Quasi state process - Thermodynamics Video Lecture - Class 11

hello friends this video on
thermodynamics part 10 is brought to you
by exam feel calm no move here from exam
please make sure that you have watched
all the videos from part 1 to 9 before
going ahead with part 10 now that we
have studied the basics of
thermodynamics we have also learnt about
the zeroth law and the first law of
thermodynamics
this is time to discuss some of the
special thermodynamic processes we will
discuss or I will introduce as well as
discuss in detail some of the
thermodynamic processes so before we go
into those processes it is necessary
that we study about certain
terminologies related to thermodynamics
so we will start with something called
quasi static process what is quasi
static static process in simple English
want the static means static means
something which is at rest something
which is not moving and dynamic means
something which is moving right so if I
say quasi static quasi means the term
quasi means something which is partial
that is semi you can say semi static
that means something which is not purely
stationary or it is not purely fixed or
not purely moving so what do I mean what
do I mean exactly mean by a quasi-static
process the first thing is quasi static
process is a hypothetical construct what
do I mean by hypothetical construct it
means that in reality there is nothing
called quasi static right in reality we
don't see any such thermodynamic process
which is semi static neither fully
static nor fully dynamic so it is a
hypothetical construct now you will say
if it does not exist in reality then
what is the point of studying this or
why was this process avoided this
process ever evolved that is because
when you go for your higher studies in
science
when you go into your research fees
there are many things which you have to
consider or which you have to consider
as a hypothesis because using that you
reach to many other things because using
that concept you build many other
concepts which are real-life concepts
so this quasi-static process also has a
significance of that sort as we go ahead
I will tell you what is the significance
of quasi-static process so quasi-static
process is an infinitely slow process so
basically what happens in a quasi-static
processes whenever a system departs
infinitesimally from its original state
such that the depart such that the way
it is moving from its original position
that is not at all visible then we call
that process as a quasi-static process
let me give you an example from your day
to day life let us suppose that a person
let us suppose this is our Ronny Ronny
is standing at this position now Ronny
has to go from this point to this point
now suppose Ronny take big steps in once
he takes big big steps and reaches that
point in just four steps right
so when Ronny moves from this position
to this position you can very clearly
see his movement you can very clearly
visualize his change in position right
so the process is quite fast I mean you
can see that he is changing his position
and he reach that piece because he is
moving fast now consider another
situation where Ronnie is initially
standing again at the same point
he moved very little he moved so little
that he moved from this point to say
this point now when you see Ronnie
moving from this point to this point you
will not even observe that he changed
his position
and you will not even realize that he
moved but actually he moved but for
somebody who is saying the movement or
the change is so small that it is not
observed now suppose if Ronnie keeps
taking such small small steps and keeps
moving ahead it will be something like
you will not realize his change in
position because he is moving very
slowly but at one point of time you will
realize that he reached his destination
right so who are static process is
something very similar to this this
process takes place very slowly and the
way it departs from his original state
it is almost not observable you don't
realize that it is moving from one state
to another state now here it was
position but when we talk of a process
it talks about state it is moving from
one state to another state what does a
state a state is nothing but a condition
which is defined by the variables
pressure volume or temperature so system
changes its variables pressure
temperature volume so slowly that it
remains in equilibrium with its
surroundings
throughout because the your variables
don't change that rapidly because you
change your state in such a way that the
difference of the new variables with the
initial variables is negligible so all
the time your system remains in
equilibrium with the surroundings that
is for example if you want to take a gas
from some initial state say PT which is
defined by it as suppose you have any
gas whose initial state is defined by
pressure P temperature T you want to
take this gas to a new state defined by
P - P - so what you can do you change
the surrounding pressure by very small
amount by very infinitesimal amount then
allow the system to reach that pressure
and continue till P
justbe - let us suppose you make the
surrounding pressure to increase by very
very small amounts so your system will
gradually reach that pressure and in due
course of time it will reach the final
pressure now the question is which
processes are approximated to
quasistatic process so which kind of
processes fall under such category or
what are the characteristics that what
are the characteristics that show that
this process is a quasi-static process
the first criteria is extremely slow
processes quasistatic processes are
extremely slow any fast process cannot
be quasi static because a process which
takes place rapidly it will not be in
equilibrium with the surroundings
because there will be marked difference
in its macroscopic variable so it will
not be in equilibrium process not
involving accelerated motion because
wherever you will have accelerated
motion you will have change in the
macroscopic variables and accelerated
motion also signifies fast processes
processes not involving large
temperature gradient this is very
important what do you mean by
temperature gradient temperature
gradient is nothing but the difference
in temperature let us suppose you have a
process where the initial temperature is
t1 and the final temperature is t2 right
so when if the difference between t1 and
t2 is very high and your process and the
system is changing from its initial
state t1 to final state t2 then it
cannot be a quasi-static process because
as I told you quasi static process
should take place in such a way that
there is very small temperature gradient
the difference between temperature
should be very small so these are the
three things which characterize a
process as a quasi-static process thank
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