PROFESSOR: OK, let's begin our unit on thermodynamics.
Before we get started, we have to define heat.
It's a lot trickier than you think.
A lot of people use the word "heat" and "hot" interchangeably.
In physics, heat refers to thermal energy that moves.
Usually, we think of if something is hot, then it contains heat.
But it doesn't contain heat unless that thermal energy moves to another spot.
So a nice, short definition of heat-- energy
that moves from an object of high temperature
to an object of lower temperature.
It's measured in joules.
We've seen that before.
A lot of stuff is measured in joules-- kinetic energy, potential energy, work.
They're all measured in joules.
Definition of energy-- it's the ability to do work.
And in our mechanics section, you probably
should have learned that work was the force that's
parallel to some displacement.
Well, here's a cool little mathematical trick.
If we take that force and divide by area and multiply it by area,
technically, nothing should happen.
We multiplied by it.
We divided by it-- should cancel out.
But we learned in fluids pressure is force over area.
And for the diagram that we have here, volume
will be the cross-sectional area times the distance that it moves.
Let me show you what that looks like in this picture.
When I say area, I mean the area of this piston that's
going to compress some gas in a cylinder, that's our area.
We're going to move that piston from here over to here.
When we do, we move to that area A, a distance, D.
And this is the change in volume.
That's where volume comes from.
So another way to think of work is not only a change in energy,
but also pressure times volume.
That's going to come in handy in section three, when we talk about gas laws.
But for now, we're going just prove that work
is equal to pressure times a change in volume.
All right.
Moving right along.
Well, now that we have our definition of heat and thermal energy,
let's talk about the internal energy of a substance.
Just like everything else-- work, and energy, and kinetic energy,
and potential energy-- this is also measured in joules.
It's just a couple of those things added together.
It's the sum of the molecular and the kinetic energy of all the molecules,
plus the potential energy of all the molecules.
We don't really deal with that too much in this unit.
Those are the forces between the molecules.
That's more of a chemistry thing.
There are a couple other kinds of forces.
If things spin, that's a rotational kinetic energy.
If things shake back and forth, there'll be
a vibrational energy associated with it.
But we're more concerned with this first one,
the sum of all the molecular and kinetic energies of the molecules.
That's almost another way to understand what heat is.
Let me go back to this diagram for a second.
If we could see all the little molecules that were in here,
the speed that they're moving would tell us the thermal energy.
That's what heat and thermal energy is.
If these are moving faster, it has more kinetic energy,
which would give it the ability to transfer more heat into something.
We pretend like we can see these.
They collide with each other.
The collisions are perfectly elastic, and it's just
a different way of transferring energy.
That's what heat is.
If this were a hot object and I grabbed a hold of it, it would feel hot to me
because collisions of those molecules are hitting my molecules, giving
my molecules more kinetic energy, and I would perceive that as heat.
OK-- brings us to this formula.
It's the internal energy of a substance.
Well, it's Q, which is how much thermal energy it has,
and plus or minus W. This gets a little bit tricky.
Different books sometimes use a plus, sometimes use a minus.
If it's a plus, it's the work that's done on the system.
If it's a minus, it's done by the system.
Let me show you what that means.
In that diagram that we had a second ago,
I set it up so that if we apply a force over here,
I'm doing work on the system.
Now what's it done if it means by the system.
Well, if I took that same cylinder and heated it up,
that would be doing work on the system.
But then, the system moves the same distance this way
to have a change in volume.
So if you do work on the system, positive.
If work is done by the system, that's negative.
Next, well, now that we know what heat is, let's talk about temperature.
It's just a way to measure the average kinetic energy of all those molecules.
If you could see them moving, the faster they go, the hotter it would be.
There's three common temperature scales-- the one
we use here in the United States, Fahrenheit, Celsius, and Kelvin.
Kelvin brings in this idea of absolute 0.
Believe it or not, there is a limit to how cold things can get.
If we made a graph of pressure on the y-axis,
and temperature here on the x-axis, we'd see that as the temperature increases,
the pressure goes up.
If we work backwards, as the temperature gets lower, and lower, and lower,
we could connect the points on this graph and work backwards,
and find that there would be a point where the pressure would theoretically
be 0, and the kinetic energy of those molecules would be 0.
Essentially, they would stop moving.
Now, they can get really close to this temperature.
If they made this graph with different gases,
there'd just be a different slope, but they would all
converge right back there at that point.
And that temperature is negative 273.15 degrees Celsius.
And then, to simplify life, they just said let's just call that 0 Kelvin.
A little note about Kelvin-- it doesn't have a degree mark by it.
Fahrenheit has a degree mark.
Celsius has a degree mark.
Kelvin is just Kelvin.
Here's a couple common ones.
Everybody knows water freezes at 32 Fahrenheit.
Water boils at 212.
On the Celsius scale, that would be 0 and 100.
On the Kelvin scale, that would be 273 and 373.
Notice, in Celsius, there's 100 degree difference
between water freezing and water boiling.
On the Kelvin scale, there's also 100 degree difference
between water freezing and water boiling.
So a change in one degree Celsius is equal to a change of one degree Kelvin.
That's not to say that 1 Celsius is equal to 1 Kelvin.
It means that the change-- if I moved up one degree Celsius,
that'd be the same as moving up one degree Kelvin.
All right.
That's where absolute 0 comes from.
I thought it might be fun take a little practice
and see if we could convert between a couple of these temperatures.
I have the formulas up here for you.
We're going to take 1.8 and multiply by the degrees Celsius and then add 32.
That'll turn Celsius into Fahrenheit.
If you want to turn Fahrenheit into Celsius, here's the formula for that.
Kelvin is the easiest one.
You just take Celsius and add 273, and that's what Kelvin is.
OK, for this first one, let me find my calculator, and we'll be all set.
OK, first one-- 0 Fahrenheit.
Well, our formula says if you want to go from Fahrenheit to Celsius,
start with Fahrenheit.
Subtract 32.
Divide by 1.8.
I got negative 17.7.
And then, to go from Celsius to Kelvin, let me just add 273.
255.
All right.
Next, we're going to go from 33 Celsius to Fahrenheit.
So if I got to go from Celsius to Fahrenheit, multiply by 1.8
and then add 32.
91 degrees-- 91.4 degrees Fahrenheit.
And then, to go from Celsius to Kelvin, let me just add 273.
And I got 306-- 306 degrees Kelvin.