Video: Thermal Convection

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? [A] Diagram (a) [B] Diagram (b) [C] Diagram (c) [D] Diagram (d)

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Video Transcript

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.

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