Good morning I welcome you to the session
of basic thermodynamics what we are discussing last class is the thermodynamic equilibrium
of a system just to again repeat it. For a system to be in thermodynamic equilibrium
the property should remain invariant with time and that should be uniform within the
system. This can be done by two ways either by allowing
the system boundary to be such that system cannot interact with the surrounding. Another way of obtaining this that when the
system properties are same as those of surroundings that means there is no imbalance between the
properties of the system and the surrounding. To meet this thermodynamic equilibrium of
a system three types of equilibrium are necessary these are like this. If we write thermodynamic equilibrium, it
has got three types of equilibrium, one is thermal equilibrium another is mechanical
equilibrium another is chemical equilibrium. These three equilibriums have to be maintained
simultaneously to have thermodynamic equilibrium. Let us first discuss on thermal equilibrium. Regarding thermal equilibrium there are three
issues which come into picture. One is thermal equilibrium, what is thermal
equilibrium? In this context very important law of classical
thermodynamics is zeroths law and the concept of temperature. It is the thermal equilibrium and zeroths
law which defines the temperature. Today, we know the concept of temperature. What is temperature if I ask you today that
you can tell there are two ways of defining temperature? One is the microscopic way of defining, it
is the average kinetic energy of the molecules that gives rise to the temperature of a body
and this kinetic energy mostly composed of the translational kinetic energy. But if you define temperature from the classical
physics point of view or classical thermodynamic point of view today we know the definition
is very simple. It is the property of the system. By virtue of the difference of this property
heat flows from a system of higher temperature to a system of lower temperature. This is the definition of temperature which
we can sense, we can measure property and the difference of which is responsible for
heat to flow. Again we define heat as that form of energy
which flows because of the temperature difference from one system to other. What is thermal equilibrium this is also very
simple we know we have read it earlier also? Thermal Equilibrium is the equilibrium relating
to the flow of heat that means if the two bodies are in contact so that heat can flow
between these two. So thermal equilibrium can only be achieved
if their temperatures are equal that means no heat will flow, so thermal equilibrium
means the equilibrium of temperature. All these things are known today. Off-course zeroeth law is the different way
of expressing these two but we now discuss how these things originated in the classical
physics at the being though we know these thing there is nothing new that what is temperature
we know thermal equilibrium means the two system have to be at the same temperature. So that they are in thermal equilibrium no
heat will flow. But how this concept originated let us discuss
that, the concept of thermal equilibrium and from which zeroeth law evolve and finally
we arrived at a definition of temperature in classical thermodynamics. This is same definition how it was there I We will first discuss the concept of thermal
equilibrium the way it originated in the classical physics this is important in studying thermodynamic
to discuss this. Let us consider the two systems, one is system
A and let us consider this system that it can be characterized by two independent intensive
properties that mean the single component single phase system XA and YA. Let us consider another system which can be
defined by any two such independent properties to define its states that is XB and YB and
let us connect these two systems by an adiabatic wall, what is adiabatic wall?. That is no heat can flow. All the terminologies are known today but
it is true that this is known today but the way it originated in the classical physics
I will describe that adiabatic wall. What is an adiabatic wall? The adiabatic wall is a wall which will be
sufficiently strong to resist any stress developed because of the imbalance of the properties
between system A and system B and it is a kind of wall in practice made of wood, felt,
asbestos because originally it was conceived like that the experiments were done like that. Then people came in to the definition of adiabatic
process? What is the adiabaticness? What is the diabaticness? All these things come afterwards. Initially it originated. Let us have a boundary of that sort wood,
felt, asbestos which does not allow the heat to flow and it is sufficiently strong to withstand
the stresses developed because of the imbalance of the properties. This type of wall is adiabatic wall. If you separate them by adiabatic wall it
has been found that now if entire thing is isolated from the surrounding then it has
been observed that if number of combinations of XA YA and XB YB are available that means
system A can assume any pairs of its XA YA or any arbitrary values of XA YA. Similarly any arbitrary values of XB YB for
system B they can assume any properties. So any values of XA YA and XB YB are available
if they are separated by an adiabatic wall. If the same two systems, system A with the
same XA YA and the system B with XB YB they are available and they are separated
by a metallic wall which are good conductor of heat for example. That is known as diathermic wall. If they are insulated from the surroundings,
then it has been found that some process will take place by virtue of which XA YA and XB
YB will change and the equilibrium will be attained by the two systems only certain restricted
values of XA YA and XB YB that means we cannot have arbitrarily all possible value of XA
YA XB YB like this. Some restricted values of XA YA and XB YB
will be possible so that no further change will take place between the two systems if
they are separated by a diathermic wall this is the classical concept of thermal equilibrium. That means this gives rise to the definition
