Unit 1(complete)

BASIC CONCEPTS OF
THERMODYNAMICS
Structure
Introduction
Objectives
Thermodynamic Systems
Classification of Systems
Classification of Boundarics
Thermodynamic State of a System and Thermodynamic Variables
Intensive and Extensive Variables
Thermodynamic Equilibrium
Thermodynamic Processes
Reversible and Irreversible Process
Quasistatic Process
Representation of a Process on an IndiFator Diagram
The Zeroth Law of Thermodynamics
The Equation of State
Deductions from the Equation 07~tare
Summary
Terminal Questions
Solutions and Answers
1.1 INTRODUCTION
In your daily life you might have observed that when you usc a bicycle purnp to inllnte
the tyre, the pump gets hot. Similarly when you rub your hands together you gct the
feeling of warmth. In these examples, the heating is not caused in the conventional
way by putting a flame or something hot underneath the pulrlp or the hr~nd.But it
arises as a result of the mechanical work done in compressing the gas in the pump or
forcing the hand to move against friction. These examples show that there is a relation
between mechanical and thermal effects. The study of the relationships bctwcen
mv-hanical and thermal energy is known as thermodynamics.
The s.udy of thermodyna~nicsis based on Four empirical laws, that is, thern~odynamics
is a phenomenological science. This means that the laws are derived from experience
and they need no proof. These laws find application in design of intcr~alcombustion
engines, refrigeration and air conditioning systems, power stations etc. With the help
of these laws one can determine the efficiency of all types of mechanical devices such

1ssteam engines, electric power plants and automobiles. These laws are also used to
analyse energy transforrri tior%in bio-chemical and geological systems,
In this unit we will discuss the basic concepts which will be used in thc study of
thermodynamics. We )kill also introduce the concept of temperaturc through thc
zeroth law of thermod namics. Finally, you will learn to use the equation of state for
deriving mathematicsl'relations between quantities such,as coefficient of volume
expansion, compressibilities, elasticity, etc.
In the next unit you will raid about the pri~~ciplcsof measurement of temperature.
Objectives
After studying this unit, you should be able to
identify several thermodynamic systems with its surroundit~gsand boundaries
explain what are thermodynamic variables and identify these variables for scverai 1
systems
expl4in thermal, mechanical and chemical equilibria
distinguish between a reversible and an irreversible process ,
ul'lderstand the importance of a quasistatic process

The ZEroth Law nnd the
First Law' ,
...
8 identify and represent different kinds of thermodynamic processes
explain the concept of temperature usi~lgthe zeroth law of thermodynamics
@ apply the equation of state of u system to solve problems in thermodynamics.
Surroundings
i 'system
Fig. 1.1 : System, surroundings
and boundary
1.2 THERMODYNAMIC SYSTEMS
Whenever we read a biography our attention gets drawn towards an individual. We
get to know about his achievements, how he has interacted with other individualsand
many such other things. To the reader's mind this individual assumes the role of
someone who is distinct from others and around whom the biography develops almost
like a story. Likewise, for any kind of study in thermodynamics we need toset aside
a certain quantity of matter or a region of space which is considered distinct and
separated from everything else, which can inl'luence it. We refer to this as
thermodynamic system or simply a system. Every such system is enclosed by an
arbitrary surface which is called its boundary. The boundary may be real or
imaginary, either in rest or in motion and may change its size or shape. The region
of space that lies outside the boundary of a system is called its surroundings. Strictly
speaking everything other than the system is its surroundings. But in thermodynamics
we consider that portion as the effectivesurroundings of a system which can influence
it. In Unit 3, you will read about the different manners in which the surroundings
can influence a system. Any study in thermodynamics begins with the identification
of the system, its boundary and its surroundings. These are schematically illustrated
in Fig. 1.1.Let us now take up some specific examples of systems with different kinds
of boundaries.
Weight
A-----------I A
I I
I Gas
I
I I B
'Boundary
Fig. 1.2: Several tl~ermodyearnic,systemsand their boundaries. In each case the boundary is shown by
means of a dotted line. (a)The chairsystem, (b) Compressedgas in ncylinder, (c) Flowof liquid througi~
a plpe.
For the chairsystem (Fig. 1.2a) the boundary is real and fixed. Everything other than
the chair constitutes the surro~ndings.For the system of a compressed gas in a
cylinder (Fig. 1.2b) the boundary is real. It is movable as the inner surface of the
piston, which is a part of the boundary, can be moved. Everything outside the gas,
including the piston and the cylinder, is the surroundings. For the svstem of a certain
mass of liquid flowing along a pipe as shown in Fig. 1 . 2 ~the boundary is imaginary.
The inside surface of the pipe may forin a part of the boundary as it presents an
obstruction to the flow of liquid. But there.& no real obstruction at th . .wo ends AB
and CD.So a real boundary does not exist. However, the dotted line is one possible
choice df a boundary, lixed in space. Such type of boundary is imaginary. Everything
8 lying outside this imaginary boundary is the surroundings.

