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PROFESSOR: Thermodynamics,
all right, let's start.

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Thermodynamics is the science
of the flow of heat.

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So, thermo is heat, and
dynamics is the motion of heat.

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Thermodynamics was developed
largely beginning in the

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1800's, at the time of the
Industrial Revolution.

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So, taming of steel.
 

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The beginning of generating
power by burning fossil fuels.

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The beginning of the problems
with CO2 and [NOISE OBSCURES]

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global warming.
 

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In fact, it's interesting to
note that the first calculation

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on the impact of CO2 on climate
was done in the late

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1800's by Arrhenius.
 

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Beginning of a generation of
power moving heat from fossil

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fuels to generating energy,
locomotives, etcetera.

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So, he calculated what would
happen to this burning of

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fossil fuels, and he decided in
his calculation, he basically

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got the calculation right, by
the way, but he came out that

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in 2,000 years from the time
that he did the calculations,

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humans would be in trouble.
 

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Well, since his calculation,
we've had an exponential growth

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in the amount of CO2, and if
you go through the calculations

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of -- people have done these
calculations throughout times

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since Arrhenius, the time that
we're in trouble, 2,000 years

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and the calculation, has
gone like this, and so now

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we're really in trouble.
 

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That's for a different lecture.
 

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So, anyway, thermodynamics
dates from the same period

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as getting fossil fuels
out of the ground.

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It's universal.
 

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It turns out everything around
us moves energy around

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in one way or the other.
 

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If you're a biological
system, you're burning

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calories, burning ATP.
 

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You're creating heat.
 

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If you're a
warm-blooded animal.

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You need energy to move your
arms around and move around --

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mechanical systems, obviously,
cars, boats, etcetera.

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And even in astrophysics, when
you talk about stars, black

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holes, etcetera, you're
moving energy around.

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You're moving heat around
when you're changing matter

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through thermodynamics.
 

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And the cause of some
thermodynamics have even been

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applied to economics, systems
out of equilibrium, like big

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companies like Enron, you
know, completely out of

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equilibrium, crash and burn.
 

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You can apply non-equilibrium
thermodynamics to economics.

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It was developed before people
knew about atoms and molecules.

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So it's a science that's
based on macroscopic

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properties of matter.
 

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Since then, since we know about
atoms and molecules now, we

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can rationalize the concepts
of thermodynmamics using

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microscopic properties, and if
you are going to take 5.62,

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that's what you'd learn about.
 

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You'd learn about statistical
mechanics, and how the

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atomistic concepts
rationalize thermodynamics.

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It doesn't prove it, but it
helps to getting more intuition

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about the consequences
of thermodynamics.

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So it applies to macroscopic
systems that are in

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equilibrium, and how to go from
one equilibrium state to

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another equilibrium state, and
it's entirely empirical

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in its foundation.
 

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People have done experiments
through the ages, and they've

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accumulated the knowledge from
these experiments, and they've

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synthesized these experiments
into a few basic empirical

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rules, empirical laws, which
are the laws of thermodynamics.

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And then they've taken these
laws and added a structure of

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math upon it, to build this
edifice, which is a very solid

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edifice of thermodynamics as a
science of equilibrium systems.

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So these empirical observations
then are summarized

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into four laws.
 

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So, these laws are,
they're really depillars.

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They're not proven, but
they're not wrong.

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They're very unlikely
to be wrong.

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Let's just go through these
laws, OK, very quickly.

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There's a zeroth law The zeroth
law every one of these laws

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basically defines the quantity
in thermodynamics and then

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defines the concept.
 

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The zeroth law
defines temperature.

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That's a fairly common-sense
idea, but it's important to

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define it, and I call that
the common-sense law.

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So this is the
common-sense law.

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The first law ends up defining
energy, which we're going to

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call u, and the concept of
energy conservation, energy

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can't be lost or gained.
 

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And I'm going to call this
the you can break even law;

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you can break even law.
 

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You don't lose energy,
you can't gain energy.

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You break even.
 

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The second law is going to
define entropy, and is going to

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tell us about the direction
of time, something that

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conceptually we, clearly,
understand, but is going to put

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a mathematical foundation
on which way does time go.

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Clearly, if I take a chalk like
this one here, and I throw it

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on the ground, and it breaks
in little pieces, if I run

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the movie backwards, that
doesn't make sense, right?

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We have a concept of time going
forward in a particular way.

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How does entropy play into
that concept of time?

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And I'm going to call this
the you can break even at

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zero degrees Kelvin law.
 

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You can only do it at
zero degrees Kelvin.

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The third law is going to give
a numerical value to the

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entropy, and the third law is
going to be the depressing one,

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and it's going to say, you
can't get to zero degrees.

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These laws are
universally valid.

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They cannot be circumvented.
 

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Certainly people have tried to
do that, and every year there's

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a newspaper story, Wall Street
Journal, or New York Times

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about somebody that has
invented the device that

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somehow goes around the second
law and makes more energy than

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it creates, and this is going
to be -- well, first of all,

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for the investors this is going
to make them very, very rich,

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and for the rest of us, it's
going to be wonderful.

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And they go through these
arguments, and they find

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venture money to fund the
company, and they get

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very famous people to
endorse them, etcetera.

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But you guys know, because you
have MIT degrees, and you've,

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later, and you've taken 5.60,
that can't be the case, and

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you're not going to get
fooled into investing money

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into these companies.
 

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But it's amazing, that every
year you find somebody coming

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up with a way of going around
the second law and somehow

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convincing people who are very
smart that this will work.

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So, thermo is also a big
tease, as you can see

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from my descriptions
of these laws here.

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It makes you believe,
initially, in the feasibility

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of perfect efficiency.
 

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The first law is very upbeat.
 

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It talks about the
conservation of energy.

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Energy is conserved
in all of its forms.

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You can take heat energy and
convert it to work energy and

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vice versa, and it doesn't say
anything about that you have

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to waste heat if you're going
to transform heat into work.

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It just says it's energy.
 

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It's all the same thing, right?
 

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So, you could break even if you
were very clever about it,

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and that's pretty neat.
 

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So, in a sense, it says, you
know, if you wanted to build a

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boat that took energy out of
the warmth of the air, to sail

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around the world,
you can do that.

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And then the second law
comes in and says well,

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that's not quite right.
 

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The second law says, yes,
energy is pretty much the same

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in all this form, but if you
want to convert one form of

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energy into another, if you
want to convert work, heat into

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work, with 100% efficiency,
you've got to go down to zero

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degrees Kelvin, to absolute
zero if you want to do that.

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Otherwise you're going to waste
some of that heat somewhere

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along the way, some
of that energy.

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All right, so you can't get
perfect efficiency, but at

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least if you were able to go
to zero degrees Kelvin,

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then you'd be all set.
 

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You just got to find a good
refrigerator on your boat,

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and then you can still
go around the world.

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And then the third law
comes in, and that's the

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depressing part here.
 

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It says, well, it's true.
 

