Video: Conduction and Convection

In this lesson, we will learn how to describe heat transfer through solid materials via conduction and define the factors that determine the effectiveness of insulation.

11:26

Video Transcript

In this video, we’re learning about conduction and convection. These are two thermal processes that help us understand how heat is transferred among solids, liquids, and gases. Let’s start out by considering conduction. This is the transfer of heat between objects that are in contact, that are touching one another.

To get an idea for how conduction works, let’s say that we have a big chamber which is divided into two parts by a sliding partition. The two sides of the chamber have different gases in them. The gas on the left is heated to a high temperature so the molecules are moving much more quickly than the gas on the right which is a much cooler gas. We could say that the temperature of the gas on the left — what we can call 𝑇 sub 𝐿 — is greater than the temperature of the gas on the right, 𝑇 sub 𝑅. And we know this because the average speed of the molecules in the gas on the left is higher than the average speed of the molecules in the gas in the right. So on one side of the chamber, we have a relatively hot gas. And on the other side, we have a relatively cool gas.

Now, let’s say that we slide our partition out of the way so that the chamber is now one big undivided space. When we do this, these two gases will start to mix. And eventually, like we see going on down here, one of the particles from the one gas will collide with the particle from the other gas. When this happens, the more energetic particle in — this case the pink one — will transfer some of its energy to the less energetic particle — the gold one. This means that after the collision the pink particle will have lost some of its energy and the gold particle will have gained that energy. It will be moving faster.

As this happens on a macroscopic scale for the collisions of many thousands of gas particles, the overall effect is that the temperature within this chamber starts to even out. The hot, fast-moving particles are slowed down and the slower slow-moving particles are speeded up. Because this heat transfer happens between objects that are in contact with one another — in this case colliding gas molecules — this is an example of conduction the transfer of heat between objects in contact.

Now, in this example, we’ve shown conduction taking place among gas molecules. But actually, conduction is much more efficient when it operates in a solid. To see why that is, let’s replace the gas in the right side of our chamber with a solid material, something with a regular atomic structure. In this case, each of these green dots represents a single atom in the structure of this solid. In this case, when one of our hot gas molecules collides with one of the atoms in this solid, just like before that incoming relatively hotter gas molecule loses a bit of energy in the interaction, while the other atom involved in the collision — the one in the solid — gains energy.

So far, this is no different than what happened when we had two different gases colliding with one another. The difference comes in when our newly energized atom in the atomic lattice starts to vibrate and transfer some of its energy to the atoms in the lattice nearby. And then, those atoms once they’ve been energized start to vibrate and transfer that energy to their surrounding atoms in the lattice. Over time, there is a net flow of energy from the front of the solid to the back. And it happens fairly efficiently because all the links in the chain so to speak are so close.

We start to see then why conduction works better in solids, that is, more efficiently, than it does in gases or liquids. When conduction happens in a gas, two colliding molecules — say this one and this one — transfer energy between them. But they need then then have another collision to pass that energy transfer on to any other molecules. In a solid material, on the other hand, one single collision can energize many different atoms through a domino effect. That’s because all of these atoms are connected in an orderly lattice structure.

If we continue to build off this idea that conduction involves heat transfer between objects in contact, we can see that the more contact there is within an object, the better it will conduct heat. And in fact, there is a way we can increase even further the level of contact between objects in the solid. That’s because we know of a class of solid materials known as conductors. The important property of atoms which are conductors is that they have electrons which are free to move away from that atom. This means that when we put a bunch of conductor atoms together, say in a lattice like we have here, then what we get is a whole bunch of mobile objects — these free electrons — moving about within the solid.

All these moving electrons mean that heat is able to be transferred even more easily through this material. That’s because now for any given atom in the lattice to be energized, say this one here, it no longer needs to be in direct contact with another energized atom. Instead, it could be energized by a mobile electron which is just bounced back from encountering another energized atom at the front or the heated part of the solid. The net effect of these mobile electrons is that they speed up even more the transfer of heat throughout this material.

If we think about this idea that having lots of mobile charges in a solid helps to speed up the flow of heat through that solid, it makes sense. Atoms that have these mobile electrons are metals. And if we’ve ever put our hand on the metal handle connected to a metal cooking dish which is hot, then we know from experience that handle can be painfully hot. But at the same time, we know that depending on the material the handle is made of, it might be too hot to touch or it might be perfectly fine to hold onto. All of that depends on what’s called the thermal conductivity of a material; that is, the material’s ability to conduct heat.

We’ve seen that in general the thermal conductivity of gases is fairly low. They’re not very good at conducting heat. And liquids typically aren’t very good either. Solids are much better and metals are best of all. Even though solids are better conductors in general than liquids and gases, there are still solid materials we can make a handle out of and make it safe to touch. For example, if you’ve ever worked with a wooden cooking spoon, you know that this solid material is comfortable to hold onto even when part of it gets hot.

