Video Transcript
In this lesson, we’re talking about
heat. Heat is the transfer of thermal
energy, and it’s something we’ve all experienced. In this video, we’ll learn about
three different types of heat, conduction, convection, and radiation. Each one shows us a different way
that thermal energy is transferred.
Let’s start in with conduction. And we can imagine that we’re
outside enjoying a nice campfire. Nearby, there are two long thin
rods; one rod is made of metal and one is made of wood. If we pick up both rods and hold
one end of each over the heat source, we know from experience what will happen. Very quickly, the metal rod will
heat up and become too hot to hold. The wood rod, on the other hand,
won’t become too hot to touch. Over time, it may even catch fire,
but still it’s not too hot.
The difference between the wood rod
and the metal rod has to do with conduction. This is the transfer of thermal
energy across a solid object. If we considered our metal rod
while one end was being heated, the thermal energy transferred to this end of the
rod would move across this solid object by conduction. This happens as tiny particles in
the rod vibrate back and forth and touch one another. This allows thermal energy to
spread all throughout the metal rod and raises the entire rod’s temperature.
This is different, we saw, from
what happened with the rod of wood. Even as we heated one end and made
it hot, that thermal energy didn’t transfer across the wood to the other end. When a solid object doesn’t
transfer thermal energy well, it’s called a thermal insulator. Wood is a good example of this. On the other hand, objects that do
easily transfer thermal energy are called thermal conductors. Our metal rod is an example of
this.
So, conduction describes the
heating of solid objects. But what about liquids and
gases? The name for that heating process
is convection. To see how this works, say that we
have a container with a liquid and another container with a gas inside, and that
along with this, we have a nice, sturdy pair of tongs. If we use these tongs to pick up
the liquid, say that it’s water, and hold it over our heat source, a fire, then over
time thermal energy will be transferred throughout the water by convection. Here’s how that works.
Looking at an up-close view of our
water and how it’s heated, we know that first the water around here in our container
will heat up. When that happens, this liquid will
start to expand. That makes it become less dense
than the other water around it. And so over time, this water rises
to the top. With the less dense water out of
the way, more dense water comes to fill its place. But then that water too gets
heated, becomes less dense, and rises upward. In this way, a cycle of movement
develops. This is convection.
And we see something similar if we
heat our gas. Say that we set down our water and
pick up the container with gas in it, holding it over the fire. By the way, holding an enclosed gas
over a heat source like this is very dangerous. We wouldn’t want to do this in real
life, and even here we’ll just heat the gas for a short time. When we transfer thermal energy to
it, the gas near where the heat is applied increases in temperature. This makes the gas expand so it’s
less dense than the gas around it. That causes this hotter part of the
gas to rise. And then cooler, more dense gas
fills its place. And then this gas heats as well,
rises upward, and a cycle develops. This is convection in a gas.
Now that we’ve seen this, we
quickly set down our gas container. We do this before the gas inside
heats up so much and expands so much that the container explodes. Now we’ve seen how thermal energy
transfers through solids, liquids, and gases. But there’s one last type of
heating for us to consider. To see what it is, say that we put
our hand near but not touching the hot container of the gas, our hand could feel
warm from this. And that’s because of
radiation.
Radiation is the transfer of
thermal energy between objects that are not in contact. Radiation is why our hands feel
warm when we stand near a campfire. It’s also why sunlight feels
warm. Thermal energy from the sun travels
through empty space, a vacuum, and reaches us warming our skin. This is heating by radiation. Knowing about these three different
heat transfer methods, conduction, convection, and radiation, let’s look now at an
example exercise.
Fill in the blank. Thermal energy is transferred from
the higher temperature end to the lower temperature end of a solid rod. This is an example of heat
blank. (A) Convection, (B) conduction, (C)
radiation.
In this example, we have an object
where one end is at a high temperature and the other is at a low temperature. We’re told that thermal energy is
transferred from the higher temperature end to the other one. We want to know what kind of heat
transfer is this. One important clue is that our
object is solid. That is, it’s not a liquid, and
it’s not a gas. The way that thermal energy is
transferred across solid objects is by the process called conduction.
Convection, option (A), is for
liquids and gases, while radiation applies when two objects exchange thermal energy
but aren’t in contact. Since we have a solid rod and
thermal energy is moving across it, this is an example of heat conduction.
Let’s look now at a second
example.
Fill in the blank. The base of a tank of liquid is
heated. Thermal energy is transferred from
the bottom of the tank to the top by the circulation of the liquid. This is an example of heat
blank. (A) Radiation, (B) conduction, (C)
convection.
In this example, we have a liquid,
and the liquid is being heated from the bottom. When we add thermal energy to the
liquid at the bottom of the container, its temperature goes up, the liquid expands,
and it begins to rise. We see this motion shown in the
first part of the arrows in our diagram. Then, liquid from elsewhere in the
tank comes in to replace the liquid that’s risen.
By this process, thermal energy is
circulated throughout the liquid. When thermal energy is transferred
like this, either through a liquid or through a gas. That’s called convection. For our other two options,
conduction applies to solid objects. And radiation takes place when the
thermal energy is transferred between two objects that are not in contact.
We’ll choose convection as our
answer. So, when thermal energy is
transferred from the bottom of this tank of liquid to the top, this is an example of
heat convection.
Let’s review what we’ve learned
about heat. In this lesson, we learned that
heat is the transfer of thermal energy. Heating can happen through
conduction, the transfer of thermal energy through solids; convection, which is that
same transfer, but through liquids or gases; and radiation, the transfer of thermal
energy between objects that are not in contact. Lastly, we saw that different
materials make conduction easier or harder. A thermal conductor like a metal
rod is good at transferring thermal energy. A thermal insulator, on the other
hand, such as wood is not. This is a summary of heat.