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If you could get to zero
degrees Kelvin, you'd get

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perfect efficiency, but you
can't get to zero degrees

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Kelvin, you can't.
 

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Even if you have an infinite
amount of resources,

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you can't get there.
 

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Any questions so far?
 

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So thermodynamics, based on
these four laws now, requires

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an edifice, and it's a very
mature science, and it

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requires that we define
things carefully.

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So we're going to spend a
little bit of time making sure

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we define our concepts and our
words, and what you'll find

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that when you do problem sets,
especially at the beginning,

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understanding the words and the
conditions of the problem sets

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is most of the way into
solving the problem.

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So we're going to talk
about things like systems.

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The system, it's that part
of the universe that

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we're studying.
 

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These are going to be fairly
common-sense definitions, but

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they're important, and when you
get to a problem set, really

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nailing down what the system
is, not more, nor less, in

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terms of the amount of stuff,
that's part of the system, it's

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going to be often very crucial.
 

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So you've got the system.
 

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For instance, it
could be a person.

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I am the system.
 

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I could be a system.
 

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It could be a hot
coffee in a thermos.

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So the coffee and the milk and
whatever else you like in your

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coffee would be the system.
 

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It could be a glass of
water with ice in it.

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That's a fine system.
 

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Volume of air in a
part of a room.

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Take four liters on this
corner of the room.

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That's my system.
 

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Then, after you define what
your system is, whatever is

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left over of the universe
is the surroundings.

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So, if I'm the system,
then everything else

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is the surroundings.
 

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You are my surroundings.
 

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Saturn is my surroundings.
 

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As far as you can go in
the universe, that's part

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of the surroundings.
 

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And then between the system
and the surroundings

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is the boundary.
 

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And the boundary is a surface
that's real, like the outsides

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of my skin, or the inner wall
of the thermos that has the

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coffee in it, or it could
be an imaginary boundary.

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For instance, I can imagine
that there is a boundary that

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surrounds the four liters of
air that's sitting in

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the corner there.
 

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It doesn't have to be a real
container to contain it.

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It's just an imaginary
boundary there.

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And where you place that
boundary becomes important.

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So, for instance, for the
thermos with the coffee in it,

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if you place the boundary in
the inside wall of the glass or

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the outside wall of the glass
and the inside of the thermos,

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that makes a difference;
different heat

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capacity, etcetera.
 

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So this becomes where
defining the system and the

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boundaries, and everything
becomes important.

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You've got to place the
boundary at exactly the right

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place, otherwise you've got a
bit too much in your system

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or a bit too little.
 

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More definitions.
 

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The system can be an open
system, or it can be a closed

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system, or it can be isolated.
 

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The definitions are
also important here.

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An open system, as the name
describes, allows mass and

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energy to freely flow
through the boundary.

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Mass and energy flow
through boundary.

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Mass and energy -- I'm
an open system, right?

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Water vapor goes
through my skin.

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I'm hot, compared to the air
of the room, or cold if

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I'm somewhere that's warm.
 

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So energy can go
back and forth.

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The thermos, with the lid on
top, is not an open system.

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Hopefully, your coffee is
going to stay warm or

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hot in the thermos.
 

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It's not going to get out.
 

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So the thermos is
not an open system.

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In fact, the thermos is
an isolated system.

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The isolated system is the
opposite of the open system,

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no mass and no energy can
flow through the boundary.

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The closed system allows energy
to transfer through the

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boundary but not mass.
 

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So a closed system would be,
for instance, a glass of ice

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water with an ice cube in
it, with the lid on top.

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The glass is not
very insulating.

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Energy can flow across the
glass, but I put a lid on top,

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and so the water can't get out.
 

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And that's the closed system.
 

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Energy goes through the
boundaries but nothing else.

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Important definitions, even
though they may sound really

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kind of dumb, but they are
really important, because when

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you get the problem, figuring
out whether you have an open,

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closed, or isolated system,
what are the surroundings?

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What's the boundary?
 

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What is the system?
 

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That's the first thing to
make sure that is clear.

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If it's not clear, the
problem is going to be

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impossible to solve.
 

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And that's also how people find
ways to break the second law,

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because somehow they've messed
up on what their system is.

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And they've included too much
or too little in the system,

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and it looks to them that the
second law is broken and

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they've created more energy
than is being brought in.

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That's usually the case.
 

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Questions?
 

273
00:16:42 --> 00:16:44
Let's keep going.
 

274
00:16:44 --> 00:16:48
So, now that we've got
a system, we've got

275
00:16:48 --> 00:16:49
to describe it.
 

276
00:16:49 --> 00:16:57
So, let's describe
the system now.

277
00:16:57 --> 00:17:02
It turns out that when you're
talking about macroscopic

278
00:17:02 --> 00:17:06
properties of matter, you don't
need very many variables to

279
00:17:06 --> 00:17:11
describe the system completely
thermodynamically.

280
00:17:11 --> 00:17:14
You just need a few macroscopic
variables that are very

281
00:17:14 --> 00:17:18
familiar to you, like the
pressure, the temperature, the

282
00:17:18 --> 00:17:22
volume, the number of moles of
each component, the

283
00:17:22 --> 00:17:25
mass of the system.
 

284
00:17:25 --> 00:17:27
You've got a magnetic field,
maybe even magnetic

285
00:17:27 --> 00:17:30
susceptibility, the
electric field.

286
00:17:30 --> 00:17:32
We're not going to worry about
these magnetic fields or

287
00:17:32 --> 00:17:34
electric fields in this class.
 

288
00:17:34 --> 00:17:37
So, pretty much we're going
to focus on this set

289
00:17:37 --> 00:17:40
of variables here.
 

290
00:17:40 --> 00:17:42
You're going to have to know
when you describe the system,

291
00:17:42 --> 00:17:48
if your system is homogeneous,
like your coffee with milk in

292
00:17:48 --> 00:17:52
it, or heterogeneous, like
water with an ice cube in it.

293
00:17:52 --> 00:17:55
So heterogeneous means
that you've got different

294
00:17:55 --> 00:17:56
phases in your system.
 

295
00:17:56 --> 00:17:59
I'm the heterogeneous
system, soft stuff, hard

296
00:17:59 --> 00:18:02
stuff, liquid stuff.
 

297
00:18:02 --> 00:18:04
Coffee is homogeneous,
even though it's made

298
00:18:04 --> 00:18:06
up of many components.
 

299
00:18:06 --> 00:18:08
Many different kinds of
molecules make up your coffee.

300
00:18:08 --> 00:18:11
There are the water molecules,
the flavor molecules, the

301
00:18:11 --> 00:18:12
milk proteins, etcetera.
 

302
00:18:12 --> 00:18:14
But it's all mixed up
together in a homogeneous,

303
00:18:14 --> 00:18:17
macroscopic fashion.
 

304
00:18:17 --> 00:18:20
If you drill down at the level
of molecules you see that

305
00:18:20 --> 00:18:22
it's not homogeneous.
 

