From early childhood, we can tell by touch
whether an object is hot or whether it is
cold.
If you want to heat an object, you bring it
in contact with a hot object, for instance
a flame.
If you want to cool an object, you bring it
in contact with a cold object.
When objects are heated or when they're cooled,
then--
and the temperature changes--
then some of their properties change, and
those properties are called thermometric properties:
ther-mo-metric properties.
One very characteristic thermometric property
is that most substances when you heat them,
they expand, and when you cool them, they
shrink.
We'll talk more about it later.
If you take a gas in a closed volume and you
heat it, the pressure goes up.
That's a thermometric property.
If you take an electric conductor and you
heat it, in general the electric resistance
will change.
If you heat an iron bar, it will expand.
And if you place it in contact with another
iron bar which is cold, then the one that
is hot will shrink and the one that is cold
will heat up and will get longer, and this
process will go on up to the point that the
hot one will not get shorter and that the
cold one will not get longer anymore.
And that is when the two objects, as we say,
are in thermal equilibrium with each other,
and that is when the temperature of the two
objects are the same.
And so you can define a temperature scale
by looking at the length of an object.
For instance, here is a bar, some material,
I clamp it in here, has length L, and I increase
the temperature by an amount delta T, and
it gets longer by a certain amount delta L.
I could put the whole thing in melting ice...
melting ice...
and I could say, "Aha." The length then is
L1.
Then I could put the whole thing in boiling
water and I do that at one atmosphere pressure,
and then I say, "Aha." I call that the length
L2.
And those are my reference points for my temperature
scale.
Celsius did just that.
The idea that he used melting ice, which is
now called zero degrees centigrade, and he
used boiling point of water, which was his
100 degrees centigrade.
He was a Swedish astronomer; in 1742 he introduced
this temperature scale.
So you could make yourself a plot now of the
temperature versus the length of that bar,
and you could say, okay, 100 degrees centigrade...
if the length of the bar... L2, zero degrees
centigrade if the length of the bar is L1.
And now you can draw a straight line--
you can always draw a straight line through
two points, you have one point here, you have
one point there--
and you can define temperature now by saying,
if my bar has this length, L of T, then this
will be the temperature.
So you can introduce a linear scale in this
fashion, and the thing, in principle, could
act like a thermometer.
I'll show you a demonstration of this shortly.
Centi in Greek means "one hundredth," and
therefore we also call this scale often "centigrade."
One degree centigrade is often called...
one degree Celsius is often called one centigrade,
for the reason that it divides the scale from
zero to one hundred in equal portions.
So we call them degrees centigrade, degrees
Celsius.
Fahrenheit, a German scientist, invented the
mercury thermometer.
We'll talk about the mercury thermometer a
little later.
In 1714 he introduced a new scale.
He lived in Holland at the time, he lived
there most of the time, and he used as his
reference point body temperature, which he
called 100 degrees Fahrenheit, and he used
a mixture of salt and ice at zero degrees.
Now, neither one of these two are very reproducible.
If you pick one person, the temperature today
may be a little higher than tomorrow.
A person may have fever.
In fact, the one that he picked probably did
have a little bit of fever.
And so the Fahrenheit scale, in that sense,
is not very reproducible, and it has been
redefined now in such a way that zero degrees
centigrade is 32 degrees Fahrenheit, and 100
degrees centigrade is 212 degrees Fahrenheit.
And so if you convert--
if you want to convert from Fahrenheit to
centigrade or the other way around--
then the temperature in Fahrenheit is 9/5
times the temperature in Celsius plus 32.
If you take room temperature, the temperature
is 20 degrees centigrade, what I was growing
up with--
in Europe, everyone uses centigrade there--
then you can see that in terms of Fahrenheit,
that becomes 68 degrees Fahrenheit.
9/5 times 20 gives you 36, and then you add
32.
Minus 40 degrees centigrade is the same as
minus 40 degrees Fahrenheit.
Check that.
That's where the two scales cross over.
