CHAPTtrR1
INTRODUCTION
1.1 What is thermodynamics?
Thermod.ynamics is the science which has evolved from the original investigations in the 19th century into the nature of "heat." At the time, the leading
theory ofheat was that it was a type offluid, which could flow from a hot body
to a colder one when they were brought into contact. we now know that what
,,heat,' is not a fluid, but is actually a form of energy - it is
was then called
the energy associated with the continual, random motion of the atoms which
compose macroscopic matter, which we can't see directly'
This type of energy, which we will call thermal energal can be converted
(at least in part) to other forms which we can perceive directly (for example,
or electrical energy), and which can be used to do useful
things such as propel an automobil e or a 747 . The principles nf thermodynamics
govern the conversion of thermal energy to other, more useful forms.
kinetic, gravitational,
For example, an automobile engine can be though of as a device which flrst
converts chemical energ'y stored in fuel and oxygen molecules into thermal energy by combustion, and then extracts part of that thermal energy to perform
the work necessary to propel the car forward, overcoming friction. Thermodynamics is critical to all steps in this process (including determining the level of
pollutants emitted), and a careful thermodynamic analysis is required for the
design of fuel-efficient, Iow-polluting automobiie engines. In general, thermodynamics plays a vital role in the design of any engine or power-generating plant,
and therefore a good grounding in thermodynamics is required for much work
in engineering.
only governed the behavior of engines, it would probably
be the most economically important of all sciences, but it is much more than
that. since the chemical and physical state of matter depends strongly on how
If thermodynamics
much thermal energy it contains, thermodynamic principles play a central role
in any description of the properties of matter. For example, thermodynamics
allows us to understand why matter appears in different phases (solid, liquid,
or gaseous), and under what conditions one phase will transform to another.
CHAPTERI.
IN"RODIJCTION
2
Thecompositionofachemically.reactingmixturewhichisgivenenoughtime
,,equilibrium" is also fully determined by thermodynamic principles
to come to
fasi it will get there) For
(even though thermodynamics alone can't tell us how
materials science, chemistry,
these reasons, thermodynamics lies at the heart of
and biology.
Thermodynamics
in its original form (now known as class'i'calthermodynam-
ics)isatheorywhichisbasedonasetofpostulatesabouthowmacroscopic
19th century, before the
matter behaves. This theory was developed in the
atomicnatureofmatterwasaccepted,anditmakesnoreferencetoatoms.The
and the impospostulates (the most important of which are energ'y conservation
work) can't be derived within
sibility of complete conversion of heat to useful
if one accepts thern' a very
the context of classical, macroscopic physics, but
with experiment'
powerful theory results' with predictions fully in agreement
Whenattheendofthelgthcenturyitfinallybecameclearthatmatterwas
showed that the postucomposed of atoms, the physicist Ludwig Boltzmann
from consideration of the
lates of classical thermodynamics emerged naturally
trying to track the atoms inmicroscopic atomic motion. The key was to give up
approach, averaging over
dividually and instead take a stal,istical, probabilistic
thebehaviorofalargenumberofatoms.Thus,theverysuccessfulpostrrlatesof
c]assicalthermodynamicsweregivenafirmphysicalfoundation.Thescienceof
statisticalrnechanicsbegunbyBoltzmannencompasseseverythinginclassical
combined with quantum methermodynamics, but can do more also' When
explain essentialiy all observed
chanics in the 20th century, it became possible to
physics' including esproperties of macroscopic matter in terms of atomic-level
superconductors, etc'
oteric states ofmatter found in neutron stars, superfluids,
in biology,
contributions
important
statistical physics is also currently making
forexamplehelpingtounravelsomeofthecomplexitiesofhowproteinsfold.
Eventhoughstatisticalmechanics(orstatisticalthermodynamics)isina
,,more fundamental" than classical thermodynamics, to analyze practical
example' to carry out
problems we usually take the macroscopic approach' For
its more convenient to think
a thermodynamic analysis of an aircraft engine'
fluid with some specified
a
continuum
of the gas passing through the engine as
sense
propertiesratherthantoconsiderittobeacollectionofmolecules'Butwe
dousestatisticalthermodynamicsevenheretocalculatewhattheappropriate
be'
property values (such as the heat capacity) of the gas should
CHAPTER 1. INTRODUCTION
L.2 Energyand EntroPY
and entropg' Most
The two central concepts of thermodynamics are energy
and prestemperature
other concepts we use in thermodynamics, for example
entropy' Both energy
sure, may actually be defined in terms of energy and
have very different
they
but
systems,
physical
and entropy are properties of
nor destroyed,
produced
characteristics. Energy is conserved: it can neither be
Entropy has a
although it is possible to change its form or move it around'
produce more entropy
different character: it can't be destroyed, but it's easy to
energy' entropy too
Like
does)'
(and almost everything that happens actually
can appear in different forms and be moved around'
Ac]earund.erstandingofthesetwopropertiesandthetransformationsthey
undergoinphysicalprocessesisthekeytomasteringthermodynamicsandlearnbook is focused
ing to use it confidently to solve practical problems. Much of this
their origins in
on developing a clear picture of energy and entropy, explaining
to analyze
methods
effective
developing
and
the microscopic behavior of matter,
to energy
happens
what
complicated practical p.ocessesl by carefully tracking
and entropY.