306
00:18:22 --> 00:18:25
But thermodynamics takes
a bird's eye view.

307
00:18:25 --> 00:18:27
It looks pretty, beautiful.
 

308
00:18:27 --> 00:18:32
So, that's a homogeneous
system, one phase.

309
00:18:32 --> 00:18:35
You have to know if your
system is an equilibrium

310
00:18:35 --> 00:18:38
system or not.
 

311
00:18:38 --> 00:18:40
If it's an equilibrium
system, then thermodynamics

312
00:18:40 --> 00:18:41
can describe it.
 

313
00:18:41 --> 00:18:44
If it's not, then you're going
to have trouble describing it

314
00:18:44 --> 00:18:46
using thermodynamic properties.
 

315
00:18:46 --> 00:18:51
Thermodynamics talks about
equilibrium systems and how

316
00:18:51 --> 00:18:53
to go from one state of
equilibrium to another

317
00:18:53 --> 00:18:55
state of equilibrium.
 

318
00:18:55 --> 00:18:56
What does equilibrium mean?
 

319
00:18:56 --> 00:18:59
It means that the properties of
the system, the properties that

320
00:18:59 --> 00:19:06
describe the system, don't
change in time or in space.

321
00:19:06 --> 00:19:10
If I've got a gas in a
container, the pressure of the

322
00:19:10 --> 00:19:12
gas has to be the same
everywhere in the container,

323
00:19:12 --> 00:19:14
otherwise it's not equilibrium.
 

324
00:19:14 --> 00:19:17
If I place my container of gas
on the table here, and I come

325
00:19:17 --> 00:19:20
back an hour later, the
pressure needs to be the

326
00:19:20 --> 00:19:22
same when I come back.
 

327
00:19:22 --> 00:19:25
Otherwise it's not equilibrium.
 

328
00:19:25 --> 00:19:29
So it only talks about
equilibrium systems.

329
00:19:29 --> 00:19:30
What else do you need to know?
 

330
00:19:30 --> 00:19:32
So, you need to know
the variables.

331
00:19:32 --> 00:19:36
You need to know it's
heterogeneous or homogeneous.

332
00:19:36 --> 00:19:39
You need to know if it's an
equilibrium, and you also need

333
00:19:39 --> 00:19:47
to know how many components
you have in your system.

334
00:19:47 --> 00:19:52
So, a glass of ice water with
an ice cube in it, which is a

335
00:19:52 --> 00:19:54
heterogeneous system, has only
one component, which

336
00:19:54 --> 00:19:57
is water, H2O.
 

337
00:19:57 --> 00:20:00
Two phases, but one component.
 

338
00:20:00 --> 00:20:04
Latte, which is a homogeneous
system, has a very, very large

339
00:20:04 --> 00:20:07
number of components to it.
 

340
00:20:07 --> 00:20:09
All the components that
make up the milk.

341
00:20:09 --> 00:20:12
All the components that make
up the coffee, and all the

342
00:20:12 --> 00:20:15
impurities, etcetera. cadmium,
heavy metals, arsenic,

343
00:20:15 --> 00:20:22
whatever is in your coffee.
 

344
00:20:22 --> 00:20:26
OK, any questions?
 

345
00:20:26 --> 00:20:29
All right, so we've
described the system

346
00:20:29 --> 00:20:31
with these properties.
 

347
00:20:31 --> 00:20:33
Now these properties
come in two flavors.

348
00:20:33 --> 00:20:37
You have extensive properties
and intensive properties.

349
00:20:37 --> 00:20:43
The extensive properties are
the ones that scale with

350
00:20:43 --> 00:20:44
the size of the system.
 

351
00:20:44 --> 00:20:47
If you double the system,
they double in there

352
00:20:47 --> 00:20:48
numerical number.
 

353
00:20:48 --> 00:20:51
For instance, the volume.
 

354
00:20:51 --> 00:20:54
If you double the
volume, the v doubles.

355
00:20:54 --> 00:20:55
I mean that's obvious.
 

356
00:20:55 --> 00:20:58
The mass, if you double
the amount of stuff

357
00:20:58 --> 00:21:02
the mass will double.
 

358
00:21:02 --> 00:21:06
Intensive properties don't care
about the scale of your system.

359
00:21:06 --> 00:21:10
If you double everything in the
system, the temperature is not

360
00:21:10 --> 00:21:12
going to change, it's
not going to double.

361
00:21:12 --> 00:21:14
The temperature stays the same.
 

362
00:21:14 --> 00:21:16
So the temperature is
intensive, and you can make

363
00:21:16 --> 00:21:19
intensive properties out of
the extensive properties by

364
00:21:19 --> 00:21:24
dividing by the number
of moles in the system.

365
00:21:24 --> 00:21:27
So I can make a quantity that
I'll call V bar, which is the

366
00:21:27 --> 00:21:32
molar volume, the volume of one
mole of a component in my

367
00:21:32 --> 00:21:37
system, and that becomes
an intensive quantity.

368
00:21:37 --> 00:21:41
A volume which is an
intensive volume.

369
00:21:41 --> 00:21:51
The volumes per mole
of that stuff.

370
00:21:51 --> 00:21:54
So, as I mentioned,
thermodynamics is the science

371
00:21:54 --> 00:22:05
of equilibrium systems, and it
also describes the evolution

372
00:22:05 --> 00:22:07
of one equilibrium to
another equilibrium.

373
00:22:07 --> 00:22:09
How do you go from
one to the other?

374
00:22:09 --> 00:22:13
And so the set of properties
that describes the system --

375
00:22:13 --> 00:22:16
the equilibrium doesn't change.
 

376
00:22:16 --> 00:22:20
So, these on-changing
properties that describe the

377
00:22:20 --> 00:22:23
state of the equilibrium
state of the system are

378
00:22:23 --> 00:22:24
called state variables.
 

379
00:22:24 --> 00:22:37
So the state variables describe
the equilibrium's state, and

380
00:22:37 --> 00:22:41
they don't care about how this
state got to where it is.

381
00:22:41 --> 00:22:44
They don't care about the
history of the state.

382
00:22:44 --> 00:22:48
They just know that's if you
have water at zero degrees

383
00:22:48 --> 00:22:53
Celsius with it ice in, that
you can define it as a

384
00:22:53 --> 00:22:58
heterogeneous system with a
certain density for the water

385
00:22:58 --> 00:23:01
or certain density for the
ice, etcetera, etcetera.

386
00:23:01 --> 00:23:04
It doesn't care how
you got there.

387
00:23:04 --> 00:23:06
We're going to find other
properties that do care about

388
00:23:06 --> 00:23:09
the history of the system, like
work, that you put in the

389
00:23:09 --> 00:23:11
system, or heat that you put in
the system, or some

390
00:23:11 --> 00:23:13
other variables.
 

391
00:23:13 --> 00:23:18
But you can't use those to
define the equilibrium state.