So almost the entire world uses the Celsius
scale; it's part of our metric system.
United States is one of the very, very few
countries who still, in a rather stubborn
way, uses degrees Fahrenheit.
And it is really a pain in the neck, degrees
Fahrenheit--
at least for me.
I have very little feeling for it.
I just happen to know that room temperature
is 68, because that's the way I set my thermostat
at my home, but that's about all.
I can't think in terms of degrees Fahrenheit.
There is no limit to high temperature, but
there is a limit to the low temperatures.
There is an absolute zero.
This absolute zero below which you cannot
go is about minus 273 degrees Celsius...
And if you take a system that cannot transfer
energy to any other system that it is in thermal
contact with, then it is at that lowest possible
temperature.
This is the way we define it.
It's about minus 460 degrees Fahrenheit.
And so we now have a third scale, which was
introduced by Lord Kelvin, was a British scientist.
He did a lot of research on heat, and he introduced
the absolute scale whereby he uses the lowest
possible temperature as zero degrees Kelvin.
But the increments in terms of increase of
one degree, he uses the same as the Celsius
scale.
So an increase of two or three degrees Kelvin
is the same as an increase of two or three
degrees centigrade.
So if we now compare the three scales--
Celsius, Fahrenheit and Kelvin--
then 20 degrees centigrade would be 68 Fahrenheit,
and that would be 273.15 plus 20.
Let's round it off and make it 293, and if
we take zero Kelvin, then we would have minus
273.15, but let's leave that off for now,
and it is approximately minus 460.
We will almost always work with degrees Kelvin
in physics and we'll discuss that in more
detail Friday.
Most substances expand when you heat them,
and if we start with an object which has length
L and I heat it up delta T degrees, it gets
longer by an amount delta L.
And that delta L can be expressed in a very
simple way.
It is alpha times L times delta T, and alpha
is called the linear expansion coefficient.
And the units are one over degrees centigrade,
or one over degree Kelvin, which is the same,
because it's the increments that matter.
The various values for alpha differ a great
deal.
Give you some values for alpha.
I'll give you copper, I'll give you brass,
I'll give you Pyrex, I'll give you Invar and
I'll give you steel, and they are in units
of ten to the minus six per degree centigrade,
and we will use some of them today.
Brass is about 19.
Copper is 17.
Pyrex 3.3; Invar 0.9; and steel is roughly
12, but there are many different kinds of
steel.
Invar was a great invention.
Notice it has a very low expansion coefficient.
It was very important in the 19th century,
even today, to make instruments very precise,
like clocks.
Clocks are affected by the expansion of the
gears.
And so the invention of Invar, which is a
mixture of 64% iron and 36% nickel, was invented
by a physicist Guillaume in 1898, and for
this discovery, he received the Nobel Prize
in 1920.
It tells you something how important it was
to get an alloy that has a very low expansion
coefficient.
If we use these numbers, let us look at the
expansion of, for instance, a railroad.
We take a railroad, and we take a piece, a
stretch of rail which is, say, a thousand
meters.
We take steel, iron--
so this is the expansion coefficient, roughly--
and we compare a cold day, not extremely cold,
but a cold day with a hot summer day.
A cold winter day, minus 15 degrees centigrade,
and a hot summer day, plus 35 degrees centigrade.
So delta T would be about 50 degrees centigrade.
So what is delta L? Well, that would be 12
times ten to the minus six times ten to the
third, times 50, and that is about 0.6 meters,
which is about 60 centimeters.
So what are you going to do with that now?
How is that solved? If the rail wants to get
longer and can't get longer, it will start
to bulge either in this way, or sideways,
whichever is the easiest.
But the way this is solved is actually quite
simple.
When you look at rails, there are openings
between them.
They're very distinct.
They're about five centimeters.
If you walk along the rail, you can see the
openings.
And if you make these openings, say, five
centimeters, then you would need 12 of them
in thousand meters, so every 80 meters you
would need a gap.
And you can hear these gaps when the train
goes over these gaps.
It's a very typical sound.
