer is that heat
transfer is a disorganized form of energy transfer whereas work is an organized form
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Recognizing this and trying
to obtain the most amount of work from any process is what this subject aims to do
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We use such conservation
principles implicitly in our daily lives
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If, at a
later time, we saw that there were only 6
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From practical experience we would know that the 4 oranges did
not spontaneously disappear
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Of
course, unlike oranges, there are many forms of energy and there is transformation between them
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This adds to the complexity in practical application of the First law
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5

A basic statement of the first law is:
Energy is neither created nor destroyed
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For practical applications, this statement is not very useful
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Not all the oranges
in the world
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We
would say that we gave away 6 mangoes to a friend on the way home
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Then we would recall that we started
with 10, gave away 6 and obtained 2 more
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In other words, for practical
application of the conservation of the oranges in the bag, we look at the events of removal
and addition of oranges and count the total number remaining
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A useful form in this case would be applied to all forms of energy:
The difference in the amount of energy added to a system and the amount of energy
removed from the system is the change in energy stored in the system
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6



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