Video: Thermal Convection

In this lesson, we will learn how to describe the origins and effects of thermal convection currents in liquids and gases.

16:03

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.

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