Because imagine when the wheel goes over it,
you hear a certain click.
You can see them and you can hear them, and
that's the way they correct for this expansion
and contraction.
Bridges can be many kilometers long, and they
have, of course, the same problem of expansion,
and the way that that is dealt with, also
very clever, is as follows.
This is a picture that I copied actually from
your book.
It's called an expansion joint, and there
are many of them in bridges, and so what it
allows the bridge is to do this--
to breathe, so to speak, adjust to the temperature.
There is a bizarre picture whereby the claim
is made that this railroad became so warped
because of an extremely hot day.
I trust it, although it's hard to believe
that it could be so bad.
It must have been extraordinarily hot.
As I mentioned, if a rail cannot expand, then
all it can do is either bulge in this way
or this way, whichever way is easiest for
it, and apparently here, the easiest way is
to go sideways, so you see a remarkable destruction,
actually, due to an unexpected high temperature.
I have here a brass bar which is about 36
centimeters long, and I'm going to heat that
brass bar.
You'll see it there, too.
The brass bar is right here, and we have a
way of showing you the extension, even though
it is extremely small--
only a fraction of a millimeter.
We can show that to you very easily.
The way we do that is we have some kind of
an amplifier.
If this is the rod, and I would put here a
hand, and pivot that hand here, then it's
easy to see that if you push against it here,
that this hand will go like this.
So very little extension here will give you
a large extension there.
We do it twice, so we have two levels.
And that is this arm.
I have here a set screw, and I can make...
I can move the bar in this direction.
I'm not making it longer, but I can move it,
and you will see what effect it has.
If I move the bar one way, it goes up.
Move the bar back, it goes down.
You can think of this as a thermometer.
It will be 70 degrees, as it is now, and if
I heat it up, then it will get longer and
you will see this end go up.
We could try that.
[blowtorch hissing]
There it goes.
Doesn't take very much.
So we have brass.
And L is 36 centimeters.
Delta L would be about one millimeter for
a temperature increase of only 150 degrees
centigrade, which of course is trivial for
us with that blowtorch.
It's still hot.
I could cool it.
I could force it to cool it, and then I could
even go below this point.
This was our 70 degrees, remember? It was
the room temperature.
I can cool it with some liquid and see whether
I can get it down quickly, and even past this
point, which would indicate that it is shorter
than it was when the lecture started.
[air whooshing]
You see, it's now shorter.
Now, what you're looking at is only something
like maybe a millimeter or even a little less.
But we amplify it and we show that in a quite
convincing way, then.
A very important implication--
an application of the expansion of metals--
is what we call bimetals.
They're all around you.
A bimetal is the following.
Say I have a strip here, length L, of Invar.
And I have another strip here which is attached
to it so that they cannot slip relative to
each other, and let this be copper.
And suppose I make, just as a working example,
I make the thickness of each one of them two
millimeters.
And I'm going to heat it.
I'm going to heat it, increase in temperature
delta T.
I'm not interested really in knowing how long
the copper gets and how long the Invar gets,
but I'm very interested in knowing the difference
in the length between the two.
Because what's going to happen when one gets
longer than the other, something got to give.
What do you think will happen? And they're
stuck together, they can't slip relative to
each other.
What will they do? They will bend.
And since the Invar is not going to expand
very much, but the copper will, if you heat
it up, it will go like this.
And we'll see that there are many applications
of that.
So what I'm interested in is really the delta
L of the copper minus the delta L of the Invar.
That's what I'm after.
And that is the alpha of copper minus alpha
of Invar times L times delta T.
So it is the difference that matters.
So the difference is 17 minus one, so that
is 16 times ten to the minus six times L times
delta T.
And if I take a length of ten centimeters
and I increase the temperature by 100 degrees
centigrade, then this difference, which you
can easily calculate, is .16 millimeters--
0.16 millimeter.
Very little.
And yet, this one will curve substantially.
For those of you who are mathematically oriented,
I would advise you to make an attempt to calculate
that.