When we encounter a material such as wood which doesn’t do a very good job conducting heat, we call that material an insulator. An insulator is a material that resists heat conduction. Examples of fairly good insulators include paper, air, and water. Now that we’ve talked about conduction, the transfer of heat between objects that are in contact, let’s talk about the second mode of heat transfer.

Convection is the transfer of heat by the movement of fluids from hot to cold. And when we talk about fluids, we know that this covers two types of matter: gases and liquids. Here’s an example of convection. Let’s say that we have a room filled with air molecules which are all basically at the same temperature. Then, we put a space heater in the room and turn it on so it starts to radiate heat to its surroundings. What this means is that the air molecules near the space heater will start to heat up. And as they heat up, they’ll start to move more quickly.

As their overall speed increases, they’ll tend to move upward in the room towards the ceiling. When they do that, the space around the space heater is now fairly empty. And this creates an opportunity for other average temperature room molecules to come closer in. Now that they are closer to the space heater, these molecules also start to heat up. And once again, thanks to their higher relative temperature, they then move off up towards the ceiling. And once again, this creates an opportunity for cooler molecules in the room to approach the heater.

What we’re seeing then is an overall motion of higher-temperature air molecules towards the top of the room and cooler air molecules towards the bottom of the room. This overall or bulk motion of these air molecules is known as convection. And we can see now why we said that convection has to do with the motion of fluids, gases, and liquids, but not solids. The atomic structure of a solid prevents the motion of individual atoms within that lattice. On the other hand, there is no such restriction for gases and liquids.

Convection is one of the primary heat transfer methods for large bodies of fluids such as the atmosphere or the oceans. It relies on the mobility of individual molecules and the fact that harder higher-temperature molecules tend to move one way, while cooler or slower molecules tend to move another way. In this way, fluids mix and heat is transferred by convection. Now that we’ve talked a bit about these two heat transfer methods, let’s consider an example exercise.

Amelia wraps three beakers in different materials. She then fills the beakers with water at 100 degrees Celsius. She measures the temperature of the water in each beaker 30 minutes later. Her results are shown in the table. What material was the best insulator? What material was the worst insulator?

Okay, so what we have here is an experiment involving three beakers of water. So let’s say that here are our beakers. Now we’re told that they’re wrapped in different materials. One of the beakers is wrapped in felt, another is wrapped in cardboard, and the third beaker in aluminum foil. Following this, all the beakers are filled with water at 100 degrees Celsius, that’s boiling hot. Then, 30 minutes later, the temperature of the water in each of the three beakers is measured. That measure temperature is recorded in the bottom row of this table of data. Based on all this, we want to know which of these three materials was the best insulator and which one was the worst.

To figure this out, we want to know what an insulator is in the first place. An insulator is a material that resists the transfer of heat. We’re all familiar with insulators. For example, each one of us has probably used an oven mitt, which is an insulating material in order to handle hot dishes on the stove. So of these three materials — felt, cardboard, and aluminum foil — the best insulator will be the one that resists the transfer of heat out of the water the most. The water in the beaker with the best insulator will be the water that decreases in temperature the least; that is, it will retain the most of its heat energy. That means that the highest water temperature will indicate the best insulated beaker.

Looking at our table, we see that in the bottom row 38 degrees is the highest temperature of the water after 30 minutes. That tells us that the beaker wrapped in this material cardboard is best insulated. So we’ve write that down as our answer for the best insulator. It’s cardboard. And then, we move on to consider what material was the worst insulator. The worst insulator will do the opposite of a good insulator. It will let lots of the heat of the water disperse. The result of that heat dissipation will be that the water temperature will be lowest.

Looking again at the bottom row of our data table, we see that the lowest measured water temperature is 25 degrees Celsius. That corresponds to the beaker wrapped in aluminum foil. Because this water has cooled the most over the half-hour interval, 75 degrees Celsius, that means it’s insulated the least. Or in other words, it’s surrounded by the worst insulator. And that worst insulator is aluminum foil based on the fact that the water in the beaker wrapped in aluminum foil cooled down the most.

Let’s take a moment now to summarize what we’ve learnt about conduction and convection. In this lesson, we’ve learnt that conduction is the transfer of heat between objects that are in contact. We saw that gases and liquids aren’t very good conductors in general. But solids are much better conductors and metals better still. The measure of a given material’s ability to conduct heat is known as its thermal conductivity. This is a value we can look up in a table or be told about a given material.

We saw further that an insulator is a material that resists the transfer or the flow of heat. A good insulator is the opposite of a good conductor. And finally, we saw that convection — another method of heat transfer — is when heat is moved by the movement of liquids and gases — that is, fluids — from areas of higher temperature to areas of lower temperature.

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