of thermal equilibrium. Then the definition of thermal equilibrium
comes in this way. This gives rise to the definition of thermal
equilibrium which can be written now this way. This is the original definition in the classical
physics of thermodynamics the thermal equilibrium. The equilibrium states achieved by two or
more systems, characterized by restricted values of the thermodynamic properties of
the systems, after they have been in communication with each other through a diathermic wall. This is an equilibrium state of a system between
the systems which they achieve with restricted values of thermodynamic coordinates so in
they are in contact by a diathermic wall that means all arbitrary values are not possible
that is the concept of thermal equilibrium. If we go little further what we will see now
this experiment was made like this if there are two systems, system A with similarly XA
YA properties, system B with properties XB YB. They are separated by an adiabatic wall. Let us consider a separate system C
with properties designated by XC YC all these systems are conceived for simplicity in understanding
that they are specified by two independent properties and if they are separated that
means system A and B are in communication with system C through diathermic wall. That means this wall is diathermic wall already
I have shown by this color diathermic wall, while this is adiabatic wall and everything
is insulated that means if system A or B in communication by adiabatic wall and system
A and B with C are in communication with diathermic wall then what happens that XA YA and XB YB
changes as XC YC change. That means this properties change XC YC because
of the interaction with system A and system B and system A and system B the properties
also change and they attain some restricted values. For examples at some equilibrium we get some
properties XA YA some property values XB YB for system B and XC YC for system C. If we remove this and put these two systems
after achieving this equilibrium system A and system B then we will see that by a diathermic
wall then what we will see that the system property that means the property XA YA and
XB YB will not change. That means when they are in thermal equilibrium
what I will not use here that means when they come to equilibrium with system C by a diathermic
wall both system A and system B separately and individually then their properties will
come to a uniform and invariant values they need they are in communication with each other
by a diathermic wall their system does not change. This gives rise to the definition of zeroeth
law. This phenomenon is known as zeroth law of
thermodynamics which is very important in classical thermodynamics which gives the definition
of temperature. What is zeroeth law? When two bodies or systems are in thermal
equilibrium with a third one then they are in thermal equilibrium with each other. Today this is very known fact if you tell
this to school boys at class seven eight level then they will tell this is known fact. But this is the way how it originated in the
classical physics. We have to go through this origin so this
is the basically zeroeth law. The concepts of these two things that the
thermal equilibrium and zeroeth law gives rise to the definition of temperature in classical
thermodynamics. If one ask you what is the definition of temperature
in classical thermodynamics, how do you define? The temperature has to be defined without
the definition of heat without going to the molecular level that it is not the average
kinetic energy of the molecules then how do you define? That is giving by the classical thermodynamics. Afterwards we will define heat with the help
of temperature as the property. Therefore without taking the concept of heat
or without consideration of the molecular events we define temperature by zeroeth law
in classical thermodynamics like this. It is a thermodynamic property that determines
whether or not a system is in thermal equilibrium with other system. This is in short the concept of the thermal
equilibrium. We come back to this thing that a system to
be in thermodynamic equilibrium it has to fulfill the three equilibriums; thermal equilibrium,
the mechanical equilibrium and chemical equilibrium. Thermal equilibrium is the equilibrium of
temperature that means when the temperature as same between two bodies there will be thermal
equilibrium. What is mechanical equilibrium? Mechanical equilibrium is the equilibrium
with respect to work transfer between the system and the surrounding that means when
there will be no imbalance of properties any properties that will cause work transfer. For example mechanical work transfer there
has to be an imbalance in the pressure between the system and the surrounding. In terms of one of the primitive properties
pressure, sometimes many books tell like that that if there is no imbalance between the
pressure of the system and the surrounding the system attends the mechanical equilibrium. Mechanical equilibrium in a broader aspect
is the equilibrium when system does not go with any process involving work transfer but
in a limited sense we can tell is a mechanical work transfer this is only responsible if
there is the disbalance of pressure between the system and the surrounding. So there is no disbalance of pressure between
the system and the surrounding then there is no question of mechanical disequilibrium
that means the system is in mechanical equilibrium. Another is the chemical equilibrium. Now you see all these non equilibrium is responsible
for change of certain class of properties when there is no thermal equilibrium the property
temperature goes on change. An equilibrium criterion is that all property
should be invariant with time. Similarly for mechanical disequilibrium some
other some properties will change that is pressure will change, volume will change some
other associated properties also will change. This is sensed basically through the change
in these properties automatically other properties will change because they are related depending