So you have, you assume that it is a perfect
circle--
that's a reasonable approximation--
and so you have an outer circle which is longer
by .16 millimeters than the inner one, and
you try to solve and find what that D is.
And I went through that exercise, and you
want to do that, too, perhaps, and I found
that it's four millimeters for these dimensions.
Four millimeters--
that's substantial.
So this thing is being used for thermostats.
Um... you break and make a contact in a heating
system, which could, for instance, be as follows.
Here would be your bimetal, very schematically.
You plug it in the wall here, your 110 volts.
Here is your heater.
And you let it sit like that, and when it's
cold, this is down.
It's not curled, and the heater works.
And when the room temperature goes up, this
starts to curl, it breaks, and that's a thermostat.
That is the basic idea behind a thermostat.
And you have them in your cars, you have them
at home.
Your central heating system, air conditioner
and your heaters--
they're all over the place.
They're also being used for safety devices.
If you have a gas hot water heater, in the
pilot light, in the flame of the pilot light,
is a bimetal.
And when that bimetal is hot, your gas valve
is open.
But when that bimetal gets cold, it shuts
off the gas valve, which is a safety device.
In fact, in Europe, all gas stoves are protected
that way by law.
Strangely enough, not in the United States,
which is very surprising.
If I open my gas valves at home of my stove,
the gas will just come out, just like that.
There's no prevention of that happening.
In Europe, that's not possible.
There is always a pilot light somewhere with
a bimetal that senses that there is a flame
nearby to ignite the gas.
And if that flame is out, the gas valve will
be closed.
So bimetals can also be used very effectively
for safety devices.
I have here a bimetal.
One side I believe, we believe, is aluminum,
and the other side we believe is iron.
And when I heat that, you will see that it
starts to bend.
[blowtorch hissing]
Here we go.
I think you get the idea.
You could use that as a thermometer--
very crude one, but this is the idea of the
thermometer, of course.
Mark Bessette, who is the person who is preparing
always these demonstrations in a fabulous
way, told me he had at home a coffee maker.
And the coffee maker is designed in such a
way that there is a bimetal at the bottom
of the water reservoir.
You heat up the water reservoir, and when
the water heats to a certain temperature,
the bimetal opens and the water comes out
and it goes through the coffee.
I'd like to show that to you--
it's really cute.
Here is that coffee machine.
It's not working anymore, it's a very old
one, but I want to show you at least that
bimetal.
This is that bimetal strip.
Water goes in here, you heat it, and when
it's hot enough, the bimetal lifts up and
you can see there is a hole there.
This is the hole.
And the water comes out, and when the bimetal
is closed, then it closes off that hole.
So it's an amazing, simple idea.
The criterion for letting the water go through
the coffee is simply when the water reaches
that temperature close to boiling and you
have your bimetal control it.
So bimetals in many ways control matters.
Thermostats, in this case they act like...
like a valve.
Bimetals can be used as thermometers.
In fact, the one that you see right here is
driven exclusively by a bimetal.
If you look in the back of this thermometer--
and I will show that to you shortly because
I broke one open for you--
then it looks like this.
There's a coil.
This end of the coil is attached to the plastic
casing.
And here is the red hand.
It's a pivot here--
the red hand.
But this is fixed, this cannot move.
I heat it up.
What will happen when I heat it up? This is
already a bimetal, it's already curved.
But when I heat it up, it will tighten even
more, it will curl up even more.
And when this one curls up even more, it will
go into this direction.
So this is when the temperature increases.
And when it, of course, gets colder, it will
uncurl, and so here is cold.
And that's what the whole thermometer is based
upon.
And in the back is this coil.
And so I broke one open for you in order to
make you see that coil.
Now, I have that here.
This is the part that is in the back, and
this is that bimetal--
the bimetal coil.
This is the bottom part, this is the top,
and this is just the hand that is attached
to it.
And then it goes into this case.
Let's make sure that the pivot goes in there--
yeah.
If I increase the temperature, try to see
that.
You see the coil tightens up.
