Quasistatic and reversible processes Video Lecture - Class 11

SAL: In the last video, where
we talked about macrostates,
we set up this situation where
I had this canister, or the
cylinder, and had this
movable ceiling.
I call that a piston.
And the piston is being kept
up by the pressure from the
gas in the canister.
And it's being kept down by, in
the last example I had, a
rock or a weight on top.
And above that I had a vacuum.
So essentially there's some
force per area, or pressure,
being applied by the
bumps of the
particles into this piston.
And if this weight wasn't here--
let's assume that the
piston itself or this movable
ceiling itself, it has no
mass-- if that weight wasn't
there it would just be pushed
indefinitely far, because
there'd be no pressure from
the vacuum.
But this weight is applying
some force on
that same area downwards.
So we're at some equilibrium
point, some stability.
And we plotted that on this
PV diagram right here.
I'll do it in magenta.
So that's our state 1 that
we were in right there.
And then what I did in the last
video, I just blew away
half of this block.
And as soon as I blew away half
of this block, obviously
the force that's being applied
by the block will immediately
go down by half, and so the
gas will push up on it.
And it happened so fast that,
al of a sudden the gas is
pushing up.
Right when it happens, the gas
near the top of the canister
is going to have lower pressure,
because it has less
pushing up against it.
The molecules that are down here
don't even know that I
blew away this block yet.
It's going to take some time.
And essentially the gas is going
to push it up, and then
maybe it'll oscillate down,
and then push it up, and
oscillate down a little bit.
It'll take some time eventually
until we get to
another equilibrium state, where
we have a new, probably,
or definitely lower pressure.
We definitely have
a higher volume.
I won't talk too much about it
yet, but we probably have a
lower temperature as well.
And this is our new state.
And our macrostate's pressure
and volume are defined once
we're at the new equilibrium,
so we're right here.
So my question in the
last video was,
how did we get here?
Is there any way to have defined
a path to get from our
first state-- where pressure and
volume were well defined,
because the system wasn't
thermodynamic equilibrium-- to
get to our second state?
And the answer was no.
Because between this state
and this state
all hell broke loose.
I had different temperatures
at different
points in the system.
I could have had a different
pressure here
than I had up here.
The volume might have
been fluctuating
from moment to moment.
So when you're outside of
equilibrium-- and I had
written it down over there--
you cannot define, or you
can't say that those macro
variables are well defined.
So there was no path that you
could say how we got from--
erase this-- how we got from
state 1 to state 2.
You could just say, OK, we
were in some type of
equilibrium.
So we were in state 1.
Then I blew away
half the rock.
The pressure went down,
the volume went up.
The temperature also
probably went down.
And so I ended up in this other
state once I reached
equilibrium.
And that's all fair and good,
but wouldn't it have been nice
if there was some way?
If we could have said, look,
you know, there's some way
that we got from this
point to this point?
If we could perform my little
rock experiment in a slightly
different manner, so that all
this hell didn't break loose,
so that maybe at every point in
between my macro variables
are actually defined?
So how could I do that?
Remember, I said that the
macro variables, the
macrostates, whether it's
pressure, temperature, volume,
and there are others, but I said
these are only defined
when we are in a thermodynamic
equilibrium.
And that just means that
things have reached a
stability point.
That, for example, the
temperature is consistent
throughout the system.
If it's not consistent
throughout the system, I
shouldn't be talking about it.
If the temperature is different
here than it is up
here, I shouldn't say that
the temperature of
the system is x.
It's different at different
points.
I really can't make a
well-defined statement about
temperature, similar for
pressure or for volume,
because the volume is
also fluctuating.
But what if I perform that
same experiment?
That same process,
I should call it.
Let me draw it again.
So I have my canister.
And instead of starting with a
rock, just one big rock-- let
me draw, this is my piston right
here, at the top of the
movable ceiling of
the cylinder.
And I have some gas
inside of it.
Instead of having just one big
rock like I had over here, how
about I start with an equal
weight of rock?
But let's say I have a bunch of
small pebbles that add up
to that same rock.