A system may be simple, such as water in a vessel; or complex like a Daniel ;ell,
which has Zn and Cu electrodes, dilute H2SO4electrolyte and CuS04 depolariser.
A system may be completely uniform with respect to its chemical composition and
physical conditions (homogeneous) or otherwise (heterogeneous). For instance, air
in a cylinder, air being a mixture of gases (N2, 02,C02,etc.) is a homogeneous
system. This is because the constituent gases havc the same composition throughout.
Whereas ice and water in a beaker ibrni a heterogeneous systelid'of a solid a.-d a
liquid.
The classification of systems from the point of view of its interaction ;it11 its
surroundings is of more importance in thermodynamics. We shall do that now.
1.2.1 Classification of Systems
Closed System : A closed system consists of a fixed amount of mass. It is also known N0
as control mass. No mass can cross its boundary. But energy can cross the boundary
and the volume of such a system need not be fixed (Fig. 1.3).
For example, let us again consider the case of the enclosed gas in a cylinder fitted
with a piston (Fig. 1.2b). In this case the gas is a system. The inner surfaces of the
piston and the surface of the cylinder form the boundary. As no mass can cross this
boundary it is a closed system. YOUmust note that energy may cross the boundary. Fig. 1.3 : Charuclcristics of a
closed syslcni
OpenSystem: An open sysle~nis a properly selected region in space which has a fixed
volume. It is also known as control volume. Both mass and energy can cross its
-boundary (Fig. 1.4).
As an example of such a system let us consider the water heater shown in Fig. 1.5.
Suppose the water in the tank is heated in order to get a steady supply of stream of
hot water. Since hot water will leave the tank and will get replaced by cold watcr, it
is not convenient to choose a fixed Inass of water as our system. Instead we call direct
our attention on the volume formed by the inner surfaces of the tank. Then we call
consider the hot and cold water streams as Inass leaving and entering the control
YES
volume or the open system. 1 + p g Y Y 6 S
L- -------T-Isolated system : An isolated system is that fcr which there call be no interchange of
energy withathe surroundingi, The contents of an ideally sealed thermos flask is an ~ i g .1.4 : Characleristicsof an
example of such a system. open syfitern
While reading aboutthe classification of systems did you realise that the
characteristics of the boundary plays an important role in determining the nature of
the system. So let us now study about the different kinds of boundary.
1.2.2 Classification of Boundaries Hot - - - - - - - . -
(i) Diathermal Boundary: If the boundary is such that heat call flow between the
system and its surroundings, then we say that it is diather~nalor tllerrnally yl:rl-FHeater
conducting and the system is in therrnal contact with thc surroundings. A (control
metallic tea pot provides a diathermal boundary to its conlents. volurnc)
I
I
I Cold
(ii) Adiabatic Boundary : If the boundary is such that it docs not allpw any heat flow I
watar
across it, then we refer to it as an adiabatic boundary and the system is said to L - --- -- in
be thermally isolated. The walls of an ideal thermos flask are adiabatic and a
filled thermos is an example of a thermally isolated system.
(iii) Rigid Boundary :If the boundary is such that it cannot be moved, however great ~ i g .1.5: An opeglsystem wit11one
an external force acting on it might be, we call it rigid. A system having such a inlet and one exit
boundary can neither be colnpressed nor expanded. Strictly speaking, an ideal
rigid boundary does not exist. The surface of a heavy spherical ball (used for
"shot put") is a close approximation to a rigid boundary.
(iu) Permeable Boundary :If the boundary allows exchange of matter, wc rcl'er to it
as permeable.
(v) Semi-permeable Boundary : The baundary which permits some components to
pass through and stops others is known as semi-permeable. For example, hot
quartz allows helium to pass but stops other gases.
Now you may like to work out an SAQ based on what you have learnt so far. 9

The Zerorh Low and the
First Law
SAQ 1 '
a) Cite an example (different from what you have read just now) each for a system
with (i) a real fixed boundary, (ii) a real movable boundary, and (iii)an imaginary
Water in boundary.
b) The schematic diagram of a car engine radiator is shown in Fig. 1.6. Should it be
modelled as a closed or an open system?
c) A bottle of soft drink at room temperature is put into the refrigerator so that it
can cool. Would you model the bottle of soft drink as a closed or anopen system?
d) A rose plant in a garden is an example of an open system. Comment.
e) Mention the nature of the system (i.e. open, c l ~ e dor isolated) that is enclosed
by a (i) diathermal boundary, (ii) permeable boundary.
Rig. 1.6 : Car Engine Radiator
Now, you learnt what is meant by a system and its surroundings. For a particular
problem these must be properly defined as per its requirement and the boundary must
fulfil the conditions imposed by it. I
As an illustration of the statement made above, suppose you want to assess the
performance of a refrigerator kept in your dining room. Here the refrigerator along
with its contents forms a system, its walls the boupdary and the room in which it is
placed constitutes its surroundings. However, if we consider the problem of air
conditioning of the same dining room, then the room itself becomes the system while
its walls, roof, doors, windows etc. constitute the boundary and its neighbouring
rooms, verandah, etc, are its surroundings.
Thus you have learnt to identify a thermodynamic system, its surroundings and its
boundary:!,Qnce we haveselected a systemfor study, we must be able to describe it
in-tefms of precise numerical quantities. For example in mechanics we need to define
primarily the,position and velocity of a particle in order to describe its motion. We
say that the variables viz. position and velocity specify the mechanical state of the
particle. Likewise we define the thermodynamic state of a system about which you
will study now.
1.3 THERMODYNAMIC STATE OF A SYSTEM*lANp
THERMODYNAMIC VARIABLES
--
Fi l x ~ i f f e r e n tsetof valuesof
In thermodynamics a system is described by specifying its physical properties such as
k.presbureand volume in (a)and (b) pressure, volume, temperature, mass, density etc. For example, the state of a gas is
shovs two differentstates of the characterised by the value of its temperature, pressure and volume. The state of a
same system stretched wire is specified by its length and tension in it. These physical properties
or parameters are called thermodynamic variables. The value of a property, which
You know that the pressure of a uniquely defines the state of a system, is always fixed. This value depends on the
gas can be to the average condition of the system at that particular instant at which the property is beingrate of change of momentum due
to the collision of the molecules
measured. For example, the pressure and volume of a system of a gas kept in cylinder
madeon a unit areaof of have fixed but different values in the two conditions shown in Fig. 1.7a and 1.7b.
the container in which it is kept. Therefore, the state of a system means specifying those prbperties of the system
Higher the rate of change of which ensure that it is uniquely defined.
momentum, higher the pressure.
Similarly, the temperature of a You must note that the properties mentioned above can be measured directly using-
gas can be related to the average calibrated devices. In otherwords they can be perceived by our senses. ~ h e s e
kinetic energy of trans'ation of jts
properties which represent the gross characteristics of the system are calledmolecules. This means that the
system can also be described in
macroscopic properties.
lenr of the propenics Of the
There are two classes of thermodynamic variables - intensive and extensive. Let usI atoms and molecules that
constitute the system. These stufiy the difference between them.
I properties are not directly
perceptible and are referred to as
! microscopic properties. You will
*
be required to describea system
in the above manner while
studying its thermodynamic
, behaviour using statistical
i methods. You will do that in
Block 4 of this course.
1.3.1 Intensive and Extensive Variables
Let us consider a system A asshown in Fig. 1.8. Suppose itis divided into two parts
Al and A, having equal masses. Now if you measure the temperature of system A
and then of the systems Al and A2 you will find that its value is the same. But the
volume of the systemsA, and A2is different from that of system A. Those variables
which have the same values when the system is sub-divided or multiplied in size are
called intensive and those variablesfor which values are altered are called extensive.
Here, pressure and temperature are intensive whereas mass and volume are

