Video Transcript
In this video, we’re talking about
the process of thermal convection. Convection is one of several ways
of transferring heat energy. And looking into this picture on
screen, we see a few of those different methods. In this example, we have an
electric heater being used to heat a pot of water. Heat travels through the air to get
to the pot; that’s called radiation. And if we consider our hand holding
onto the pot handle, if that handle is not insulated, our hand will be heated by a
process known as conduction. And then, along with all of this,
the water in the pot is heated primarily by convection. Convection is a primary way that
liquids and gases are heated. So let’s look into it more
closely.
Let’s start out with a
definition. Convection is the transfer of heat
through liquids or gases being moved through warming. The first thing we can notice about
this definition is that it only applies to certain states of matter, liquids or
gases, but not solids. To see why this is, let’s consider
the three different states of matter and how they respond to heating. Say, over here we have some solid
material, and we know that a solid is characterized by an orderly arrangement of
atoms and molecules.
What’s more, the particles that
make up a solid are fixed in place. Except for some jiggling and
vibrating when they’re heated, these particles won’t move relative to one
another. This means that if we were to apply
heat to this solid material, then the particles closest to the heat source would
heat up first and become energized. And then as they vibrate and come
in contact with surrounding particles, some of the heat energy would be transferred
to those. But notice that this transfer
happens through physical contact. This is what’s known as
conduction.
This is what takes place when, for
example, we put our hand on a hot piece of cookware and heat is transferred to our
hand. That transfer happens by the means
of conduction, from one solid object to another. When it comes to solids, because of
the way that the particles are fixed in place relative to one another. Even when one of the particles is
heated, say this one right here, there’s no way for that particle to move throughout
the solid and transfer that heat to some particle farther away. Rather, it can only transfer heat
to its nearest neighbors through the process of conduction. Because the particles of a solid
are fixed in place, it’s not possible for a solid material to transfer heat through
convection. This would require more movement
within the material, than a solid is capable of.
This brings us to liquids and
gases. In these states of matter,
particles are much more free to move past one another to other spots within the
liquid or the gas. For example, let’s say that this
liquid is some water that we have in a cup. If we were to take hold of the cup
and move it around gently, then we could easily imagine that any particular
particle, say this one here, could travel past other particles to any location
within the cup. It might be able to end up in this
corner, for example. And when we move from liquids to
gases, gas particles are even more free to move relative to one another. Liquids and gases then, which
together are known as fluids, have this property that the particles in them are able
to move relative to one another.
And since convection is the
transfer of heat through gases or liquids being moved through warming, this process
can only occur in liquids and gases, not solids. Now, to see how convection actually
works in a liquid or a gas, let’s take an up close look at our liquid and imagine
that we’re heating it. All right, so here’s our
liquid. And let’s say that this is tap
water in a glass and that the water all has the same temperature at the start. And then, like we said, we start to
heat this liquid, say, from underneath. What will start to take place here
is a process. And we can write down the steps of
that process. The first thing that will happen is
that the particles nearest to our heat source will start to gain heat energy. So that’s step one. And let’s say that the particles
that gain energy are these particles right here.
Next, thanks to this increased
level of energy, these particles start to pick up speed; they move faster. When this happens, the particles
move farther apart from one another. Now, let’s consider this particular
region of our liquid, the region that’s heated, as compared to the region that’s
not. The particles in the heated region
are farther away from one another. So the liquid in this pocket,
compared to the rest of the liquid in the glass, is less dense. That is, inside this heated region,
there are fewer water molecules per unit volume than there are outside the
region. So then, if this fluid is less
dense than the fluid around it, what happens to it?
What happens is this less dense
region starts to rise up. As this happens, as the less dense
region moves up and up in the liquid, then other cooler particles in the liquid sink
down and take their place. And once this happens, notice what
will take place next. Those cooler particles that sank
down to the bottom of our cup are now the closest particles to the heat source, and
so they heat up themselves. And that takes us back to step one
in our process. Just like our earlier set of
particles, these particles are heated and then they move farther apart from one
another, so they’re less dense, which makes them rise in our liquid. And, as they do cooler particles
from the top of the liquid sink down to take their place. And then as those particles are
heated, we go back to step one. What we have then is a cycle of
transferring heat through our liquid by heating the liquid and having it move
around.