392
00:23:18 --> 00:23:20
You can only use the state
variables, independent

393
00:23:20 --> 00:23:22
of history.
 

394
00:23:22 --> 00:23:25
And it turns out that for a one
component system, one component

395
00:23:25 --> 00:23:30
meaning one kind of molecule in
the system, all that you need

396
00:23:30 --> 00:23:37
to know to describe the system
is the number of moles for a

397
00:23:37 --> 00:23:43
one component system, and to
describe one phase in that

398
00:23:43 --> 00:23:46
system, one component,
homogeneous system, you

399
00:23:46 --> 00:23:53
need n and two variables.
 

400
00:23:53 --> 00:23:58
For instance, the pressure
and the temperature, or the

401
00:23:58 --> 00:24:00
volume and the pressure.
 

402
00:24:00 --> 00:24:05
If you have the number of moles
and two intensive variables,

403
00:24:05 --> 00:24:07
then you know everything there
is to know about the system.

404
00:24:07 --> 00:24:11
About the equilibrium
state of that system.

405
00:24:11 --> 00:24:17
There are hundreds of
quantities that you can

406
00:24:17 --> 00:24:19
calculate and measure that are
interesting and important

407
00:24:19 --> 00:24:23
properties, and all you need is
just a few variables to get

408
00:24:23 --> 00:24:26
everything out, and that's
really the power of

409
00:24:26 --> 00:24:29
thermodynamics, is that it
takes so little information to

410
00:24:29 --> 00:24:32
get so much information out.
 

411
00:24:32 --> 00:24:42
So little data to get a lot of
predictive information out.

412
00:24:42 --> 00:24:51
As we're going on with our
definitions, we can summarize a

413
00:24:51 --> 00:24:56
lot of these definitions into a
notation, a chemical notation

414
00:24:56 --> 00:25:00
that that will be
very important.

415
00:25:00 --> 00:25:04
So, for instance, if I'm
talking about three moles

416
00:25:04 --> 00:25:08
of hydrogen, at one bar
100 degrees Celsius.

417
00:25:08 --> 00:25:12
I'm not going to write, given
three moles of hydrogen at one

418
00:25:12 --> 00:25:14
bar and three degrees,
blah, blah, blah.

419
00:25:14 --> 00:25:17
I'm going to write it
in a compact notation.

420
00:25:17 --> 00:25:21
I'm going to write it like
this: three moles of hydrogen

421
00:25:21 --> 00:25:28
which is a gas, one bar
100 degrees Celsius.

422
00:25:28 --> 00:25:30
This notation gives you
everything you need to

423
00:25:30 --> 00:25:31
know about the system.
 

424
00:25:31 --> 00:25:33
It tells you the
number of moles.

425
00:25:33 --> 00:25:34
It tells you the phase.
 

426
00:25:34 --> 00:25:37
It tells you what kind of
molecule it is, and gives

427
00:25:37 --> 00:25:41
you two variables that
are state variables.

428
00:25:41 --> 00:25:44
You could have the volume
and the temperature.

429
00:25:44 --> 00:25:47
You could have the volume
and the pressure.

430
00:25:47 --> 00:25:48
But this tells you everything.
 

431
00:25:48 --> 00:25:51
I don't need to write
it down in words.

432
00:25:51 --> 00:25:55
And then if I want to tell you
about a change of state, or

433
00:25:55 --> 00:25:57
let's first start
with a mixture.

434
00:25:57 --> 00:26:02
Suppose that I give to a
mixture like, this is a

435
00:26:02 --> 00:26:06
homogeneous system with two
components, like five moles of

436
00:26:06 --> 00:26:13
H2O, which is a liquid, at one
bar 25 degrees Celsius, plus

437
00:26:13 --> 00:26:21
five moles of CH3, CH2, OH,
which is a liquid, and one

438
00:26:21 --> 00:26:26
bar at 25 degrees Celsius.
 

439
00:26:26 --> 00:26:30
This describes roughly
something that is fairly

440
00:26:30 --> 00:26:36
commonplace, it's 100-proof
vodka 1/2 water, 1/2 ethanol

441
00:26:36 --> 00:26:41
-- that describes that
macroscopic system.

442
00:26:41 --> 00:26:43
You're missing all the
impurities, all the little the

443
00:26:43 --> 00:26:47
flavor molecules that go into
it, but basically, that's the

444
00:26:47 --> 00:26:50
homogeneous system we were
describing, two component

445
00:26:50 --> 00:26:53
homogeneous systems.
 

446
00:26:53 --> 00:26:56
Then you can do all sorts
of predictive stuff

447
00:26:56 --> 00:26:59
with that system.
 

448
00:26:59 --> 00:27:01
All right, that's the
equilibrium system.

449
00:27:01 --> 00:27:04
Now we want to show a notation,
how do we go from one

450
00:27:04 --> 00:27:07
equilibrium state like this
describes to another

451
00:27:07 --> 00:27:16
equilibrium state?
 

452
00:27:16 --> 00:27:18
So, we take our two equilibrium
states, and you just put an

453
00:27:18 --> 00:27:24
equal sign between them, and
the equal sign means go

454
00:27:24 --> 00:27:25
from one to the other.
 

455
00:27:25 --> 00:27:31
So, if we took our three moles
of hydrogen, which is a gas at

456
00:27:31 --> 00:27:38
five bar and 100 degrees
Celsius, and, which is a nice

457
00:27:38 --> 00:27:40
equilibrium state here, and we
say now we're going to change

458
00:27:40 --> 00:27:43
the equilibrium state to
something new, we're going to

459
00:27:43 --> 00:27:48
do an expansion, let's say.
 

460
00:27:48 --> 00:27:50
We're going to drop the
pressure, the volume

461
00:27:50 --> 00:27:51
is going to go up.
 

462
00:27:51 --> 00:27:53
I don't need to tell you the
volume here, because you've

463
00:27:53 --> 00:27:56
got enough information
to calculate the volume.

464
00:27:56 --> 00:28:01
The number of moles stays
the same, a closed systems,

465
00:28:01 --> 00:28:03
gas doesn't come out.
 

466
00:28:03 --> 00:28:05
Stays a gas, but now the
pressure is less, the

467
00:28:05 --> 00:28:06
temperature is less.
 

468
00:28:06 --> 00:28:10
I've done some sort of
expansion on this.

469
00:28:10 --> 00:28:12
I've gone from 1 equilibrium
state to another equilibrium

470
00:28:12 --> 00:28:15
state, and the equal sign
means you go from this

471
00:28:15 --> 00:28:16
state to that state.
 

472
00:28:16 --> 00:28:17
It's not a chemical reaction.
 

473
00:28:17 --> 00:28:20
That's why we don't have an
arrow here, because we could

474
00:28:20 --> 00:28:22
go back, this way too.
 

475
00:28:22 --> 00:28:24
We can go back and forth
between these two

476
00:28:24 --> 00:28:25
equilibrium states.
 