extenswe. The i~ltlt~isivevariables are independent of mass whereas the extensive
variables are dependent on mass. We have given below a list of systems along with
intensive and extensive variabtes describing them. Some places are kept vacant for
you to fill up.
Table 1.l: Tl~ermodynamlcVariables
System Intensive variahle(s) Extensive varieble(s)
Gas in acylinder pressure @)
temperature (%)
density (p)
mass ( m )
volume (V)
Paramagneticsolid flux density (B) intensityof
magnetisation (M)
Stretched wire tension (F) --
Surfacefilm -- area (A)
Electriccell -- --
SAQ 2
a) Fill in the gaps in Table 1.1.
b) "Iff and g are two extensive variables, flg is intensive." Justify,thisstatement with
the help of an example.
c) What are the thermodynamic variables required for specifying the E~~llowins
systems :
(i) Air inside a bicycle pump
(ii) A dielectric substance placed in an electric field.
~t times the state of a system may be completely identified even from a knowledge
of somc of its properties. This is because the values of all remaining properties can
be obtained using the known properties. For example, say, we know the values of
pressure (p),volume (V) and the number of ~nolecules(n) of an ideal gas. Then we
1 can obtain its temperature (73by using the ideal gas equation
i pV = nRT (1.1)
I An equation of the type (1.1) is called an equation of state. It is a relation between
the values of the variables p, V, n and T,when the system has attained equilibrium,
In fact the properties of a system are truly defined only when it is in equilibrium, We
refer to this as thermodynamic equilibrium. Let us learn about this in some detail
1; ,l.J.Z Thermodynamic Equilibrium
Suppose you have some water at 60°C in a container. If the container filled with water
is left to itself it would finally attain the room temperature. This means that the water
in the container interacts with the surroundings, and its temperature decreases with
time. When it attains the room temperature, i.e. a temperature equal to that of the
surroundings there occurs no further change in its temperature. We say tha the water
in the container has attained thermal equilibrium with the surroundings.
If within the system, there are variations in pressure or elasticstress then parts of the
system may move or expand or contract. Eventually when these expansions or
contractions will cease, i.e. when there will be no unbalanced forces or torques acting
on the system, it will be in mechanical equilibrium. For example, during the
formation of the earth it was in a molten state. So the centrifugal force due to the
axial rotation of the earth was greater than the cohesive force. Since the centrifugal
force is maximum at the equator and decreases with latitude becoming minimum at
the poles, the earth bulged out at the equator in the process of attaining mechanical
I equilibrium.
Finally, suppose that a system contains substances that can react chemically. After a
sufficient time when all possible chemical reactions will have taken place, the system
is then said to be in chemical equilibrium. In other words, a mixture of substances'ls
in chemical equilibrium i'f there is no tendency for a chemical reaction to occur.
A system which is in thermal, mechanical and chemical equilibrium is said to be in
thermodynamic equilibrium. Under this condition the macroscopic properties of a-- .
Basic Concepts of
l'hermodyna~nlcs
Fig. 1.8 : Illustrating the
difference between Intensiv:~and
extensive variables.
Strictly speaking, in addition to
thermal,mechanical and chemical
.equilibria, one must include the
aspect of phase equilibrium.
However we shall not consider
this now. You will get to know
about this in Unit 9, Block 2.

