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.
