SPEAKER 1: Hi this is section 2.2 of thermodynamics.
We call this heat transfer.
In this section, we're going to learn how energy, thermal energy,
goes from an area of high temperature to an area of low temperature.
We're going to learn the three methods of heat transfer.
There's conduction, radiation, and convection.
We're going to talk about how a thermos works.
How does it manage to keep thermal energy in,
without traveling through it?
And we're going to learn how aluminum does the exact opposite.
How is it aluminum goes and gets rid of its heat super fast?
All right, so let's get started with these things.
This is section 2.2 of thermodynamics, heat transfer.
SPEAKER 2: this next section of thermodynamics,
we're going to talk about how heat is transferred from one area to another.
There's three methods to transfer heat.
We've got radiation, convection, and conduction.
Our first one, radiation, is how energy moves
in the form of electromagnetic waves.
It's how thermal energy travels from the sun to the Earth.
It doesn't require a medium.
This is different than our previous definition of heat.
We said it was the kinetic energy of the molecules.
Well these aren't molecules.
These are waves.
Luckily, that's all the more we need to know about it.
It can also travel through a vacuum.
The second method of heat transfer is convection.
It's how thermal energy is moved from place
to place by the bulk movement of a fluid.
Simply put, hot stuff rises, cool stuff sinks.
That's how a lava lamp works.
It's why a hot air balloon rises.
What happens here is when stuff is heated, its volume increases.
It expands.
And we learned in a fluid section that the density
is something-- it's how much mass there is in a certain amount of volume.
Well if you heat something, its mass doesn't change.
But if you heat something, its volume increases.
So if I take the same amount of mass and divide by a bigger number,
density decreases.
If density decreases, a hot air balloon can rise.
That's why warm stuff floats.
The opposite it's also true.
If you cool something down, it shrinks.
Its volume decreases.
If I take the same amount of mass and I divide by a smaller volume,
that makes the density increase.
Which is why when things cool down they become more dense and sink.
The third method heat transfer is called conduction.
It's how heat is move directly through a material.
Going back to our definition of thermal energy
being the kinetic energy of the molecules--
If I had a hot thing right here, it's got a bunch of molecules
and they're moving really fast, and bouncing around,
colliding into each other.
If I take that and put it right next to a cold object,
it's molecules are going to be moving as fast.
That's what it is to be cold.
So if we could see the molecules, they wouldn't be moving around as much.
Well these warm molecules are going to have collisions with these smaller
molecules, and transfer kinetic energy, and eventually speed them up.
So the hot object transfers thermal energy
to the cool object because of the kinetic energy of the molecules.
Now I know this formula looks complicated.
I hope it doesn't scare you.
I'll simplify it as much as I can.
Q is how much thermal energy there is.
K is a physical constant called thermal conductivity.
Different objects have a different ability
to allow heat to transfer through them.
A is the cross sectional area between the two objects that are touching.
Change in temperature is what's the difference in temperature
from the hot side to the cold side.
The little t is time.
And L is the length that it has to travel through the material.
All right let's get into some of these.
I found a table of some common thermal conductivities online.
Silver, one of the best thermal conductors
there is, it can transfer 420 joules per second meter degree Kelvin.
And a little note, this degree Celsius could also be Kelvin.
Aluminum very good.
Copper even better.
And then these are thermal insulators.
Styrofoam one of the best.
All right, let's take a look at this formula.
We said K was a thermal conductivity.
I have a table over here of little k.
I've got to be careful not to get those mixed up.
Little k, thermal conductivity.
Big K, Kelvin.
So little k is the material's ability to transfer heat.
A is the cross sectional area that they move through.
Delta T is change in temperature.
And L is the distance that it has to travel through the object.
I made a little diagram to help illustrate this for you.
If we had a hot object over here, and a cold object over here,
the kinetic energy of those molecules is going
to have to travel this distance, L, to get through there.
The cross sectional area is how big this object
is that the thermal energy can travel through.
And time is just how much time it takes to get through there.
OK so let's try one.
Let's make up a couple numbers and see if we can just figure this out.
If the hot side of something is 150 degrees Celsius or Kelvin,
and the cold side is 50 degrees Celsius or Kelvin,
that gives us a change in temperature of 100.
Let's just make up a cross sectional area.
Let's say this was a square, and each side was 0.2 meters.
So I've got 0.2 times 0.2 meters thick.
It's a cross sectional area of 0.04 meters squared.
If this object was aluminum, it would have a thermal conductivity 240.
And let's see.
If the length was half a meter, we could figure out the rate
that this block of aluminum could transfer heat through it.
What I'm going to do is take time and move it to the other side.
And what that'll do is I'll tell us how many joules of energy per second
can travel through here.
So I plug in my thermal conductivity, 240 for aluminum,
cross sectional area of point 0.04, change in temperature of 100 degrees,
and the length between them 0.5 meters.
Throw that into my calculator, and it comes out to be 1,920.
And that's how many joules could travel through there per second.
That's pretty high.
Aluminum is an excellent conductor of thermal energy.
Now the College Board, they are supposed to be getting away
from a whole bunch of number stuff, and trying
to ask more conceptual questions about this.
I could foresee them asking a lot of questions.
Like what would happen to this rate if the area was twice as big?
Well that's going to make the rate twice as big.
Well what if the length was four times as long?
Well now I've got 2 divided by 4.
It's only going to be half as fast at transferring heat
from one spot to another.
I'll show you how these equations work.
I know the first time if I multiply these out
I'll get the amount of heat that's transferred.
If stuff changes, if the area is suddenly seven times as much,
and the length is one half as much, well then how
does that affect the rate that energy can travel through it?
Let me cross out the T, put it over here, now that's how many
joules per second travel through it.
Well if I have a seven upstairs, and a one half in the denominator,
that's going to make this rate 14 times as much as it was before.
All right, that's conduction.
It's how thermal energy is transferred through a material.
SPEAKER 1: What we just in that last section
was that different elements and different materials
have the ability to gain and lose heat at different rates.
Styrofoam is one of the best thermal insulators.
It holds onto its thermal energy for a really long time.
Aluminum is one of the best thermal conductors.
It can send heat really, really fast.
So what we have here is a beaker full of ice water,
and a few aluminum cans with some boiling water inside.
I'm going to take this aluminum can, stick it in the ice water,
and it is going to get rid of all it's thermal energy in under a second.
Let's see what happens.
Notice how fast that happened.
It takes less than a second, and this aluminum
can is already at room temperature.
So the reason it happened so fast was because it's aluminum.
Notice what happened to its volume.
We're going to learn about that in the next section,
about how temperature changes affect the volume changes.
Want to see it again?
There we go.
The collapsing can.