This four-step process applies not
just to liquids, but it could also apply to gases. For the process to work for gas,
though, we would need some sort of lid on our container. That would keep the gas in the
system and allow it to circulate through the cycles of convection. To see how convection involves
cycles of fluid motion, let’s consider the particles in our container once more. We’ve seen that heating the
particles in this container causes them to expand and, therefore, rise up towards
the top. But what happens, we might wonder,
when these particles here reach the top of the container? Does that mean particle motion
would stop once are initially heated particles get to this point?
It turns out that particle motion
doesn’t stop because the particles that are closer to our heat source are still
warmer than those that are farther away, even if those that are farther away were
heated some time ago. No matter how long the cycle runs,
we’ll always have a high temperature region here, where the particles are spaced far
apart and less dense, which rises up and pushes the more dense, cooler particles up
here out of the way. And these cooler particles, as they
sink, will come around the sides and enter the area that is being heated directly by
our source. No matter how long the heating
process goes on, convection continually involves a cycle of movement with the
particles in the fluid following this four-step process we saw earlier. Knowing all this about convection,
let’s get some practice with these ideas through an example.
A container holds a liquid that is
steadily heated from the base of the container. Which of the following diagrams
most correctly shows the motion and temperature change of the liquid in the
container due to convection?
Alright, so in this example, we
have some kind of container with a liquid in it. So there’s our container; there’s
our liquid. And we’re told that this liquid is
being steadily heated from the bottom of the container. So say that we have some heat
source doing that. We want to find out which of these
four diagrams, a, b, c, and d, most correctly shows the motion and temperature
change of the liquid in the container as it’s being heated. And in particular, we’re told the
heating method to consider is the method of convection. Convection, we can recall, is the
transfer of heat through a liquid or a gas of fluid that occurs thanks to heated
portions of the fluid moving throughout the liquid or the gas.
So here’s what that might look like
in our particular example. The heat energy source below our
container heats up the particles in that container closest to it. Then, because of the added energy
given to these particles, they start to spread out. As they do, this portion of the
liquid that they occupy becomes less dense than the surrounding parts of the
liquid. Because of that, because this
pocket is less dense, it starts to rise to the top of the liquid. As it does this, it vacates the
space that it occupied At first. This gives cooler, more densely
packed particles the chance to fill in that space. And then these particles, now that
they’re the closest to the heat source, themselves are heated, expand, and rise.
What’s created, then, is a cycle of
hotter, less dense particles rising to the top and the relatively cooler particles
that were at the top sinking down to occupy that space closer to the heat
source. It’s important to realize that the
cycle will continue so long as heat as being added to our liquid through the base of
the container. The particles in the liquid then
move in a cycle. And that brings us to our
four-answer choices. In particular, let’s consider
options b and c. Starting with option b, here we
have our heat source, creating two rising columns of hotter liquid. But then notice what happens.
Once this liquid gets away from the
heat source and starts to cool down, according to this diagram, it simply stops
moving. It moves to the surface of the
liquid, and it never comes back down. There’s no continuous cycle of
movement. But this can’t accurately represent
the motion and temperature change of the liquid, because it ignores the fact that as
hotter less dense liquid rises, cooler more dense liquid must descend to take its
place. That aspect of the cycle is missing
from option b. So we won’t choose that as our
answer. And then considering option c, we
see something similar is going on here. Once more, liquid is being heated
and rising to the top of the container. But based on the diagram, none of
the liquid of the top is coming back down. This would indicate that heated
fluid, once it rises to the top, stays there motionless. There isn’t a cycle of motion going
on.
For the same reason we didn’t
choose option b then, we also won’t choose option c. Next, let’s take a look at option
a. This diagram says that hot liquid
rises from the bottom of our container and then, as it moves towards the top of the
container, cools down. But then we can see that, at the
top of this loop, the liquid from either side collides with one another. It’s as though two separate streams
of liquid collide head on at this point in the cycle. Now, practically if this were the
case, these two streams would likely curl around so that they could continue in
cyclical motion. But that’s not what we see in
diagram a. Rather, this diagram simply has the
two streams colliding head on. That’s not what actually happens to
liquid being heated through convection. So we won’t choose option a
either.
So option d is our last choice. Here, we see heated liquid from the
bottom of the container rising towards the top. And notice that the heated liquid
could be rising like this up through the middle of the container. Or it could be rising like this
along the outside edges. Either one is a possibility. But since it seems the heating is
confined to the central portion of the base of our container, we’ll say that the hot
liquid moves up this way through the center of the container. As this heated liquid rises to the
top of the container, cooler liquid follows in behind it and gets closer to the
heating source. That liquid is then heated and
follow suit, rising up towards the top of the container. At that point, the relatively
cooler fluid at the top follows in behind it, and the cycle continues on.
