Hi. It's Mr. Andersen and this is AP Physics essentials video 18. It is on thermal conductivity
which is a property of matter that either allows it to transfer heat readily from a
hot to a cold object or it does not. And so the materials that I use every morning when
I make coffee, it is important that they have varying amounts of thermal conductivity. So
the electric kettle that I use, the element that heats up the water is metallic because
we want to transfer as much of that heat to the water as we can. But the handle that I
use to pour it is plastic. And that is because it has low thermal conductivity. We do not
want to transfer a lot of heat to my hand. Likewise the press that I press the water
through the grounds is plastic. So I am not burning my hand. And so the ability of matter
to transfer heat from a hot to a cold object is called its thermal conductivity. And it
is based on the structure of that material, both at the atomic and molecular level. And
so if we have something like a metal that is a good conductor of electricity it is probably
also a very good conductor of heat. And the reason why is there are these delocalized
electrons and it allows that heat to move through the material. Now non-metals can also
pass heat. And they do that not using these delocalized electrons but using phonons which
are waves in the lattice itself. And so you should be able to calculate thermal conductivity
of different materials in an AP Physics class. And so what is thermal conductivity? Well
we measure it in watts per meter kelvin. But what it is really showing us is how heat can
readily be transferred from a hot object to a cold object. And so if we have these two
objects in isolation there is not going to be heat flow between them. But if I move this
metal in the middle there is going to be the transfer of this kinetic energy from the hot
object to the cold object. And the ability of that material to transfer that heat is
called its thermal conductivity. And metals are very good at conducting that heat. And
that is because they share their electrons in this delocalized cloud. And so the electrons
are actually transferring heat. If we were to take something like wood, it is going to
have low thermal conductivity. If I were to put it between the hot and the cold objects,
heat is still going to be transferred from the hot object to the cold object, but since
there are not these free electrons it is not going to be transferred as readily. How is
it moving then? Well this whole wood is a lattice and you get these waves inside the
lattice called phonons that are actually transferring that heat. Now you can experience this right
now. So if you are sitting at a table like I am, I am going to put my hand on something
that is a non-metal. So that is going to be the surface of the table. And then I am going
to put my other hand on the bottom of my chair which is metal. And so it is going to be a
really good conductor. And the question I ask you is which one is colder? Which one
is warmer? And so when I feel those both I feel like the table is warmer than the chair.
But they are both exactly the same temperature. I was not touching them so they both are at
room temperature. So why does the metal feel colder than the wood? It is because it has
better thermal conductivity. And so if we were to look at a few objects, copper, which
is a good conductor of electricity is also a good conductor of heat. And so we would
say it has a value of 401 watts per meter kelvin. If we were to look at something like
brick, it is going to have lower thermal conductivity. Styrofoam, you know would have really low.
And so it is a fractional amount. And then a vacuum would have 0 thermal conductivity.
We should not be able to move heat through a vacuum. Now in an AP Physic lab you should
be able to measure thermal conductivity. And this is kind of a pretty ingenious set-up
on how you could do that. So what we have over here on the right is a steam generator.
So it is generating steam. That is going to move over here. So that is going to be our
heat source. And it is just going to start to build up in the steam chamber. The steam
chamber is enclosed on the top. And then some of the water that condenses can leak down
off the bottom. And so what you do is you put some kind of material on the surface of
that. So we have a plate here. In this case it is metal. And then we put some ice on the
top. That ice has a consistent size and it also, since it is frozen is going to have
a consistent temperature. You turn on the steam chamber and what is going to happen
is it is going to melt that ice. It is collected right here. And then that water can be measured.
So we could look at the time and the amount of water that is being converted from a solid
to a liquid. So we could measure the thermal conductivity from the steam chamber to the
ice. We could then try a different object. Maybe some wood. And we would see that it
is going to have lower thermal conductivity. And then we could try some kind of an insulating
plate as well. And we could see that that is going to be even lower a lower amount of
thermal conductivity. So it is a really quick way to measure how quickly heat is being transferred
between these objects. And so did you learn to design an experiment and to analyze data
that will show you thermal conductivity? This is a great example. And I hope that was helpful.