he 7aroth L ~ Wand the system do not change with time. Thus, from now onward, by the phrase 'state of a
First Law
system' we will mean an equilibrium state.
Now th& you have learnt about an equilibrium state, we shall now'discuss how a
system goes from one equilibrium state to another.
Room Temperature
WI1c.n Coolcd
Fig. 1,9: Tension is developed in
thewire when it is allowed to cool.
Thewire issaid to haveexecuted a
thennodynamic process.
I
Fig. 1.10 : Graphical
representationof several processes
occurrln~between A and B
1.4 THERMODYNAMIC PROCESS
When any of the thermodynamic variables associated wit11a system undergoes change
in its value while going from one equilibrium state to another the system is said to
execute a thermodynamic process. Thus, a process dgnifies change of a system from
one equilibrium state, called the initial state, to another equilibrium state, called the
final state: For example, the expansion of a gas in a cylinder at constant pressure due
to heating is a therm~dynamicprocess. Again suppose a wire is stretched taut
between two rigid subports (Fig. 1.9a). It has a certain tension and length. Now, if
this wire is allowed to cool it would shrink. Since the wire is not permitted to shrink,
a tension is developed in the wire to stretch it (Fig. 1.9b). So it will have a different
tension and length, and will be said to have executed a thermodynamic process.
In the subsequent units, as you will see, our aim will be to analyse thermodynamic
processes. In order to do that, first we would like to represent a thermodynamic
process graphically. Suppose two variables x and y specify a system. Refer to Fig.
1.lo. Here A is the initial equilibrium state represented by the coordinates (xA, yA)
and B is the final equilibrium state represented by the coordinates (xB,ye). The
points A and B are joined. Such a graphical representation of the tl~ermodyl~amic
process is called a path. Notice that you can join the points in several ways as shown
in Fig. 1.10. But in whichever way you draw a path, you would like that the
intermediate points must also be defined by specific coordinates. What does this
imply? It implies that all the intermediate stages between A and B must be
equilibrium states. But is it possible in practice? To answer this question we shall first
define two kinds of thermodynamic processes.
1.4.1-' Reversible and Irreversible ProcesBes
If a process is executed in a very slow and controlled manner so that all the
intermediate states between the initial and final one are in equilibriu and if
necessary the process may be executed back along the same eyilibrirm states from
the final to the initial state then it is called reversible. If the above conditions are not
satisfied then it is called irreversible.
Now let us take an example to see whether a reversible process can be executed in
practice or not.
(dl
Fig. 1.11: Demonstrationshowing that a reversible pr&%-cannot be achieved in practice; it can only be
12, ideallsed
Suppose a cylinder containing gas is fitted with a piston (Fig. 1.lla). The volume
occupied by the gas is V. The weight Wplaced on the piston is such that on its removal
the piston takes up the position A'B' and the volume of the gas becomes 2V. Now
suppose we want to increase the volume of the gas to 2V. If we do it just by removing
W, the process would be irreversible as it occurs quickly. (The volume of gas will
become 2V immediately on removing W.) -
D
A B A
C-------Q
- - - - - - B
B
,

Now instead of hav~nga single weight W, if we take two weights, each equal to W/2 Basic Concepts or
(Fig. l.llb), the situation is similar to that of Fig. 1.1la. Now, on removing one such
IThermodynnmics ,
weight the piston will take up a position CD midway between AB and A 'B' (Fig.
1.11~).Then on removing the riext weight it goes to A 'B' (Fig. 1.lld). Thus we get
one intermediate equilibrium state (denoted by CD) between the initial and final states.
Likewise by using three weights, each equal to W/3,we can generate two intermediate
equilibrium states. By using n equal weights, each equal to Wln, we can generate
(n-1) intermediate equilibrium states. But for this expansion process to be reversible
all the infermediatestates must be in equilibrium. For this n has to be infinity, which
is impossible. This indicates that a reversible process can only be idealised and cannot
be achieved in practice.
You must note that a system can be restored to its initial state either by a reversible
or an irreversible process. But, for a reversible process, this restoration is made
without leaving any net change in the surroundings, whereas for an irreversible
process, there occurs some change in the surroundings. For example suppose a
pendulum bob (Fig. 1.12) is released from the position A. It goes upto B and then
again comes to A, i.e. the initial state. In doing so the pendulum bob is restored to
its original state but there occurs a slight change in the temperature of the
surroundings because the bob has to overcome air resistance in course of its motion.
So the process is ~rreversible.Had the pendulum been set up in perfect vacuum, air
resistance would have been totally absent and hence there would have been no /Ichange in the te~peratureof the surroundings. Thus the process would have been A - - _ _ _ I _ C -
U
reve2sible.But qsydu know perfect vacuum can never be created. This again shows
that a reversible process can only be idealised and never be achieved in practice. Fig. 1.12 : When the pendulum
bob swings froni lhe position A !r
Thus we understand that all natural processco are irreversible. B and then back to A, there is a
slightchange in the temperatureo
This means that for such processes, the' intermediate stages are not in equilibrium the surroundings due to air
and hence such processes cannot be repfesented by a path. If il:cannot be represer~ied resistance showing that the
by a path then its analysis from thermodynalnic point of view is not possible, This process is irreversible.
raises a very vital question. Can we not at all analyse natural processes ,
thermodynamically? The answer lies in the description of nquasistatic process which
we shall study now.
1.4.2 Quasistatic Process
If a process is so executed that it passes through states which are not cquilibri~uin
states but deviates only infinitesimally from equilibrium, then it is called quasistatic.
Thus a quasislatic (i.e., almost static) process closely approximates a succession of
equilibrium states. If there are finite departures from etluilibrium, the process is
non-quasistatic.
Suppose we wish to increase the temperature of a system froin an initial value TIto
a final value T2.The temperature could be increased by enclosing the system in a
diathermal boundary and maintaining the surroundings of the system at the
temperature T,. But this process would not be quasistatic because the temperature
of the system near its boundary would increase more rapidly than at the internal
points. To increase the temperature quasistatically, the temperature of the
surroundings be kept at the initial temperature TI and then this temperature should
be increased sufficiently slowly so that at all times it is only infinitesiinally greater
than that of the system.
Now, all actual processes are non-quasistatic because during the process, there always
exists a finite difference of pressure, temperature, etc. between several parts of a
system. So for dealing with such a process we shall visualise it as being executed
quasistatically. This will enable us to analyse the process. You will be able to
appreciate the above statement when you solve actual problems.
You may now like to work out an SAQ on the concepts of reversible and irreversible
processes.
SAQ 3
aj Classify the following processes as reversible or irreversible.
(i) A gas encloscd in a cylinder provided with a frictionless piston is quickly
compressed (ii) Mixing of sugar in milk (iii) Oscillations,of an ideal simple
pendulum with a frictionless support.