So just a bunch of, well, you
know, just a pile of pebbles.
You know, maybe they're sand.
They're super duper small.
Instead of just blowing away
half of the sand all at once,
like I did with that rock over
there, and immediately jumping
to that state and throwing the
whole system into this
undefined state of
non-equilibrium.
Instead of doing that, let me
just do things very slowly and
very gently.
Let me just take out one grain
of sand at a time.
So if I just take out
one grain of sand.
And so I took out an
infinitesimal amount of weight.
So what's going to happen?
Well this piston's going to
move up a little bit.
And let me draw that.
So let me copy and paste it.
So I just took out one
little piece of sand.
The force pushing down will
be a little bit less.
The pressure pushing down
will be a little less.
And so my piston-- let me see
if I can draw this-- it will
have moved up-- let me erase
it-- it will have moved up a
very infinitesimal--
infinitesimal means an
infinitely small amount-- it
would have moved an infinitely
small amount of time.
And so you wouldn't have thrown
that system into this,
you know, havoc that I
did this last time.
Of course, we haven't moved
all the way here yet.
But what we have done is, we
would have moved from that
point maybe to this other point
right here that's just a
little bit closer to there.
I've just removed a little
bit of the weight.
So my pressure went down
just a little bit.
And my volume went up
a just a little bit.
Temperature probably
went down.
And the key here is I'm trying
to do it in such small
increments that as I do it, my
system is pretty much super
close to equilibrium.
I'm just doing it just slow
enough that at every step it
achieves equilibrium
almost immediately.
Or it's almost in equilibrium
the whole time I'm doing it.
And then I do it again,
and do it again.
And I'll just draw my drawings
a little less neat, just for
the sake of time.
Let's say I remove another
little dot of sand that's
infinitely small mass.
And now my little piston will
move just a little bit higher.
And I have, remember I have one
less sand up here than I
had over here.
And then my volume in my gas
increases a little bit.
My pressure goes down
a little bit.
And I've moved to
this point here.
What I'm doing here is I'm
setting up what's called a
quasi-static process.
And the reason why it's
called that is
because it's almost static.
It's almost in equilibrium
the whole time.
Every time I move a grain of
sand I'm just moving a little
bit closer.
And obviously even a grain of
sand, the reality is if I were
to do this in real life, even
a grain of sand on a small
scale is going to reek
a little bit
of havoc on my system.
This piston is going to
go up a little bit.
So say, let me just do even a
smaller grain of sand, and do
it even a little bit slower
so that I'm always in
equilibrium.
So you can imagine this is kind
of a theoretical thing.
If I did an infinitely small
grains of sand, and did it
just slow enough so that it's
just gently moved from this
point to this point.
But we like to think of it
theoretically, because it
allows us to describe a path.
Because remember, why am I
being so careful here?
Why am I so careful to make
sure that the state, the
system is in equilibrium the
whole time when I get from
there to there?
Because our macrostates, our
macro variables like pressure,
volume, and temperature, our
only defined when we're in
equilibrium.
So if I do this process super
slowly, in super small
increments, it allows me to keep
my pressure and volume
and actually my temperature
of macrostates at
any point in time.
So I could actually
plot a path.
So if I keep doing it small,
small, small, I could actually
plot a path to say, how did I
get from state 1 to state 2 on
this on this PV diagram.
And you might say, hey, you
know, Sal, this is all-- And
I'll take a little
step back here.
I always found this
really confusing.
You know, you'll see a lot of
talk in thermodynamic circles,
or even in your book about--
it has to be a quasi-static
process, and I always used to
wonder, why are people going
through these pains to describe
this process where
you're removing sand
after sand?
And the whole point is because
you want to get as close to
equilibrium the whole time
you're doing it as possible so
that your pressure and volume
are defined the whole time.
The reality is, in the real
world you can never get
something that's continuously
defined, but you can just do
really, really, really
small increments.
So that at each small increment
you're at some
equilibrium.
And if you're not happy with
that, you can do even smaller
increments.
So at some point, at some
limiting point, you do have
some type of continuous state
change, while you're always in
equilibrium.