477
00:28:25 --> 00:28:25
They're connected.
 

478
00:28:25 --> 00:28:27
This means they're connected.
 

479
00:28:27 --> 00:28:30
And when I put this, I
have to tell you how

480
00:28:30 --> 00:28:31
they are connected.
 

481
00:28:31 --> 00:28:33
I have to tell you the
path, if you're going

482
00:28:33 --> 00:28:34
to solve a problem.
 

483
00:28:34 --> 00:28:36
For instance, you want to know
how much energy you're going to

484
00:28:36 --> 00:28:39
get out from doing
this expansion.

485
00:28:39 --> 00:28:42
How much energy are you going
to get out, and how far are you

486
00:28:42 --> 00:28:44
going to be able to drive a car
with this expansion, let's

487
00:28:44 --> 00:28:46
say, so that's the problem.
 

488
00:28:46 --> 00:28:49
So, I need to tell you how
you're doing the expansion,

489
00:28:49 --> 00:28:51
because that's going to tell
you how much energy you're

490
00:28:51 --> 00:28:53
wasting during that expansion.
 

491
00:28:53 --> 00:28:56
It goes back to the second law.
 

492
00:28:56 --> 00:28:57
Nothing is efficient.
 

493
00:28:57 --> 00:28:59
You're always wasting energy
into heat somewhere when you

494
00:28:59 --> 00:29:03
do a change that involves
a mechanical change.

495
00:29:03 --> 00:29:07
All right, so I need to tell
you the path, when I go from

496
00:29:07 --> 00:29:09
one state to the other.
 

497
00:29:09 --> 00:29:12
And the path is going to be the
sequence, intermediate states

498
00:29:12 --> 00:29:15
going from the initial
state the final state.

499
00:29:15 --> 00:29:22
So, for instance, if I draw a
graph of pressure on one axis

500
00:29:22 --> 00:29:27
and temperature on the other
axis, my initial state is at a

501
00:29:27 --> 00:29:34
temperature of 100 degrees
Celsius and five bar.

502
00:29:34 --> 00:29:42
My final stage is 50 degrees
Celsius and one bar.

503
00:29:42 --> 00:29:46
So, I could have two
steps in my path.

504
00:29:46 --> 00:29:49
I could decide first of all to
keep the pressure constant

505
00:29:49 --> 00:29:53
and lower the pressure.
 

506
00:29:53 --> 00:29:55
When I get to 50 degrees
Celsius, I could choose to

507
00:29:55 --> 00:29:59
keep the temperature constant
and lower the pressure.

508
00:29:59 --> 00:30:01
I'm sorry, my first step would
be to keep the pressure

509
00:30:01 --> 00:30:04
constant and lower the
temperature, then I lower the

510
00:30:04 --> 00:30:07
pressure, keeping the
temperature constant.

511
00:30:07 --> 00:30:08
So there's my intermediate
state there.

512
00:30:08 --> 00:30:12
This is one of many paths.
 

513
00:30:12 --> 00:30:15
There's an infinite number
of paths you could take.

514
00:30:15 --> 00:30:19
You could take a continuous
path, where you have an

515
00:30:19 --> 00:30:24
infinite number of equilibrium
points in between the two, a

516
00:30:24 --> 00:30:28
smooth path, where you drop the
pressure and the temperature

517
00:30:28 --> 00:30:30
simultaneously in
little increments.

518
00:30:30 --> 00:30:35
All right, so when you do
a problem, the path is

519
00:30:35 --> 00:30:38
going to turn out to be
extremely important.

520
00:30:38 --> 00:30:43
How do you get from the initial
state to the final state?

521
00:30:43 --> 00:30:44
Define the initial state.
 

522
00:30:44 --> 00:30:45
Define the final state.
 

523
00:30:45 --> 00:30:47
Define the path.
 

524
00:30:47 --> 00:30:50
Get all of these really
clear, and you've basically

525
00:30:50 --> 00:30:51
solved the problem.
 

526
00:30:51 --> 00:30:56
You've got to spend the time to
make sure that everything is

527
00:30:56 --> 00:30:58
well defined before you start
trying to work out

528
00:30:58 --> 00:31:02
these problem.
 

529
00:31:02 --> 00:31:04
More about the path.
 

530
00:31:04 --> 00:31:08
There are a couple ways you
could go through that path.

531
00:31:08 --> 00:31:10
If I look at this
smooth path here.

532
00:31:10 --> 00:31:14
I could have that path be very
slow and steady, so that at

533
00:31:14 --> 00:31:18
every point along the way,
my gas is an equilibrium.

534
00:31:18 --> 00:31:21
So I've got, this piston here
is compressed, and I slowly,

535
00:31:21 --> 00:31:25
slowly increase the volume,
drop the temperature.

536
00:31:25 --> 00:31:29
Then I can go back, the
gas is included at

537
00:31:29 --> 00:31:33
every point of the way.
 

538
00:31:33 --> 00:31:35
That's a reversible path.
 

539
00:31:35 --> 00:31:36
That can reverse the process.
 

540
00:31:36 --> 00:31:39
I expand it, and reverse
it, no problem.

541
00:31:39 --> 00:31:49
So, I could have a reversible
path, or I take my gas, and

542
00:31:49 --> 00:31:53
instead of slowly, slowly
raising it, dropping the

543
00:31:53 --> 00:32:00
pressure, I go from five bar
to one bar extremely fast.

544
00:32:00 --> 00:32:01
What happens to my gas inside?
 

545
00:32:01 --> 00:32:04
Well, my gas inside is
going to be very unhappy.

546
00:32:04 --> 00:32:06
It's not going stay
in equilibrium.

547
00:32:06 --> 00:32:08
Parts of the system are
going to be at five bar.

548
00:32:08 --> 00:32:10
Parts of it at one bar.
 

549
00:32:10 --> 00:32:14
Parts of it may be even at zero
bar, if I go really fast.

550
00:32:14 --> 00:32:15
I'm going to create a vacuum.
 

551
00:32:15 --> 00:32:21
So the system will not be
described by a single state

552
00:32:21 --> 00:32:23
variable during the path.
 

553
00:32:23 --> 00:32:27
If I look at different points
in my container during that

554
00:32:27 --> 00:32:31
path, I'm going to have to use
a different value of pressure

555
00:32:31 --> 00:32:32
or different value of
temperature at different

556
00:32:32 --> 00:32:35
points of the container.
 

557
00:32:35 --> 00:32:38
That's not an equilibrium
state, and that process

558
00:32:38 --> 00:32:42
turns out then to be in
irreversible process.

559
00:32:42 --> 00:32:43
Do it very quickly.
 

560
00:32:43 --> 00:32:46
Now to reverse it and get back
to the initial point is going

561
00:32:46 --> 00:32:50
to require some input from
outside, like heat or extra

562
00:32:50 --> 00:32:53
work or extra heat or
something, because you've done

563
00:32:53 --> 00:32:54
an irreversible process.
 