upon the process constants that i will come afterwards. Similarly chemical equilibrium is the equilibrium
of the system with respect to chemical reactions and mass transfer that means if there is no
disequilibrium between the system and the surrounding by which the chemical reactions
or mass transfers do not take place then the system is in chemical equilibrium. That means a system has to be in chemical
equilibrium it should not go for any process involving transfer of masses and the chemical
reactions which causes the species density you see the multi component system. The species amount or the masses of species
to change with time they should not be invariant with time. That means chemical equilibrium is responsible
for these changes that means for chemical equilibrium to be attain a system and surrounding
has to be in equilibrium with respect to certain gradients. What is that gradients? That is the concentration gradients. That is the chemical affinity, the gradients
of chemical affinity these are the things which causes the mass transfer and the chemical
reaction to take place. Similarly again I repeat for mechanical equilibrium
pressure gradient has to be 0 and the gradients of other quantities which are responsible
for other types of work transfer also that I will come afterward should be 0. Similarly for thermal equilibrium temperature
gradient has to be 0 between the system and the surrounding. If all these things are met at a time then
the system attains simultaneously thermal equilibrium, mechanical equilibrium and chemical
equilibrium and as the whole the system attains thermodynamic equilibrium. This is the basic concept of thermodynamic
equilibrium. After this I go to the concept of
thermodynamic process which is very important. Thermodynamic process Is a simple process
that when a system interacts with surrounding. First of all define a system at an equilibrium
state when a system interacts with the surrounding a process takes place how does it interact
with the surrounding in terms of energy transfer also mass transfer may take place if the system
is not a closed one then after some time it comes to another equilibrium state. So from one equilibrium state to other equilibrium
states system comes through a natural process interacting with the surroundings. Now question is very important in this regard
process is very simple everybody knows that how to specify the process in thermodynamic
coordinate diagram. That means for a example you see that if I
express the system by two coordinate Y and X let pressure and volume here. Let this be the state one initial state and
system is allowed to interact with the surrounding in forms of mass energy whatever may be very
general open system and after sometime system comes again to an equilibrium condition. If we give infinite time the system will always
come to an equilibrium condition when the properties will be same as that of the surrounding
it will come to a dead state but sometimes we forcefully stop this process so that we
forcefully make this system at another final equilibrium state let this is two. We can tell the system has changed from state
one to state two by a process but to specify that process in the thermodynamic coordinate
by a continuous line then what we will have to do we will have to define the intermediate
states the succession of intermediate state through which the system has passed so that
we can define the process as a continuous line on a thermodynamic coordinate diagram. How to do it? Because if we tell that this is one of the
intermediate point this is one of the intermediate point that means these points we can show
only if the system is in equilibrium at that state but when a system natural process occurs
from one state to other state system is in dynamic equilibrium system is not in thermodynamic
equilibrium. All the processes are non equilibrium in nature
means the properties are varying with time and also properties may not be uniform within
the system. Therefore a natural process can never be shown
by such a specified path. To understand this let us go to this. Let us see a arrangement like this. Any difficulty you now ask me. This is very important concept. Let us consider a cylinder in practical case
and a piston and a and let us consider the piston is loaded with some weights which are
divided. You have read all those things earlier any
confusion is there you please ask me sir this is my confusion number of weights. Let us consider that there is some gas within
the cylinder and consider this gas as a system. This gases exert some pressure and at the
initial state let us consider the weight of the piston along with the external weights
balance this pressure p. Let this pressure is p1 and initial volume
is v1 the system is at state one. Let us consider there are two stops which
allows the piston not to go beyond this. Now what happens? This is a system at equilibrium state which
is specified by its pressure p1 and volume v1 how it is in equilibrium? Now this pressure p1 exerted on the piston
balances the piston weight and the external weights, P one into the pressure force. So that this is in equilibrium the gas has
a fixed volume, fixed pressure and along with that other properties are fixed if it is a
system where two properties are required to fix its states. Practically if we now release all the weights
simultaneously or a considerable portion of the weight from the top what will happen there
will be a pressure imbalance between the gas and the surrounding. So piston will move upward piston will move
upward and it will heat the stop and ultimately it will come here. If we finally make this just remove this what
will happen the piston will come here and at that time an equilibrium state will be
achieved with the gases inside and let this gas pressure will be p2 and in that case what
will happen this pressure will balance the piston weight and the reaction of the stop. Practically what will happen there will be
some oscillations here and ultimately it will be fitted like this so the reaction and the