The ~erothLaw and the b) Energy is dissipated during an irreversible proccss-illustrate this statemen't with
First Law
the help of an example.
Before moving on to next sub-section try to'do an activity which will enable.you to
understand quasistatic process more clearly.
Activity
Take any clock which has all the three hands. Observe the movement of the
second-hand. Compare its movement with the movement of minute and hour-ha&.
Comment.
Now that you have learnt about a quasistatic process, we can discuss about the
representation of an actual process.
1.4.3 Ihqresentation of a Process on an Indicator Diagram
You must have realised that the stages of a quasistatic process are a series of
approximately equilibrium slates. As you have seen In Fig. 1.10, equilibrium states
can be indicated by a series of points in the xy-plane where x and y are two
thermodynamic variables. As you know, a curve through these points represents the
path of the proccss. Such a representation is known as an indicator diagram. Let us
now learn to represent an actual process on an indicator diagram.
Consider a gas contained in a cylinder C fitted with a frictionless piston P having
diathermal walls and immersed in a constant temperature water bath as shown in Fig.
1.13a.
Pig. 1.13(a): Arrsngement for expansion of a gas at constunt temperature. (b) p vs. V diagram for
I
quasistatic isothermalexpansion of a gas. It is also referred to as an indicator diagram.
Suppose that the initial state of the gas is defined by (pi,Vi,Ti).Now, we pull out
the piston veryslowlyso that the gas expands at constant'temperaturein such a way
that at any instant the external pressure on the piston differs from the gas pressure
by an infinitesimal amount dp. Let the final state of the gas be (pf:Vf,Ti).While
undergoing change from the initial state to the final state, the system 1scharacterised
by a series of values of p apd V differing only infinitesimally from each other. The
plot of these successive values with V along the abscissa and p along the ordinate
gives us the required repreqentation (Fig. 1.13b).
So far we'have classified processes on the basis of thk pace of their execution. We
can also classify processes, on the basisof the fact that ope property of the system
remains constant during the process, That is, one of @e variables of the system
remains constant.'Wespecify these processes by using the prefix 'iso' before that
property. For example, in the abovesaid brocess, the gas has been expanded at
constant temperature. Thus, the above process is isothermal. If a process takes place
at constant pressure, we label it as isobaric. A process taking place at constant volume
is called isochoric or isovolumic or isovolumetric. A change of state (i.e. water to
steam or water to ice) takes place at constant temperature and pressure. This is an
example of isothermal-isobaricI process.
Furthermore, if the system has adiabatic boundaries and the process takes place I
without any exchange of heat between the system and the surroundings, we refer to ..-

it as an adiabatic process. For example, a single stroke of the plston 01an internal
combustion engine is very nearly adiabatic. he reason is that the duratioq of the
process is extremely small and no heat can flow in or out of the system during this"
time.
If the transformation of the system is such that it returns toits original state covering
any path, then we say that it has undergone a cyclic process. The working substances
of all heat engines and refrigerators go through several cycles of operation.
We have represented some of the above-mentioned processes on the p-V diagrams
(Fig. 1.14) below:
Fig. 1.14: (a) isobaric (b) isochoric (c) lsotl~ermal(d) c'yclic
You may now like to work out an SAQ on the representation of processes.
Draw V-T and p-T diagrams for a perfect gas undergoing following processes.
(i) Idobaric expansion (ii) Jsochoric compression (iii) lsothermc?l compression.
I
Before proceeding further it would be worthwhile to know how. the concept of
temperature got introduced. Actually, we hat; been using the term temperature
quite freely (since it is very familiar to you) but its basis lies in a law of
thermodynamics. ,Let us study this law which is known as the zeroth law.
1.5 THE ZEROTH LAW OF THERMODYNAMICS
The zeroth law of thermodynamics states :
"If two systems A and AZare in thermal equilibrium independently with a third
system A3,then Al and A2are also in thermal equilibrium with each other."
Let us discuss the meaning of this law. Refer to Fig. 1.15~1.
Rasic Concepts of
Thermodynamics
Fig. 1.15 : The Zerothlaw of thermodynamics (adiabaticwalls are denotedby shadingand dialhermar walls
by solid Lines):(a) AI and A, are in thermal contact with A,. (b) Al ant1 A2arc in thermal co~~tactwith onr
mother.

The -zeroth Law and the
Flrst Law
Here Al and AZare two systems separated by adiabatic walls. They are in thermal
contact with another system A3.Both A, and A2 individually will come in therpal
equilibrium with A,. If now A1and A, are put in thermal contact with each other
(Fig. 1.15b) then according to the zeroth law both A, and A, will be in thermal
equilibrium.
All bodies in thermal equilibrium must have a common propbrty which has thesame
value for all of them. This property is called the temperature. Thus the temperature
of a body is the property which determines whether or not the body is in thermal
equilibrium with other bodies. The phenomenon that two things in contact tand
towards a common temperature is so common that its importance has been
overlooked. When physicists finally did appreciate itsSignificance and fundamental
nature, it was decided to have it elevated to the status of a "Law of thermodynamics"
By that time the first and second laws were already enunciated. So in order to place
it ahead of these laws it is named as the zeroth law.
Now we would like you to look at Eq. (1.1) and read once again the paragraph
following it. Eq. (1.1) is quite well known to you. Can such relations, exist for other
thermodynamic systems? In fact from the zeroth law it can be established
mathematically that a relation does exist between temperature and other
thermodynamic variables associated with a system. Such a relation, as you know, is
called an equation of state. Let us study about it in some detail before rounding off
this unit.
1.6 THE EQUATION OF STATE
You know that the temperature of a system can be expressed as a function of any
other two thermodynamic parameters. If the two parameters are x and y then
mathematically it can be expressed as
T = f l (x, Y ) (1.2a)
Eq. (1.2a) can be solved for x in terms-of T and y and can even be solved for y in
terms of T and x so that we have the relations
X = fi (T Y) (1.2b)
,Y= f 3 (7'9 x) (1 . 2 ~ )
In other words Eqs. (1,2a, b and cj can be expressed as
f (4Y, T ) = 0
'Eq.(1.3) is parametric as the form off is not given. But the equation of state of an
ideal gas (Eq. 1.1) is exact. Likewise we may have equarions of state for a real' gas
or for several other systems. We will not aim to derive any such equation here. We
will only provide a few examples in Table 1.2,
-. . Table 1.2
Paramagnetic solid
e solid is placed.Cis a
L and Loare thelengthsof the wire
at tsrnp5raturestTandTo
respectively.Fis the tension in the
wire,p and q are constants.