564
00:32:54 --> 00:33:04
You've wasted a lot of energy
in doing that process.

565
00:33:04 --> 00:33:09
I have to tell you whether the
path is reversible or

566
00:33:09 --> 00:33:13
irreversible, and the
irreversible path also defines

567
00:33:13 --> 00:33:15
the direction of time.
 

568
00:33:15 --> 00:33:19
You can only have an
irreversible path go one way

569
00:33:19 --> 00:33:20
in time, not the other way.
 

570
00:33:20 --> 00:33:24
Chalk breaks irreversibly
and you can't put it

571
00:33:24 --> 00:33:25
back together so easily.
 

572
00:33:25 --> 00:33:29
You've got to pretty much take
that chalk, and make a slurry

573
00:33:29 --> 00:33:32
out of it, put water, and dry
it back up, put in a mold, and

574
00:33:32 --> 00:33:34
then you can have the chalk
again, but you can't just

575
00:33:34 --> 00:33:35
glue it back together.
 

576
00:33:35 --> 00:33:36
That would not be the
same state as what

577
00:33:36 --> 00:33:40
you started out with.
 

578
00:33:40 --> 00:33:42
And then there are a
bunch of words that

579
00:33:42 --> 00:33:43
describe these paths.
 

580
00:33:43 --> 00:33:47
Words like adiabatic, which
we'll be very familiar with.

581
00:33:47 --> 00:33:50
Adiabatic means that there's no
heat transferred between the

582
00:33:50 --> 00:33:51
system and the surrounding.
 

583
00:33:51 --> 00:33:55
The boundary is impervious
to transfer of heat,

584
00:33:55 --> 00:33:56
like a thermos.
 

585
00:33:56 --> 00:33:59
Anything that happens inside of
the thermos is an adiabatic

586
00:33:59 --> 00:34:03
change because the thermos has
no connection in terms of

587
00:34:03 --> 00:34:04
energy to the outside world.
 

588
00:34:04 --> 00:34:06
There's no heat that can
go through the walls

589
00:34:06 --> 00:34:07
of the thermos.
 

590
00:34:07 --> 00:34:10
Whereas, like isobaric
means constant pressure.

591
00:34:10 --> 00:34:15
So, this path right here
from this top red path

592
00:34:15 --> 00:34:18
is an isobaric process.
 

593
00:34:18 --> 00:34:21
Constant temperature means
isothermal, so this part

594
00:34:21 --> 00:34:23
means an isothermal process.
 

595
00:34:23 --> 00:34:28
So then, going from the initial
to final states with a red

596
00:34:28 --> 00:34:32
path, you start with an
isobaric process and then you

597
00:34:32 --> 00:34:34
end with an isothermal process.
 

598
00:34:34 --> 00:34:36
And these are words that are
very meaningful when you read

599
00:34:36 --> 00:34:42
the text of a problem
or of a process.

600
00:34:42 --> 00:34:44
Any questions before we
got to the zeroth law?

601
00:34:44 --> 00:34:49
We're pretty much done with
our definitions here.

602
00:34:49 --> 00:34:49
Yes.
 

603
00:34:49 --> 00:34:53
STUDENT: Was adiabatic
reversible?

604
00:34:53 --> 00:34:56
PROFESSOR: Adiabatic can be
either reversible or not, and

605
00:34:56 --> 00:35:01
we're going to do that probably
next time or two times.

606
00:35:01 --> 00:35:02
Any other questions?
 

607
00:35:02 --> 00:35:06
Yes.
 

608
00:35:06 --> 00:35:07
STUDENT: Is there a boundary
between reversible

609
00:35:07 --> 00:35:08
and irreversible?
 

610
00:35:08 --> 00:35:12
PROFESSOR: A boundary between
reversible and irreversible?

611
00:35:12 --> 00:35:14
Like something is
almost reversible and

612
00:35:14 --> 00:35:15
almost irreversible.
 

613
00:35:15 --> 00:35:16
No, pretty much things
are either reversible

614
00:35:16 --> 00:35:19
or irreversible.
 

615
00:35:19 --> 00:35:28
Now, in practice, it depends on
how good your measurement is.

616
00:35:28 --> 00:35:35
And probably also in practice,
nothing is truly reversible.

617
00:35:35 --> 00:35:43
So, it depends on your
error bar in a sense.

618
00:35:43 --> 00:35:45
It depends on what what you
define, exactly what you

619
00:35:45 --> 00:35:46
define in your system.
 

620
00:35:46 --> 00:35:50
It becomes a gray area, but
it should be pretty clear if

621
00:35:50 --> 00:35:57
you can treat something is
reversible are irreversible.

622
00:35:57 --> 00:36:03
Other questions, It's
a good question.

623
00:36:03 --> 00:36:06
So the zeroth law we're going
to go through the laws now.

624
00:36:06 --> 00:36:10
The zeroth law talks about
defining temperature and

625
00:36:10 --> 00:36:12
it's the common-sense law.
 

626
00:36:12 --> 00:36:14
You all know how.
 

627
00:36:14 --> 00:36:16
When something hot, it's got
a higher temperature than

628
00:36:16 --> 00:36:18
when something is cold.
 

629
00:36:18 --> 00:36:21
But it's important to define
that, and define something

630
00:36:21 --> 00:36:23
that's a thermometer.
 

631
00:36:23 --> 00:36:26
So what do you know?
 

632
00:36:26 --> 00:36:28
What's the empirical
information that

633
00:36:28 --> 00:36:29
everybody knows?
 

634
00:36:29 --> 00:36:34
Everybody knows that if you
take something which is hot and

635
00:36:34 --> 00:36:40
something which is cold, and
you bring them together, make

636
00:36:40 --> 00:36:48
them touch, that heat is going
to flow from the hot to the

637
00:36:48 --> 00:36:56
cold, and make them touch, and
heat flows from hot to cold.

638
00:36:56 --> 00:36:57
That's common sense.
 

639
00:36:57 --> 00:37:03
This is part of your DNA, And
then their final product is an

640
00:37:03 --> 00:37:11
object, a b which ends up at a
temperature or a warmness which

641
00:37:11 --> 00:37:13
is in between the
hot and the cold.

642
00:37:13 --> 00:37:16
So, this turns out to be warm.
 

643
00:37:16 --> 00:37:19
You get your new equilibrium
state, which is in between

644
00:37:19 --> 00:37:28
what this was, and
what a and b were.

645
00:37:28 --> 00:37:34
Then how do you know that it's
changed temperature, or that

646
00:37:34 --> 00:37:37
heat has flowed from a to b?
 

647
00:37:37 --> 00:37:41
Practically speaking, you need
some sort of property that's

648
00:37:41 --> 00:37:43
changing as heat is flowing.
 

649
00:37:43 --> 00:37:50
For instance, if a were
metallic, you could measure

650
00:37:50 --> 00:37:54
the connectivity of a or
resistivity, and as heat

651
00:37:54 --> 00:38:01
flows out of a into b, the
resistivity of a would change.