weight will be balanced by this p2 and let the volume is v2. So the final state will be p2v2. We know the initial state and final state
now while going so very fast the piston will move any intermediate state if you want to
observe by any instrument measuring pressure, temperature other things you will see they
are not only varying with time they may not be uniform also throughout that is very first
process because we want to make it very fast. We keep a high imbalance in the pressure remove
all the weights then it will come to it and practically all natural processes are like
that but instead if we conceive a process like this which is infinitely slow like this
instead of releasing the weight one by one if we do like this we release the weight like
this, instead of doing this if i take a small weight out that means we create a very small
amount of pressure imbalance. So that the in that case the piston will move
very slowly and come to a equilibrium position like this that means there is weight we remove
only this one stop is there as usual. This gas is there and this pressure let it
is p1 dash which is equal to p1-dp pressure is reduced by this and v1 dash which is equal
to v1+dv. Piston has moved a infinite small distance
because the imbalance is very small and if this way we remove the weights let us consider
another picture that we have removed another weight slowly if we think in terms of infinite
resolution of the weights and go on reducing the weights like this and piston
will be ultimately coming to this position slowly and in that case the third position
is that this is v1 double dash it is p1 dash minus dp similarly v1 double dash is v1 dash
plus dv. Slowly and gradually if we remove the weight
in such a way that all the time we restrict the piston to move infinite small distance,
under a infinite small imbalance of the pressure. Piston comes from an initial pressure p1 and
volume v1 to an intermediate pressure which is slightly less than p1 volume is slightly
more than v1 and then again slowly if we remove and if we do like that after a long time if
you do it with infinite resolution of this weight the piston will come here and if we
do this then what are the differences then all the intermediate state points are almost
equilibrium. They are in the limit of thermodynamic equilibrium. To conceive it in an ideal case to be exact
thermodynamic equilibrium we have to stop the process. That is just like consider the walking of
an old man he walks certain distance small distance then he stops again walk stops the
small man with the heart disease walks this is a very good example he stops walks and
so then all that means he covers the distance in such a way that infinite small distance
he travels and then it comes to an equilibrium state everything within the system becomes
uniform and invariant with that. But even if you do not stop if you do it continuously
and gradually removing this weight then in all the succession states we can think of
a sort of equilibrium in the limit equilibrium process. In this case what happen that if you now pv
that this is your state one p1v1 then all the states if you come like this you can specify
by process then let this is your p2v2 and all these succession states you can specify
and you can join this as a single curve and you can specify the process that means this
is the thermodynamic processes. The process can be specified provided the
intermediate succession states are in equilibrium. This type of process is known as quasi equilibrium
quasi static process, quasi means almost. I will discuss that length this thing reversible
at the present movement you just write it that it is known as a reversible process the
concept of reversible process we will come afterward. One of the criteria for reversible process
is the quasi equilibrium process. This process is quasi equilibrium or quasi
static that means almost static process that means process is almost static infinite small
departure then again rise but even if we it is done continuously this gap this means this
continuous there is the infinite one in finite infinite number of small elemental process
is we conceive. So that system departs from one state and
goes to the other state only under an infinite small imbalance of the properties between
the system and the surrounding. In this way this system moves. In this case what will happen infinite long
time will be required for a system to come from one equilibrium state to other equilibrium
state? This way only we can specify a process. Very important thing at this moment we must
know that whenever we specify a thermodynamic process performed by a system in thermodynamic
coordinate diagram we always consider the system the process is a quasi equilibrium
or quasi static or a reversible process. Next I will go quickly time is short for this
today’s class which I have thought that I will complete this thing. The thermodynamic concept of energy transfer
let me write thermodynamic concept of energy transfer. This is very important. In thermodynamics the concept of energy transfer
knows thermodynamic concept of energy rather you cut the transfer thermodynamics concept
of energy. In thermodynamics energy has been given two
status that I will tell you in first law. One is the energy as a state variable property
of a system, energy as a point function another is energy as a path function two forms of
energy one is energy in storage that is system can store energy for an example let us see
that a mass of gas at high temperature what do you tell that gas is storing the gas at
the system storing what type of energy because of its temperature internal energy. Internal energy stored in the gas you consider
a fuel sometimes we tell the chemical energy of the fuel which can be converted into heat
by burning then that heat can be converted into mechanical work but always we tell that
chemical energy that is the bonding energy of the molecules. There are various ways a system can store
energy by virtue of its temperature that internal energy that is more precise the intermolecular
energy chemical energy another thing is that mass of gas is there you start the gas you