/

The parametric form of the equation of state may be used to study some typical
characteristics like coe ficient of thermal expailsion, elasticity, compressibility and so
Ion of any substance. e stiall do that now. This exercise will make you competent
to handle thermodynamic relations involving partial derivatives. You will encounter
several such relations, particularly in Block 2 of this course.
1.6.1. Deductions from Equation of State
Befo~cy~aproceedto study the several characteristics using the equatlon of state
you wil, ; ed to learn a little about partial differentiation. You will find that it is an
exte~~sionof the idea of ordinary differentiation. We comeacross such differentiation
when a quantity is a function of more than one variable and it is required to find the
change in the quantity when any one of the variables changes by a small amount. For
example the temperature of ,a gaseous system is a functioli of pressure and volume.
NOW: we may like to obtain the rate of variation of temperature with respect to an
isochoric (volume = constant) variation of pressure. Likewise we may wish to obtain
the rate of variation of temperature with respect to an isobaric (pressure = constant)
variation of volume. In the former case we are seeking the partial derivative of
( :T),,, whereas in the latter caseTwith respect top at constant V,denoted by -
the partial derivative of Twith respect to V, denoted by ($$) . Notice that instead
P
of using the symbol 'd' as in the case of ordinary differentiation here we are using
thesymbol 'a' (pronounced 'del'). We shall now work out mathematical relationship
between the partial derivatives involving any three variables. These will be very
useful in doing the relevant deductions from equation of state:
Let z be a function of x and v :
.- ,
. r; , z= z ix, y,
Then total differential dz is given by the following relation :
1
Eq. (1.5) expresses the change in z which results from changes in x and y oh which
z depends.
Now if we take three variables asp, V and Tof a gaseoussystem then we may say that
v = v (P, T)
Thus usingEq. (1.5) the volume difference dV between two neighbouringequilibrium
states may be expressed as
and similarly for the pressure difference dp as
Now, substituting Eq. (1.6b) in Eq. (1.6a), we get
Now, of the three variables p, V,and T only two are independent. Choosing V and
Tas the independent variables,Eq. ,(l.7)must be true for all sets of values of dV and
I
dT. Thus for two stat9s which are at the same temperature (dT = 0) but having
different volumes (dV # 0) we get from Eq. (1..7)

'Iht ZcroUi Law and tk
Flrrl Law
or
Thisis the mathematical relation which allows us to replace any partial derir ' ' ~ e ' b * ~
' the reciprocal of the inverted derivative with the same variable(s)held constan.:Thl'ie
is another important relation which you will derive in the following SAQ.
" '
-9
SAQ 5
a) Imposing the condition dV = 0,dT # 0in Eq. (1.7)
prove that
T- 4
b) Write down the equation corresponding to Eq. (1.9) for
(i) a paramagnetic solid
m
(ii) a stretched wire.
We shall now learn to use Eqs. (1.8) and (1.9). For that we shall first define
expansivity, P (or the isobaric coefficient of volume expansion) and the bulk
modulus, ET (or the isothermal elasticity) in the language of partial derivatives.
and
You must note that forall known systems pressure decreases with volume. So
( dTis negative. Hence, we have the minus sign in Eq. (1.11) to give ETa positive val le.
Combining Eqs. (1.10) and (I.11) we get
On using Eq. (1.9), we get
An infinitesimal change in pressure may now be expressed in terms of these physicaI
quantities. Thus
At constant volume,
dp = PETdT
At constant volume, when the change in temperature is finite and small, the
quantities l3 and ET are assumed as constant. Hence, the finite change in pressure,
Ap, corresponding to a finite change in temperature, AT, at a constant volume is
given by
Ap = PET AT - (1.12)
Now, let us take up a numerical example to illustrate Eq. (1.12).
. .