652
00:38:01 --> 00:38:04
Or you could have something
that's color metric that

653
00:38:04 --> 00:38:09
changes color when it's colder,
so you could see the heat

654
00:38:09 --> 00:38:13
flowing as a changes color or
b changes color as

655
00:38:13 --> 00:38:15
heat flows into b.
 

656
00:38:15 --> 00:38:17
So, you need some sort of
property, something you can

657
00:38:17 --> 00:38:20
see, something you can
measure, that tells you

658
00:38:20 --> 00:38:21
that heat has flowed.
 

659
00:38:21 --> 00:38:26
Now, if you have three objects,
if you have a, b, and c, and

660
00:38:26 --> 00:38:37
you bring them together, and a
is the hottest, b is the medium

661
00:38:37 --> 00:38:41
one, and c is the coldest, so
from hottest to coldest a,

662
00:38:41 --> 00:38:49
b, c, -- if you bring them
together and make them touch,

663
00:38:49 --> 00:38:57
you know, intuitively, that
heat will not flow like this.

664
00:38:57 --> 00:38:59
You know that's not
going to happen.

665
00:38:59 --> 00:39:03
You know that what will happen
is that heat will flow from a

666
00:39:03 --> 00:39:07
to b from b to c
and from a to c.

667
00:39:07 --> 00:39:08
That's common-sense.
 

668
00:39:08 --> 00:39:11
You know that.
 

669
00:39:11 --> 00:39:13
And the other way in the
circle will never happen.

670
00:39:13 --> 00:39:17
That would that would give rise
to a perpetual motion machine,

671
00:39:17 --> 00:39:18
breaking of the second law.
 

672
00:39:18 --> 00:39:22
It can't happen.
 

673
00:39:22 --> 00:39:24
But that's an empirical
observation, that heat

674
00:39:24 --> 00:39:27
flows in this direction.
 

675
00:39:27 --> 00:39:29
And that's the zeroth
law thermodynamic.

676
00:39:29 --> 00:39:32
It's pretty simple.
 

677
00:39:32 --> 00:39:39
The zeroth law says that if a
and b -- it doesn't exactly say

678
00:39:39 --> 00:39:41
that, but it implies this.
 

679
00:39:41 --> 00:39:45
It says that if a and b are in
thermal equilibrium, if these

680
00:39:45 --> 00:39:48
two are in thermal equilibrium,
meaning that there's no heat

681
00:39:48 --> 00:39:51
flows between them, so that's
the definition of thermal

682
00:39:51 --> 00:39:54
equilibrium, that no heat flows
between them, and these two are

683
00:39:54 --> 00:39:57
in thermal equilibrium, and
these two are in thermal

684
00:39:57 --> 00:40:02
equilibrium, then a and c will
be also be in thermal

685
00:40:02 --> 00:40:04
equilibrium.
 

686
00:40:04 --> 00:40:07
But if there's no heat flowing
between these two, and no heat

687
00:40:07 --> 00:40:09
flowing between these two,
then you can't have heat

688
00:40:09 --> 00:40:13
flowing between these two.
 

689
00:40:13 --> 00:40:16
So if I get rid of these
arrows, there's no heat flowing

690
00:40:16 --> 00:40:18
because they're in thermal
equilibrium, then I can't

691
00:40:18 --> 00:40:20
have an arrow here.
 

692
00:40:20 --> 00:40:22
That's what the
zeroth law says.

693
00:40:22 --> 00:40:24
They're all the
same temperature.

694
00:40:24 --> 00:40:25
That's what it says.
 

695
00:40:25 --> 00:40:28
If two object are in the same
temperature, and two other

696
00:40:28 --> 00:40:30
object are in the same
temperature, then all three

697
00:40:30 --> 00:40:34
must have the same temperature.
 

698
00:40:34 --> 00:40:37
It sounds pretty silly, but
it's really important because

699
00:40:37 --> 00:40:44
it allows you to define a
thermometer and temperature.

700
00:40:44 --> 00:40:48
Because now you can say,
all right, well, now b

701
00:40:48 --> 00:40:49
can be my thermometer.
 

702
00:40:49 --> 00:40:54
I have two objects, I have an
object which is in Madagascar

703
00:40:54 --> 00:40:59
and an object which is in
Boston, and I want to know, are

704
00:40:59 --> 00:41:01
they the same temperature?
 

705
00:41:01 --> 00:41:05
So I come out with a third
object, b, I go to Madagascar,

706
00:41:05 --> 00:41:06
and put b in contact with a.
 

707
00:41:06 --> 00:41:11
Then I insulate everything, you
know, take it away and see

708
00:41:11 --> 00:41:12
if there's any heat flow.
 

709
00:41:12 --> 00:41:15
Let's say there's no heat flow.
 

710
00:41:15 --> 00:41:18
Then I insulate it, get back on
the plane to Boston, and go

711
00:41:18 --> 00:41:20
back and touch b with c.
 

712
00:41:20 --> 00:41:23
If there's no heat flow between
the b and c, then I can say

713
00:41:23 --> 00:41:27
all right, a and c were
the same temperature.

714
00:41:27 --> 00:41:29
B is my thermometer that
tells me that a and c are

715
00:41:29 --> 00:41:30
in the same temperature.
 

716
00:41:30 --> 00:41:33
And there's a certain property
associated with heat flow with

717
00:41:33 --> 00:41:35
b, and it didn't change.
 

718
00:41:35 --> 00:41:37
And that property
could be color.

719
00:41:37 --> 00:41:38
It could be resistivity.
 

720
00:41:38 --> 00:41:39
It could be a lot of
different things.

721
00:41:39 --> 00:41:41
It could be volume.
 

722
00:41:41 --> 00:41:44
And the temperature then is
associated with that property.

723
00:41:44 --> 00:41:47
And if it had changed, then the
temperature between those two

724
00:41:47 --> 00:41:50
would have changed in a
very particular way.

725
00:41:50 --> 00:41:57
So, zeroth law, then, allows
you to define the concept of

726
00:41:57 --> 00:42:05
temperature and the measurement
of temperature through

727
00:42:05 --> 00:42:09
a thermometer.
 

728
00:42:09 --> 00:42:12
Let's very briefly go
through stuff that

729
00:42:12 --> 00:42:13
you've learned before.
 

730
00:42:13 --> 00:42:16
So, now you have this object
which is going to tell you

731
00:42:16 --> 00:42:19
whether other things are in
thermal equilibrium now.

732
00:42:19 --> 00:42:21
What do you need
for that object?

733
00:42:21 --> 00:42:27
You need that object to be a
substance, to be something.

734
00:42:27 --> 00:42:31
So, the active part of the
thermometer could be water.

735
00:42:31 --> 00:42:35
It could be alcohol, mercury,
it could be a piece of metal.