See, the coil gets tighter.
And when I make the temperature go down artificially
now by uncurling the coil, it goes in this
direction.
Now, what we can do is we can actually heat
it up.
We have to set the temperature at whatever
we think it is now.
It's about 70 degrees in the room.
Is that what it is? Well, that's close enough.
This is Fahrenheit--
oh, what a terrible scale.
Yeah, by the way, you also see some civilized
scale there.
You see centigrade in addition to the Fahrenheit.
So it's close enough, it's a little bit over
70, and I can now make sure that this is stuck
to it, to the case.
I can heat it.
[motor purring]
The bimetal tightens up more.
So when I heat it, the bimetal tightens up
more, and when I blow air over it, I can actually
make it cool a little faster than it will
otherwise.
[blowing]
So as simple as that.
Very simple device with...
the bimetals have many, many applications.
Your thermostats in your dormitories of your
heating systems, and also perhaps of the air-conditioning
systems, almost all have a coil like this
in there.
It was an extremely ingenious device.
It has a coil, and at the end of the coil
is a little glass tube, only one centimeter
large, and there is mercury inside.
So the coil is like this.
Bimetal.
And at the end here is a glass container and
there is some mercury, and the mercury is
here on this side, because notice the way
I have tilted it a little.
The mercury rolls to the left.
And there are here two wires, electric wires,
one here and one here, which go to the heating
system.
And so when the mercury is here, which has
a good conductivity, the heater is on.
Now, the room gets warmer and warmer and warmer
and the bimetal will tighten up, will curl
even more.
And so there comes a time that this glass
thing will go like this and the mercury rolls
out.
And when the mercury rolls out--
here is the mercury--
these contacts here are open, and the heater
will stop.
And that's in almost every room.
Very ingenious device.
Now I want to move from linear expansion coefficients
to cubic expansion coefficients.
I will leave these numbers there, because
I will need them later on.
But now I take a block of some material and
want to discuss the volume increase--
not just the length, but the volume increase.
So here I have a solid material.
Let's make it simple, all sides L.
And I increase it by an amount of temperature
delta T.
Well, the old volume is L cubed, and then
I'm going to increase the temperature by delta
T, so all these sides will get longer by amount
delta L, and so the new volume will be L plus
delta L to the power 3.
This can also be written as L to the third
times one plus delta L over L to the power
3.
Same thing, right? Now, perhaps you remember,
or at least you should remember, that one
plus x to the power n, in case that x is much,
much smaller than one, is approximately one
plus nx.
We've used that before when we discussed the
Doppler shift--
light from receding stars.
This is called the binomial series, binomial
expansion.
It's really the first order terms of the Taylor
expansion.
If you, as an example, if you take x equals
0.05 and you calculate the exact value with
your calculator, you would find 1.158.
If you do it this way, you will find 1.15,
which is very close.
Approximation is better than one percent.
So I will use the same approximation here,
and so we're going to get L to the third times
one plus three delta L divided by L.
So that is L to the third plus three delta
L, L squared.
So the difference in terms of delta V, the
old volume was this.
And this volume is V plus delta V, so delta
V is the difference between the two, is three
delta L, L squared.
But I know that delta L equals alpha times
L times delta T.
And so I can substitute that in here, and
so I find that delta V equals three alpha
times L squared times L--
that is L cubed--
times delta T.
But L cubed was the old volume, and so I now
find that delta V equals three alpha times
the old volume times delta T, and this one
is often called beta.
Beta is the cubical expansion coefficient,
as opposed to the linear one.
So you will say, well, big deal.
I mean, why are you talking about beta? Because
if we have the values for alpha, all we have
to do if we go to a volume, to make that beta
three alpha, and we are in business.
Well, when you have liquids, in general, you
don't find in the tables values for alpha.
So when you deal with liquids, for instance,
mercury, which is the one that I want to use
today, then you find that the cubic expansion
coefficient is 18 times ten to the minus five
per degree centigrade.
If I compare that with Pyrex, you have to
take three times this value, so that's very
roughly ten to the minus five per degree centigrade.