Example1
The temperature of a block of copp'eris increasedfrom 400K to 410K. What chahge
in pressure is necessary to keep the volume constant? It is given that for copper,
p = 5 x lo-' K-' and ET = 1.3 x 10" ~m-'.
. Basic Concepts of
Thermodynamics
Solution
h Eq. (1.12), we have AT = 10 K,
So Ap = (5 x lo-' K-') (1.3 x 1011~ m - ~ )(10 K)
Before roundingoff this dnit you may like to work out an.SAQ on the use of partial
derivatives.
E
SAQ 6
4) Isothermal compressibility, K, is the reciprocal of isothermal elasticity. Now if g
be the density of a substance, show that
(i) = L (A) (ii) p = L (3)1 p a p T P d T P
b) Show that for an ideal gas, K = 1,P i 1P T
C) Prove that for a pVT-system
Cct us now sum up what we have learnt in this unit.
1.7 SUMMARY-
A region of space under study is called the system and everything else around it
is the qurroundings. An arbitrary surface enclosing the system is known as the
boundary. 1 /
Thermodynamicstate of a system is s~ecifieqt. +be properties of the systenl,"
which uniquely defines it.
The macroscopicquantitiesthatdetermine the aboxsnm-~*tiesof the system are
called thermodynamic variablgs.
Thermodynan'c variables ark classified as extensive or intensive depending on
whether these are depdnden: 011 the m'dssof th'e'systerql or not.
The variables of z system in.~nmqtl);na&icequiljbrium can be ekpressed in the
form of a mathematical relationsf'"; called ;he equatidn o!: ;rate of the system.
A system is said to be in thermodynamic equilibrium iE its state does not change
in any way (thermal, mechanical and chemical) with time
When any of the properties of a system changes, the state of the system changes
and it is said to undergo a process. .
A reversibleprocessis one that is performed in such a way that, at the conclusion
of the process,both the system and the local surroundings may be restored to their
initialstates, without producingany changein the rest of the universe. If the above
conditions are not satisfied then it is called irreversible process.
If the process iscarried out in such a way that at every instant the system departs
only infinitesimally from an equilibrium state; the process is called quasistatic.
* The zeroth law of thermodynamicsstates that if systems Al and A2are separately
in thermal equilibrium with system A3 then Al and A2 ar,ealso in thermal
equifibrium vith each other.
All systems in thermal equilibrium with each other have same temperature.--
The general equation of state for any gaseous system is represented by
f (P,V, T) = b
wheref is a single-valued function of pressure p! volume Vand the aboslute
temDerature T.

If z = z (x, y) then total differential dz is expressed as
This leads to two important mathematical relations:
a In the language of partial derivatives the coefficient of isobaric coefficient of
vo~dmeexpansion (p), isothermal elasticity (ET)and isothermal compressibiiity
(K) are defined as follows
1.8 . TERMINAL OUESTIONS
1) a) Can a system composed of 200 molecules be called a thermodynamicsystem?
Justify your answer.
D ) ' h v e that the specific value corresponding to an extensive variable is an
I
intensive property. (Hint : Specific value of any variable is its value per unit
mass.)
c) The weight qf 2 m3
0f Hg at OQCand 1 bar pressure at a place where
g = 9.80ms-2 is 2,67 x 10' N. Give two extensive and five intensivevariables
of this system.
2. Consider the following statements :
i) specikic gravityah substance is an intensive property but density is an
extensive property.
ii) Reversible processesare only an idealisation and all the natural processesare
irreversible.
iii) An isolated system is necessarily adiabatic..Choose your answer.
(a) if only (i) and (ii) are correct.
(b) if only (ii) and (iii) are correct.
(c) if only (i) and (iii) are correct.
(d) if all the statements are correct.
3) A certain stretch of railway track is laid without expansion joints in Thar desert
where day and night temperaturesdiffer by 25 K. The cross-sectionalarea of rail
is 3.6 x m2
. The Young's modulus, Y,of its material is 2 x 10" N m-2 and
its co-efficient of linear expansion, a,is 8 x K-'. (a) If the length of the
track is kept constant, what is the differency in the tension in the rails between
day and night? (b) If the track is 15 km long, and is free to expand, what is the
change in its length between day and night?
(Hint: Usef {L,F, T)= 0 alongwith Y =
where L, F and T denote length, tension and temperature respectively.)
SAQs
I) (a) (i) A dice (Fig. 1.16a) (ii) A car (Fig. 1.16b)as its doors, bonnet and dickey
'v be opened (iii3Smoke coming'out of a chimney (Fig. 1.16~).

-. -
Fig. 1.16:Diagramsfor answerto SAQ 1 (a)..Ineach case the dotted line represents the boundary.
(b) The volume of the system is constant. Hence, it is an open system.
(c) This system has a constant mass so it is a closed system.
(d) When we consider a rose plant in a garden it is a system with other plants,
soil and atmosphere assurroundings. In day time it takes up sunlight (energy)
and CO, (matter) from the atmosphere and gives out 02.It also takes
nutrients (matter) frbm the soil and increases its own matter co~tent.Thus,
exchange of energy and matter both takes place, which makes the rose plant
in a garden an open system.
(e) (i) closed (ii) Open
2) a) Intensive :surface rensim (s), e.m.f. (E)
Extensive: length (L), charge (q)
b) Let f be mass and g be volume of a substance then ffg is its density.
'
c) (i) Pressure, volume, temperature, mass and density.
(ii) Intensity of polarisation, temperature, electric field intensity.
3) a) (i) and (ii) irreversible (iii) reversible.
b) Let us consider the case of expansion of a gas contained in a cylinder. Suppose
this is done by pulling the piston outward. If there is a friction between piston
and the inner wall of the cylinder the process is definitely irreversible. In this
case a portion of mecha~licalenergy provided by the pull goes in heating of
the piston and the wall of the cylinder. This illustrates that energy is dissipated
during an irreversible process.
Activity
Angular speed of second-hand = rad s-'. = rad s-
1
60 30
1 = 3.31~X rad s-I
i
i
Angular speed of minute-hand = &a d sW1= rad s-'
1800
= 5 . 5 ~x rad s-l'
Angular speed of hour-hand = 12xgtx rad s-' = rad s-
t
21600
= 4,6v x rad s-'
The movement of second-hand can be perceived. Angular speeds of minute
and hour-hand being small their movements cannot b ~ ~ e r c e i y e d .Hence their
movement is quasistatic in nature.
4) Refer to Fig. 1.17. Here i andf represents initial and finalstate respe'ctively. Note
that in V-T diagram for (i) the extrapolated portion of the straight line must pass
through the origin as $ = constant. The case of p-T diagram for (ii) will be
similar.
5) a) Putting dV = 0 and dT #' 0 in Eq. (1.7) we get
Basic Concepb of
Thermodynamics