736
00:42:35 --> 00:42:39
You need a substance, and then
that substance has to have a

737
00:42:39 --> 00:42:41
property that changes depending
on the heat flow, i.e.,

738
00:42:41 --> 00:42:45
depending on whether it's
sensing that it's the same

739
00:42:45 --> 00:42:46
temperature or different
temperature than

740
00:42:46 --> 00:42:46
something else.
 

741
00:42:46 --> 00:42:51
And that property could be the
volume, like if you have a

742
00:42:51 --> 00:42:54
mercury thermometer, the
volume of the mercury.

743
00:42:54 --> 00:42:55
It could be temperature.
 

744
00:42:55 --> 00:42:58
It could be resistivity, if
you have a thermocouple.

745
00:42:58 --> 00:43:02
It could be the pressure.
 

746
00:43:02 --> 00:43:04
All right, so now
you have an object.

747
00:43:04 --> 00:43:06
You've got a property
that changes, depending

748
00:43:06 --> 00:43:07
on the heat flow.
 

749
00:43:07 --> 00:43:09
It's going to tell you
about the temperature.

750
00:43:09 --> 00:43:11
Now you need to define
the temperature scales.

751
00:43:11 --> 00:43:16
So, you need some reference
points to be able to tell you,

752
00:43:16 --> 00:43:22
OK, this temperature is 550
degrees Smith, whatever.

753
00:43:22 --> 00:43:27
So, you assign values to very
specific states of matter and

754
00:43:27 --> 00:43:29
call those the reference
points for your temperature.

755
00:43:29 --> 00:43:32
For instance, freezing
of water or boiling of

756
00:43:32 --> 00:43:34
water, the standard ones.
 

757
00:43:34 --> 00:43:36
And then an
interpolation scheme.

758
00:43:36 --> 00:43:41
You need a functional form that
connects the value at one state

759
00:43:41 --> 00:43:45
of matter, the freezing point
of water, to another phase

760
00:43:45 --> 00:43:47
change, the boiling
point of water.

761
00:43:47 --> 00:43:51
You can choose a linear
interpolation or quadratic,

762
00:43:51 --> 00:43:54
but you've got to choose it.
 

763
00:43:54 --> 00:43:56
And it turns out
not to be so easy.

764
00:43:56 --> 00:43:58
And if you go back into the
1800's when thermodynamics was

765
00:43:58 --> 00:44:02
starting, there were a zillion
different temperatures scales.

766
00:44:02 --> 00:44:06
Everybody had their own
favorite temperature scales.

767
00:44:06 --> 00:44:09
The one that we're most
familiar with is the centigrade

768
00:44:09 --> 00:44:12
or Celsius scale where mercury
was the substance, and the

769
00:44:12 --> 00:44:13
volume of mercury
is the property.

770
00:44:13 --> 00:44:16
The reference points are water,
freezing or boiling, and the

771
00:44:16 --> 00:44:19
interpolation is linear, and
then that morphed into the

772
00:44:19 --> 00:44:21
Kelvin scale, as we're
going to see later.

773
00:44:21 --> 00:44:24
The Fahrenheit scale is
an interesting scale.

774
00:44:24 --> 00:44:26
It turns out the U.S. and
Jamaica are the only two

775
00:44:26 --> 00:44:28
places on Earth now that
use the Fahrenheit scale.

776
00:44:28 --> 00:44:33
Mr. Fahrenheit, Daniel Gabriel
Fahrenheit was a German

777
00:44:33 --> 00:44:37
instrument maker.
 

778
00:44:37 --> 00:44:41
The way he came up with his
scale was actually he borrowed

779
00:44:41 --> 00:44:43
the Romer scale, which
came beforehand.

780
00:44:43 --> 00:44:48
The Romer scale was, Romer was
a Dane, and he defined freezing

781
00:44:48 --> 00:44:54
of water at 7.5 degrees
Roemer, and 22.5 degrees

782
00:44:54 --> 00:44:56
Romer as blood-warm.
 

783
00:44:56 --> 00:45:00
That was his definition.
 

784
00:45:00 --> 00:45:03
Two substances,
blood and water.

785
00:45:03 --> 00:45:05
Two reference points,
freezing and blood-warm,

786
00:45:05 --> 00:45:07
you know, the human body.
 

787
00:45:07 --> 00:45:09
A linear interpolation between
the two, and then some numbers

788
00:45:09 --> 00:45:12
associated with them,
7-1/2 and 22-1/2.

789
00:45:12 --> 00:45:15
Why does he choose 7-1/2 as
the freezing point of water?

790
00:45:15 --> 00:45:18
Because he thought that
would be big enough that in

791
00:45:18 --> 00:45:22
Denmark, the temperature
wouldn't go below zero.

792
00:45:22 --> 00:45:24
That's how he picked 7-1/2.
 

793
00:45:24 --> 00:45:25
Why not?
 

794
00:45:25 --> 00:45:28
He didn't want to use negative
numbers to measure temperature

795
00:45:28 --> 00:45:31
in Denmark outside.
 

796
00:45:31 --> 00:45:33
Well, Fahrenheit came along and
thought, well, you know, 7-1/2,

797
00:45:33 --> 00:45:36
that's kind of silly; 22-1/2
that's, kind of silly.

798
00:45:36 --> 00:45:40
So let's multiply
everything by four.

799
00:45:40 --> 00:45:46
I think it becomes 30 degrees
for the freezing of water and

800
00:45:46 --> 00:45:49
22.5 x 4, which I don't know
what it is, 100 or something

801
00:45:49 --> 00:45:55
-- no, it's 90 I think.
 

802
00:45:55 --> 00:45:58
And then for some reason, that
nobody understands, he decided

803
00:45:58 --> 00:46:04
to multiply again by 16/15, and
that's how we get 32 for

804
00:46:04 --> 00:46:08
freezing of water and 96 in his
words for the temperature in

805
00:46:08 --> 00:46:10
the mouth or underneath
the armpit of a living

806
00:46:10 --> 00:46:12
man in good health.
 

807
00:46:12 --> 00:46:14
What a great temperature scale.
 

808
00:46:14 --> 00:46:16
It turns out that 96
wasn't quite right.

809
00:46:16 --> 00:46:21
Then he interpolated and found
out water boils at 212.

810
00:46:21 --> 00:46:24
But, you know, his experiment
wasn't so great, and, you know,

811
00:46:24 --> 00:46:26
maybe had a fever when he did
the reference point

812
00:46:26 --> 00:46:28
with 96, whatever.
 

813
00:46:28 --> 00:46:31
It turns out that it's not
96 to be in good health,

814
00:46:31 --> 00:46:34
it's 98.6 -- whatever.
 

815
00:46:34 --> 00:46:37
That's how we got to
the Fahrenheit scale.

816
00:46:37 --> 00:46:40
All right, next time we're
going to talk about a much

817
00:46:40 --> 00:46:43
better scale, which is the
ideal gas thermometer and how

818
00:46:43 --> 00:46:45
we get to the Kelvin scale.
 

819
00:46:45 --> 00:46:45