And so now you begin to smell the idea of
a mercury thermometer.
I put some mercury in Pyrex glass, and the
Pyrex glass is not going to expand very much,
but the mercury will.
And then the mercury, which is in an enclosed
environment, will have to expand, and it expands
into my thermometer and that's the way that
we read the temperature.
So here is a glass tube which is very, very
narrow.
All this is closed here.
And let's say the radius here is only 0.1
millimeter.
And here is a reservoir, mercury, just a working
example, and suppose we take one cubic centimeter--
just simple numbers.
Just want to show you the basic idea behind
the thermometer.
I'm going to increase the temperature of this
mercury, say by ten degrees, so delta T, ten
degrees centigrade.
So by how much will this volume expand? Well,
delta V equals beta 18 times ten to the minus
fifths times the volume, which is one cubic
centimeter--
let me just put the volume in there; you may
want to do it in cubic meters--
I leave it up to you--
times the delta T.
And you will find that the expansion in terms
of cubic centimeters is 0.0018 cubic centimeters.
See, if you leave V in cubic centimeters,
which you can do, then you get your answer
also in cubic centimeters, of course.
You do not always work... have to work mks.
So this is an extraordinarily small increase
in terms of its volume.
However, the Pyrex is not expanding at all.
You can just forget that for now.
If you want to calculate how much it is, it's
fine, but it is 18 times less, so the fact
that the vessel gets a little larger of course
is important, but I will just ignore that
for now.
And so I will just assume that all this mercury
will be driven up here.
And so if the mercury, then, changes its height
by an amount delta h, and if this is a tube
with a radius .1 millimeters, then this amount
of mercury must be the same as pi r squared
times delta h, which is the volume of the
new column--
the increase in the column.
And so you take your .1 millimeters and you'll
find that delta h for this example that I
chose is 5.7 centimeters.
That's huge.
That's very easy to see.
For ten degrees increase in temperature, it's
5.7 centimeters.
So for one degree centigrade, you would get
six millimeters.
It's very easy to see, and so that's the idea
behind a mercury thermometer.
I have a mercury thermometer here, but they're
very hard to read.
But I have one here.
It's a medical one.
It says "oral," not to be confused.
And I can stick it in my mouth and measure
my temperature using this scale.
It's easier for me to show you the one that
I have in my office, which also works on liquid.
[mumbling]: This one.
So this one, you see the red liquid? Let me
be sure that we have the right light setting.
This should go off.
Okay.
So you see the red liquid.
There it is.
And also notice that it is degrees Fahrenheit,
which is unfortunate.
But it's helping.
When the temperature is here, at least you
know that you're going to get ice.
[a few chuckles]
Well, that's somewhere near 32... 32 Fahrenheit.
That must be here.
Snow.
And this makes you feel good--
sun.
Yeah.
You know, just in case you don't remember.
I could heat it up.
[blow-dryer humming]
You always should be careful when you heat
it up that you don't go too far, because the
top of the thermometer is closed.
In the case of a mercury thermometer, there
is vacuum in here, and so if you--
when I was a kid, I loved to do this--
to take the medical thermometer of my parents
and heat it up with a hair dryer and then
it would just burst through it, it would just
break the glass, because the expansion is
huge, the force, and it would just pop off.
And then I'd put it back and say nothing.
And that can happen with this, too.
So it doesn't have an opening.
It is not open, it's a closed thing, so you've
got to be quite careful.
There is a technique which is called "shrink
fitting," and shrink fitting is the following.
You have a piece of metal--
let's take just a solid cylinder as a working
example--
and you have a ring.
The ring could be itself a cylinder.
Well, this one, this opening is smaller than
this--
just a little smaller--
purposely made that way.
Just a little smaller.
You heat this one.
Expands.
And you put it over here, it will fit.
And then you let it sit.
It cools, and it tightens itself up.
That's called shrink fitting, a technique
which is often used.
I have something to show you here which is
the opposite of shrink fitting.