The Zeroth Law and the
First Law
V - T Diagrams
T
(ii)
T
(iii)
(ii) (iii)
NOW,since--- ($)p,weget
(%Ip
This relation is used to split up a derivative into a product of more convenient
derivatives. It may be written with any-derivative then following through the other
variables in cycIic order.
(ii)
(%)=(%)L (%IF = & I
mWe kdow that density = -!X&S- ; i.e. p = -
volume V
Differentiating, d (pVj = 0 (because m = a constant)
.'. pdV + Vdp = 0

(ii) We have
.
b) For an ideal gas,
Again,
c) We know from Eq. (1.6a) that
. ( ) = - KT[ and (s),,= pV
T
So, we get
Terminal Questions
1) a) No, because a thermodynamic system should be solarge that it can be treated
as a continuous collection and a system of 200 molecules does not fulfil this
condition.
b) By definition, an extensive variable, say, energy E depends 04the mass so
that it can be written as E a m or E = km
1
where k is constant of proportionality. The corresponding specific value of
this variable isElm.which is equal to k. It is independent of niass. Hence,
specific value of an extensive variable is an intcnsivb property.
c) Extensive variables: Volume = 2 m3,,mass = 2.67 " = 2.72 X lo4kg.
9.80
I
Intensive variables: Temperature = O°C, pressure = 1 bar, acceleration due
to gravity = 9.80 m sY2, density = = 1-36 x lo4 kg m-',2
Basic Concepts of
Thermodynnmics

The Zeroth Law and the
First Low
2 = 7.35 x lo-' m3 kg-'specificVolume =
2.72 x 104
(specific volume is defined as volume per unit mass)
2) (b)
3) a) Since, f (L, F, T ) = 0, from Eq. (1.9) we get,
Thus
So, if the length of the track is kept constant, the difference in tension
correspondipg to a difference in temperature A T is given by
AF = - YAu AT
Here AT is negative when we consider the change from day to night.
Hence, AF = (2 x 10" N m-') (3.6 x lo-' m2
) (9 x K-I) (25 K)
AF = 1.44 x lo5N
As the temperature changes from day to night, the track contracts. However, the
track is not permitted to contract because its length is being kept constant. Therefore,
a force would be required to expand the track and restore it to its original length and
hence AFis positive,
b) f (L, F, T ) = 0
orL=. L (F, T)
Putting dF = 0 (since the track is free to expand) and
a = 1(a)the difference in length L corresponding to a difference in
L a T ;
temperature AT is given by
AL = (wLAT
If we consider the change from day to night, A T = -22 K
.'. AL = (8 x IO-~K)(15 x 10' m) (-25 K) = - 3m
i.e, there-is a decrease in the length of the track by '3m.

TEMPERATURE
Structure
2.1 Introduction
Objectives
2.2 Principle of Measurement of Temperature
Physical Properties Considered for Measurement of Temperature
. Scale of Temperature. -
'2.3 Gas Thermometer
Constant Volume Gas Thermometer
- Perfect Gas Temperature Scale
2.4 Resistance Thermometers
- Platinum Resistance Thermometer
Thermistors
2.5 Thermocou~les
Thermocouples and Tliermoelectric Circuits
Practical Ther~nocouples
2.6 Radiation Pyrometers
2.7 The International Practical Temperature Scale
2.8 Summary
2.9 Terminal Questions
2.10 Solutions and Answers
211 INTRODUCTION
Would you like to remain outdoor on a hot summer afternoon? Preferably not.
Likewise you will not like to take bath in ordinary tap water during winter. As you
can realise that the above examples are related with the most important
. thermodynamic variable temperature. You have learnt about the concept of
temperature in the previous unit. We can have a feeling of this thermodynamic
I variable by way of our senses. But it has an element of difference from thesenses of
smell andtgste. We do not go for quantifying these senses. In other words we would
I
, never go-&measure how much higher is the smell of a rose than that of a sunflower.
I
1
Nor we question how much is a curry tastier than a salad, whereas for temperature
we need to quantify, Everyday you get to know about the temperature of the
iAPortant cities from the newspaper. If someone has a prolonged fever a temperature
chart of hislher body is maintained.
The device used for measuremel~tof temperature, as you know, is called
thermometer. For thc above measurements we normally use a liquid-in-glass
thermometer with which you are familiar. It utilises the property of variation of the
volumeof a liquid with temperature. However, the temperatures of our interest are
not only restricted to that of a place or a man's body. It ranges from as low as
0.01K, where helium solidifies, to 6000 K, the temperature of sun's surface. How do
we measure these temperatures? Certainly not by liquid-in-glass thermometers. In
fact there are many othnr kind of thermometers about which you will read in this unit.
The action of every kind of thermometer is based on the temperature-variation of
'
some physical property. In this unit you will read how gas thermometers, resistance
thermometers, thermocouples and radiation pyrometers are used for the
measurement of temperature.
You will also learn how a scale is developed fox the measurement of temperature.
We shall chiefly deal with the perfect gas scale. Finally we shall discuss briefly the
International Practical Scale of Temperature.
As you know the change in temperature of a body is brought about by heat. But what
is the nature of heat? The answer to this question will lead us to the first law of
thermodynamics. In the next unit you will read about that.

Unit 1(complete)

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Unit 1(complete)