1.3 SomeTerminologY
is cerMost fields have their own specialized terminology, and thermodynamics
here' so we can
tainly no exception. A few important terms are introduced
begin using them in the next chapter'
1.3.1 System and Environment
Inthermodynamics,Iikeinmostotherareasofphysics,wefocusattentionon
or region(s)
only a small part of the world at a time' We call whatever object(s)
the system
surrounding
of space we are studying Ihe sgstem' Everything else
(in principle including the entire universe) is t,he enu'ironment- The boundary
betweenthesystemandtheenvironmentis,Iogically'tlnesystembound"arg.
definition of the
The starting point of any thermodynamic analysis is a careful
system.
\.
',
IR-k"t
System
Boundary
engines, '
motors, chemical plants, heat pumps, pov/er plants, fuel cells, aircraft
CHAPTER 1.. INTRODUC"ION
Figure 1.1: Control masses and control volumes'
1.3.2 Open, closed,and isolatedsystems
Any system can be classified as one of three types: open, closed, or isolated'
They are defined as follows:
Both energy and matter can be exchanged with the environment. Example: an oPen cuP of coffee.
open system:
energy, but not matter, can be exchanged with the environment. Examples: a tightly capped cup of coffee'
closed system:
system: Neither energy nor matter can be exchanged with the environment - in fact, no interactions with the environment are possible at all.
Example (approximate): coffee in a closed, well-insulated thermos bottle.
isolated
no
Note that no system can truly be isolated from the envitonment, since
which
phenomena
thermal insulation is perfect and there are always physical
etc.).
can,t be perfectly excluded (gravitational fields, cosmic rays, neutrinos,
But good approximations of isolated systems can be constructed. In any case,
isolated systems are a useful conceptual device, since the energy and mass contained inside them staY constant.
1.3.3 Control massesand control volumes
ttolume.
Another way to classify systems is as either a control rnass ot a control
thermodynamrcs'
This terminology is particularly common in engineering
A control mass is a system which is defined to consist of a specified piece
mass.
or pieces of matter. By definition, no matter can entel or ieave a control
moves
boundary
system
the
then
If the matter of the control mass is moving,
with it to keep it inside (and matter in the environment outside)'
A control volume is a system which is defined to be a particular
region of
space. Matter and energy may freely enter or leave a control volume, and thus
it is an open system.
CHAPTER 1. INTRODUCTION
L.4 A Noteon Units
exclusively. If you grew up
In this book, the SI system of units will be used
very familiar with this
anywhere but the United States, you are undoubtedly
used the SI
undoubtedly
have
system. Even if you grew up in the US, you
of your
many
and probably in
system in your courses rn physics and chemistry,
courses in engineering.
Energy, no matter
one reason the sI system is convenient is its simplicity'
other systems'
some
: 1 kg-m2/s2)' In
what its form, is measured in Joules (1 J
differentunitsareusedforthermalandmechanicalenergy:intheEngiishsvsof thermal energy and a ft-lbf
tem a BTU ( "British Thermal Unit" ) is the unit
thermal enelgy is measured
is the unit of mechanical energy. In the cgs system,
incalories,allotherenerg-yinergs.Thereasonforthisisthattheseunitswere
was like mechanical energy'
chosen befbre it was understood that thermai energy
2
only on a much smaller scale'
AnotheradvantageofSlisthattheunitofforceisindentica]totheunit
choice if one knows about
of (mass x acceleration). This is oniy an obvious
as
Newton's second law, and allows it to be written
F:
ma.
(1'1)
unit derived from the 3 primary
In the SI system, force is measured in kg-m/s2, a
but given the shorthand name
SI quantities for mass' length, and time (kg, -, s),
..Newton.,, The name itself reveals the basis for this choice of force units.
of a
TheunitsoftheEnglishsystemwerefixedlongbeforeNewtonappearedon
thescene(andindeedweretheunitsNe.wtonhimselfwouldhaveused).The
of mass is the "pound mass"
unit of force is the "pound force" (ibf), the unit
So Newton's second law
(lbm) and of course acceleration is measured in fb/s2'
from &14 units (lbm ft/s2)
must include a dimensional constant which converts
to force uniis (lbf). It is usually written
F:
I
=m4
(1.2)
9c
where
Qc: 32.1739ft-1bm/tbf-s2'
(1.3)
Of course,in SI g": l.
ffisusedtoo.Americanpowerplantengineersspeakofthe
,,heat rate,, of a power p]";*h;
is defined as the thermal energy which must be absorbed
The heat rate is usually expressed in
from the furnace to produce a unit of electrical energy
BTU/kw-hr.
CHAPTER 1.. INTRODUCTION
of their
In practice, the units in the English system a,re now defined,in terms
and
one
meter,
a
of
fraction
a
certain
SI equivalents (e.g. one foot is defined as
it
is
units,
lbf is defined in terms of a Newton.) If given data in Engineering
necessary
often easiest to simply convert to SI, solve the problem, and then if
assume
implicitly
we
will
reason'
this
For
convert the answer back at the end.
law.
2nd
Newton',s
in
SI units in this book, and will not include the 9. factor