L10 PV work irreversible - Thermodynamics, Class 11 Video Lecture

hello friends this video on
thermodynamics part 10 is brought to you
by the exam fear calm no more fear from
exam before watching the video please
make sure that you have watched part 1
to part 9 we have seen examples where
the volumes were constant the volunteer
constant right but it is not always the
case
the volume is always not constant by the
volume increase or decrease and there is
a work done in the system is expanding
that new system is doing some work the
volume is increasing system is
contracting somebody is doing work on
the system that means we can see this
the volume is linked with the book there
is something called pressure volume work
because the volume is linked to the ball
the change in volume is length with the
volume is increasing living system is
doing some more volume is contracting
there is somebody is working on the
system right it was a bar like this I
have given an example also where if
there's a gas and if it expands and
there is a suppose a wood plank here it
covers is useiess it expands will go up
you put some iron rod air it will go
down right that case you go down a
putting some work here volume is
contracting system is doing works then
it will expand redo go up so the volume
pressure volume work is some kind of
work which is there so will take once in
an irreversible compression scenario
there a vegetable common base problem
because motional process another signal
to let understand the original process
was really easy actually
so let's assume I had this cylinder with
a gas here this is my guy
and this gets squeezed and the pestle
here I'm applying some pressure on this
external wrong I am doing and I am
contracting it and weeks of pressure
which I have put it's caused a constant
external pressure I'm putting I'm
contracting this so this becomes this
there is change volume I have to find
them all that in this case okay I know
the formula from physics that is work is
nothing but force into displacement this
is something I know for physics okay
that is nothing word force into L LS is
less than the suppose and suppose this
is a compression as L but I have
pressure area I have pressure here and
I'm looking for force is then in
relation with force and pressure just
there is we have seen that force is
nothing but pressure into area in this
case we are using P extended so I'll
just put P external so let us put this
force here so my work will become what
so this equation will become word force
becomes pressure into area DX into area
and L was there now you see L into area
is what if this is my area of this cross
section and you multiply with this this
becomes a volume correct this is the
volume Delta volume see this is my area
of the piston area of piston it suppose
if it's PI R square it suppose all of
the ring so this and this is the L here
so PI R square L if you see this in
other word volume of Snyder this muscle
endure this was it
changeable this this is nothing but
change whatever so you see this let's
assume this is one of my twisting like
this and it went from here to here this
was a since Ellen this is the whole area
came out to be PI R square into L that
is if a into L plus a is nothing but my
PI R square which was so a into L is 1 e
into L is nothing but Delta V please not
a to LS Delta V which becomes V
excellent
Delta change in volume ok change in
volume P external becomes the area into
L becomes Delta and Delta V here is
negative if you see Delta V is nothing
but we generally use V final minus V
initial as the convention we use right
we do not think the absolute value of
Delta V here because Delta V by formula
is we final my vacation
since the work is done on the system
that means system has gained
internalized in this case right if this
is negative this is positive or thing is
negative here I want to make it positive
I will add a negative here
why all right all right he'll put the
negative sign this becomes minus P X
Delta Y because work done on system C
because this convention is just PR
following to make our life easy to
understand things here if you take the
absolute value of Delta V you are fine
you don't need to worry about it
everything is positive but generally
we'll talk about Delta V you talk about
VF v fan - volition here we final is
less VD she'll is more this value is
negative
because that what Delta be we take here
Delta VB God was the logical one right
so Delta V is V cannot be racial
mathematically this is negative but if
that is negative this is pause the whole
thing is negative that means this says
that work is done by system but actually
the work is a non system as per our sign
convention so one negative sign here so
my avoid becomes minus P external into
delta T where Delta V is not the
absolute Delta V not the absolute change
in volume example in volume changed from
four to one I am talking about Delta V
is 1 minus 1 this is minus C if you take
the absolute value of Delta V and come
out DV 3 but not only but absolutely
always in talking about the VF minus V I
V final minus V initial this is the
formula we got work done by external
constant pressure whether it changed the
volume by Delta V will be minus P
external into Delta that is 1/4
now this formula will be applicable for
expansion synergy also if you see that K
is expansion Janerio my volume will
increase volume final will be greater
than volume ignition so VF minus VI is
will be negative so this will be
positive VF minus VA positive positive
into positive is positive plus minus
signs negative the Paulding is negative
that means system is expanding correct
this is the general form level gate will
applicable for all scenarios expansion
contraction that is work done as minus p
XML into Delta e very critical formula
presume it is work done as minus P
children into Delta V thank you visit
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