So we see in option d an accurate
representation of the motion and temperature change of the liquid in the container
due to convection. This is the only option that shows
us how this fluid might move continuously in a heating cycle.
Let’s take a look now at a second
example involving convection.
A fluid is in a container that is
steadily heated from the base. The region of the fluid that is
most directly heated thermally expands, reducing its density, as shown in the
diagram. Which of the following diagrams
most correctly shows both how lower and higher density fluid regions would be
distributed through the fluid and how these regions would heat each other?
Alright, to start out, let’s ignore
diagrams A, B, C, and D. And instead just focus on this one
here. What we see here is a heat source
being applied to a fluid that’s in a container. And from this diagram we can see
that the bottom portion of the fluid is thermally expanding, whereas the top portion
of the fluid is labeled as higher density. Along with this, we can assume that
the heating of this fluid is evenly spread out over the bottom of the fluids
container and that it goes on continuously.
Now, based on this diagram, our
question asks which of the following diagrams, A, B, C, or D, most correctly shows
two things. First, how the lower and higher
density regions in the fluid would be distributed? And, second, how these regions
would heat one another? In other words, how heat would
transfer throughout the fluid. To start answering this question,
let’s go back to our original diagram. And in particular, we can notice
that the bottom portion of our fluid is labeled as thermally expanding. Now, this tells us two things about
this region of our fluid. First, since the region is
expanding, we know that expansion will lead to lower density. And second, we’re told that this is
thermal expansion. That is, expansion due to heating,
meaning that this region will also be the hotter portion of our fluid.
Now that part is important, and
here’s why. Say that we have two objects. We have a cold object right here,
and then we have a hot object right here in contact with a cold one. In this situation, heat will travel
in a particular direction. It will travel from the hot object
to the colder one. And in fact, that’s always true of
the transfer of heat energy. It always moves from hot toward
cold. Knowing this and reviewing our four
answer options, we realized that whichever option is correct must show heat moving
from hot to cold. That is, it must be moving from a
higher temperature region to a lower temperature region and not the opposite
way. We saw that the two things we can
say about the lower portion of our fluid is that, one, it’s less dense than the
upper portion and, two, it’s hotter than the upper portion.
Now, in each one of our answer
candidate diagrams, we see heating that’s going on between these two regions,
between lower and higher density in our fluid. What we can do is go through these
options and eliminate any of them that do not show heat moving from hot to cold. And we see that, in our context,
that means from the lower density region towards the higher density region because,
as we’ve seen, the lower density portion of our fluid is also hotter.
So then, taking a look at answer
option A. We see on the left hand side that
heat is moving from the lower density region to the higher density region. That makes sense because that’s
from hot to cold. But over here on the other side,
heat is moving the opposite way from higher to lower density. That’s like saying that heat is
moving from cold to hot opposite what we saw here in our example. But that can’t be. Heat always moves from higher
temperature regions to lower temperature. Therefore, we cross off option A as
our answer choice.
Looking at option B, we see that
something similar is going on here. Heat is purportedly moving from the
higher density, that is, cooler part of our fluid, to the lower density, that is,
warmer part. But this has it backwards. Heat will move the opposite way
from lower density and hotter to higher density and cooler. And then the same thing happens
with option C. Once more, heat is purportedly
moving from the cooler region to the hotter region. But that doesn’t happen, so we
cross off option C as well.
This leaves us just with option D,
which correctly shows heat moving from the hotter to the cooler parts of our
fluid. Since this option also shows that
the lower density hotter regions would primarily expand of the sides of our
container, while the cooler higher density fluid would settle towards the
middle. Option D is our choice for our
final answer.
Let’s take a moment now to
summarize what we’ve learned about thermal convection in this lesson. Starting off, we saw that
convection is a heat transfer process involving the motion of heated material due to
that heating. Because convection involves the
motion of heated material, it can occur in liquids and gases, that is, fluids. But it can’t take place in solids,
where the particles of a solid are fixed relative to one another.
Lastly, we saw that the process of
convection involves a cycle. First, particles gain energy
through heating. Next, they move farther apart from
one another, which makes the mass of particles less dense overall, causing it to
rise, which leads cooler particles to sink down and take the place that the hotter
particles used to occupy. This is the convection cycle.