I have a ring--
I'll show you shortly on the screen there--
and that's made of brass, and I have a ball
which goes through there.
Just barely, but it goes through there.
And then I will heat up this ball, and then
it won't go through there, so it's the reverse,
but at least you get the idea.
And then if you wait and it cools, when you
bring them in contact with each other, this
one will cool and this one will get warmer.
So you catch two birds with one stone.
This one will shrink, and at the same time,
this one will become a little larger, and
so then clearly it will fall through, and
that's the idea.
And let's try that.
Get the best lighting that we can under the
circumstances, and we'll change to... is that
it? Yeah, this is the ring.
And here is that brass ball.
It goes through there quite easily.
I'll put it horizontal now.
And then I will start heating up this ball.
[flame whooshing]
Here's the ball.
It can't be too close to the ring, because
then the ring will also be heated, and then
of course if the two have the same temperature,
then the ball will still go through.
[flame whooshing]
Let's see.
Now it doesn't want to go through.
So we'll leave it like that, and we'll see
what happens.
So the ring will now expand a little because
it gets hotter, and the ball will shrink a
little because it gets colder and it shouldn't
take too long for the ball to be able to get
through.
If I heat them both with the torch, then of
course they will always be able to go through
each other.
And there it goes.
So if I heat them both...
[flame whooshing]
...so they both expand, since it's both brass,
there is no differential expansion like with
bimetals, then no matter what you do, it will
always be able to go through.
It's only when I...
...heat the ball and not the ring that you
get the effect that it won't get through.
It may actually get through now, still get
through.
Yeah.
What I've left out of my discussion with you
is water.
Why have I not mentioned to you what is the
expansion coefficient, the cubical expansion
coefficient of water? Water is so important
in our lives.
Well, there is a reason why I left it out,
because there is something very special with
water.
If you take 20-degree centigrade water and
you cool it, it shrinks.
Normal behavior.
Beta has a positive value.
But when you reach four degrees centigrade
and you go all the way down to zero, then
it doesn't shrink--
it expands.
So in that range, from zero degrees centigrade
to four degrees centigrade, water has a negative
value for beta.
When you heat it, it shrinks, and when you
cool it, it expands.
And that makes water extremely unusual.
But it's great for fish, because it means
that water of four degrees centigrade has
the highest possible density.
It's higher density than at 20 degrees, and
a higher density than at zero degrees.
And so when in the winter the ponds freeze,
the highest density water goes to the bottom,
and that's why the way... that's the way that
fish can survive.
Rather than becoming deep-freeze fish right
there, they can swim.
So most of the pond in the winter, the bottom
layers are four degrees centigrade, which
is safely from the freezing point.
Now, when you melt a solid and it becomes
liquid, in almost all cases the liquid expands.
Sort of natural.
And so the solids sink in the liquids.
If you take crystals, they sink in their own
liquid.
But not water.
Water and ice are very anomalous.
If you take water at zero degrees and you
freeze it and it becomes ice crystals, it
expands.
And the expansion is enormous, because the
density of ice is eight percent lower than
the density of water.
The density of ice is 0.92 grams per cubic
centimeter, and water per definition is one.
This is why pipes can burst in the winter
when they freeze.
The pipes freeze, people have water pipes
near the outside walls.
They cool, they freeze, the pipes burst, because
the ice expands.
They're not aware of that, and in the spring
when the water melts, when the ice melts,
all of a sudden--
they have a flood because the pipe burst.
This is why the Titanic sank.
Because ice floats on water.
Ice has a lower density.
Without ice floating, no icebergs.
This is why you can skate on ponds, because
ice has a lower density than water, so ice
floats on water.
The best way that I can demonstrate to you
that ice floats is to treat myself and give
myself a glass of something.
Today it will be apple cider.
And I have here some ice cubes.
And I put some ice cubes in here, and they
float.
And if you don't believe it, come and take
a look.
Okay, enjoy your weekend.
Oh, no, we still have a lecture on Friday.
See you then.