It's almost an oxymoron, because
you're saying you're
static, you're saying that
you're in equilibrium the
whole time, but clearly you're
also changing the whole time.
You keep removing little
pieces of sand.
But you're moving them just
slowly enough that all that
crazy up and down motion, and
all of the flux, and all of
the weird temperature changes
don't happen.
And it just, you know, just
that it slowly, slowly,
slowly creeps up.
The reason why I'm even going
through this exercise is
because it's key when we start
talking about thermodynamics
and these PV diagrams, and
we'll start talking about
carnot engines and all of that,
that we be able to at
least theoretically describe the
path that we take on this
PV diagram.
And we wouldn't have been able
to do that if we can't assume
that we're dealing with a
quasi-static process.
Now there's another term that
you'll hear in thermodynamic
circles that really, I mean, to
me it really, I don't know,
I had trouble comprehending it
the first time I heard it,
called reversible.
And sometimes these terms
quasi-static and reversible
are used interchangeably, but
there is a difference.
Reversible processes are
quasi-static, and most
quasi-static processes are
reversible, but there are a
few special cases that aren't.
But the idea of a reversible
process is something that
happens so slowly.
So in this example I took off
a grain of sand and I got to
the state, but if I assume that
no friction when, you
know, when this piston moved up
a little bit, in the real
world, let's say if this piston
was metal, when this
rubs against the canister,
there'd be a little bit of
friction generated and a little
bit of energy would be
dissipated as friction
or heat.
But in a reversible process,
we're assuming that, look,
this is frictionless.
When anything happens in the
system, when we go from this
state right here-- let's
say this the state a,
this is state b.
So this is state a,
this is state b.
When we go from this state
to this state, one, we're
infinitesimally close to
equilibrium the whole time, so
all of our macrostates
are well defined.
And even more, when we move from
one state to the other,
there's no loss or dissipation
of energy.
So those are two important
characteristics.
One, infinitely close to
equilibrium at all times, and
no loss of energy.
And the reason why that matters
for a reversible
process is because if we wanted,
if we were sitting in
state b, we could just add
another grain of sand back in,
push down this piston infinitely
slowly, at an
infinitely small increment,
and get back to state a.
So that's why it's called
reversible.
You could be at this point right
here, and take out a
little bit of sand, and get
to this point right here.
But if you want, since no energy
was lost, you could add
a little bit of sand, and get
back to this point right here.
Now the reality in the real
world is, there is no such
thing as a perfectly
reversible process.
There will always be, whenever
you do anything, there will
always be some energy or heat
lost to the process.
In the real world, if I moved
down here, if I tried to put
the sand back I would lose some
energy and probably get
to a little slightly
different point.
But you don't have to
worry about that.
The important takeaway from
this video is that, in the
situation I described there,
there was no intermediate
macrostate variables, because
our system was in flux, it
wasn't in equilibrium.
So if we wanted to get
intermediate states, we just
have to essentially do
this process slower.
And so slow, I mean, it
theoretically would take you
forever, so we can only
approximate it.
But the sand gives you an idea
of what we're talking about.
And if we did it slowly with
these infinitesimally small
particles of sand, then we can
define the state at every
point along the process.
And that's why we call it
quasi-static, because at any
point it's almost static.
It's almost in equilibrium.
So our pressures, volumes, and
temperatures can be defined.
And if we add to that the notion
that we haven't lost
any heat when we're going in one
direction or another, we
could say it's reversible,
because if we took a piece of
sand away, we can always add
a little bit of sand next.
Now, actually, with that said,
let me give you the one
example of maybe a
quasi-static-- no, actually
I'll save that for
future video.
Anyway, hopefully you understand
that these are two
concepts that used to really
confuse me, and hopefully this
clears it up a little bit.
And I think more than what it
is, I think the first time I
read about them I'm like, OK,
well what's the big deal?
The big deal is, it allows you
to define your macrostates for
every state in between
these two states
that you care about.
When you just did it as
a regular kind of
non-quasi-static process,
in between you
don't know what happened.