creates the motions of the microscopic particles. Then we consider the gas as a system then
we can tell because of the motion of the particles the system stores some energy which is the
kinetic energy of its particles another form of energy is the potential energy. What is the definition of potential energy
that means if you place a system in a conservative force field because of its position in a conservative
force field a system’s energy stored in the system because non dissipative work is
done to place the in a conservative force field that is stored in the system as an energy
always we tell that for example gravitational potential energy in the gravitational field
of what always we have some energy stored within us because of which we can shift our
self from one position to other. These are the energy that you are acquainted
with or the energies which can be stored in a system. So thermodynamics tells this energy all together
as an internal energy of a system that is system which has these stored energy internal
within it and that is the point function or state variables. Another energy comes in to picture these are
not stored within a system these are energy transfer that means these cannot be conceived
when a system is in equilibrium state. This can be conceived only in the form of
transit when there is a process takes place. These types of energies are classified into
two groups. Work transfer and heat transfer no other groups. Therefore now I can write the thermodynamic
concept of energy is that one is energy in storage very important this thing i am telling
one is energy in storage which is known as internal energy in general though we mean
by internal energy colloquially intermolecular energy by virtue of temperature not that all
form of energy which is stored internally within system. This is a point function because I know that
internal energy is a property state variable that means this is associated with the state
of the system another form of energy is energy in transit which is the energy which is transferred
between a system and the surrounding. These are path functions and these energy
in transit or energy transfer again can be divided into two groups one is work transfer
another is heat transfer. I have given the examples of energy in storage
internal energy of a system point function or state variables. Energy in transit only comes into play when
two systems are interacting with each other or systems surrounding interacting with each
other. One form of this energy in transit is heat
that is the energy in transit by virtue of a temperature difference and flows in the
direction of the negative temperature gradient means high temperature to low temperature. And all other form of energy transfers is
all work transfer. Usually mechanical engineers will be little
bias to tell work means mechanical work it is f dot ds no all other form of energy transfer
it may be an electrical energy transfer. If you put a voltage across a resistor which
allows a current to flow an electrical energy is being transferred to the resistor as a
system that is work transfer. But we use an adjective to separate it from
different categories of work transfer. That is another adjective is used mechanical
work transfer, electrical work transfer, magnetic work transfer you understand? But all are coming under the category of work
transfer only heat is the different category of transfer which takes place by virtue of
the temperature difference. These are the two different concept of energy
in thermodynamics one is the energy in storage another is the energy in transit that is the
path function. We see that there are different form of time
is up what is the time five minutes left five minutes left. So think today we are late I will stop here
today just I will ask you that if you have any queries you can ask questions only because
I will always give five minutes time for questions I will be happy to answer your questions from
whatever I have taught today of course a very less amount of things had been told today
please. That is a very simple thing why natural processes
cannot be specified by path? Because to specify a path of the process in
thermodynamic coordinate how can you specify a path or specify a continuous curve? If you have got intermediate points so you
understand because to specify a path how do you draw a curve very simple thing go back
to school level class five six level geometry that to draw a curve you know you have to
know the number of points you do not know the equations before hence. So numbers of points have to be joined that
means the number of state points have to be known. So whenever a state point is specified in
thermodynamic coordinate the thermodynamic system has to be in equilibrium. How do you specify the system pressure or
system. One example, any two properties x and y. If you have to show an intermediate points
with x and y for the thermodynamic system for any natural process in any intermediate
state this x and y are not constant within the system. You cannot specify at the beginning I told
that if you tell. My intermediate points is having this pressure
and this volume but unfortunately if you measure continuously the pressure and volume for an
example in a natural process where a gas expands we will see even the pressure equilibrium
is not there internally within the gas. How can we specify the gas by a single pressure
or a single temperature moreover they are invariant with that because the two things
are clubbed a non equilibrium states where the system changes rapidly and continuously
with time there is no equilibrium within the system itself. So it does not attain a uniform value of the
pressure. Therefore to specify any intermediate points
to draw the curve we require that particular criterion of equilibrium that should be uniform
within the system and momentarily invariant with time. That is why all processes are natural processes
cannot be specified by a path only it can be done if the process is quasi equilibrium. Thank you.
I couldn't get past the high pitched tone that's there... it seems like it's there throughout the whole video.