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Today we’re talking about heat transfer and the different mechanisms behind it. We’ll explore conduction, the thermal conductivity of materials, convection, boundary layers, and radiation.

Crash Course Engineering is produced in association with PBS Digital Studios:


Çengel, Yunus A., and Michael A. Boles. Thermodynamics: An Engineering Approach. 8th ed., McGraw-Hill Education.


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CC Kids:
Whether you’re heating up leftovers from last night, or just trying to stay cool in the summer, heat transfer is everywhere.

In some ways, the world is literally built around it. Any well-engineered building is designed and made out of materials that help keep the inside of the building at a good temperature, even during bitter blizzards or horrendous heat waves.

A furnace or A/C will only get you so far – without protecting against heat transfer, you might as well try to air condition your whole neighborhood. A big part of engineering is using your knowledge of natural processes to keep them from interfering with whatever you’re trying to do – or in some cases, taking advantage of those processes. So without a solid understanding of how and why heat moves the way it does, your designs won’t be very successful.

Especially if you’ve just discovered a tropical island. [Theme Music] So. This tropical island. It’s small, totally deserted, and seems like the perfect place to build that vacation home you’ve always dreamed of.

The tropical weather is really nice, but it’s also hot – very hot. So you’ll probably want to design your house to keep the heat out, and the cold in. In other words: you’ll want to prevent as much heat transfer as possible.

Even though heat isn’t a fluid itself, heat transfers in a way that’s kind of similar to the fluid movement we’ve talked about in past episodes. When fluids move, there’s always a driving force behind them – specifically, a difference in either pressure or velocity. With heat transfer, the driving force is a difference in temperature.

In fact, if you don’t have a temperature difference, you can’t have heat transfer. Period. If you have two boxes right next to each other, and they’re both at the same temperature, then one of the boxes won’t randomly start giving its heat energy away to the other one.

But if you're watching this video on your laptop and it’s, well, on top of your lap, then your legs might be warming up as heat is transferred to them from your laptop. That’s because right now your laptop is warmer than your skin, and when heat passively transfers, it always moves from a higher temperature to a lower one. You'd have to use the cycles and work energy we’ve talked about before – basically, an engine – to make it move in the opposite direction.

And unless you have perfect insulation, which is practically impossible in most settings, then a temperature difference will cause heat transfer. So, this house you’re building is going to heat up from the warm, tropical air no matter what. But there’s a lot you can do to slow down the process.

There are three main types of heat transfer to look out for: conduction, convection, and radiation. With conduction, heat energy is transferred by the collisions of molecules or other particles. When two things touch, the faster moving molecules of the warmer object crash into the slower moving molecules of the colder object, transferring energy that heats them up.

In your house, that will cause heat transfer through the layers of the walls. As the outer layer warms up, conduction will transfer the heat through to the cooler inner layers. To slow this down, you’ll want to build the walls out of a material with low thermal conductivity – something that doesn’t transfer heat well.

Copper, for example, has a high thermal conductivity, so if you made your walls out of it, you’d effectively be making a giant sauna. Which doesn’t sound like a great place to live. For one thing, it would probably be a burn hazard.

Materials like brick and drywall, on the other hand, have lower thermal conductivities, so they’d be much better choices. Anything that’s sold as insulation also has a very low thermal conductivity, so you’ll probably want some layers of that, too. The thicker these layers, the more resistant the walls will be to heat transfer by conduction.

Specifically, they’ll have more of what’s called thermal resistance, which is defined as the thickness of the layer of material, divided by the material’s thermal conductivity and the area of the layer. All this means is that materials with a lower thermal conductivity have a higher thermal resistance, and that a thicker layer will also have a higher thermal resistance. But if there’s more area for heat to be conducted through, that will lower the thermal resistance.

Thermal resistance is also equal to the temperature difference divided by the heat transfer rate. Which also makes sense. It means that for a given temperature difference, materials with a higher heat transfer rate will have a lower thermal resistance, and vice versa.

You can use this equation to determine the level of thermal resistance you’ll need if you want a low rate of heat transfer for the temperature difference you’d expect to have in the layers of your walls. And the resistance of the different layers adds up, so the total thermal resistance of the walls is equal to the resistance of each layer combined. It’s like putting on layers of clothing on a cold day.

The more layers you have, the lower your rate of heat loss will be, and the easier it is to stay warm. So for your house, if the drywall and brick don’t have enough thermal resistance to keep the inside cool, you can add layers of insulation to up the resistance. That takes care of the heat transfer within the walls, but there’s more going on outside of them.

Which brings us to the second method of heat transfer: convection, the transfer of energy by the physical movement of a fluid – which in engineering, can be a liquid or a gas. As a fluid moves against or across a surface, it can add or take away heat. For example, as air heats up, its particles spread out, lowering its density.

That warmer air rises, taking heat energy with it, and is replaced by cooler, denser air. That’s called natural convection, because variations in the temperature of the fluid create natural movement without any external forces. There’s also forced convection, where something external like a fan or the wind moves the fluid.

Heat transfer by convection actually involves some conduction, especially at first. When the warm outside air comes into contact with your house’s walls, it creates the no-slip condition we’ve talked about before, where the layer touching the surface comes to a stop. Right on top of that, there’s what’s called a boundary layer, where the air is moving, but slowly.

The lower velocity slows down the rate of heat transfer, creating a thermal boundary layer where there’s more conduction between the particles, and less convection. Beyond the boundary layer, though, heat transfer mostly happens by convection through the faster-moving air. You can measure how much heat is being transferred by convection using what’s known as the convective heat transfer coefficient, represented by the letter h.

It’s proportional to the thermal conductivity of the fluid over the thickness of the boundary layer, which just means that thinner fluids with higher thermal conductivity will transfer more heat. Makes sense. h also depends on how fast a fluid is moving. The less a gas moves around, the less heat is transferred through convection.

That’s why windows often have double panes. The air trapped between the layers of glass can’t move very much, reducing the heat transfer. So, double-paned windows are probably a good idea for your house.

There’s not much else you can do to minimize the effects of convection, since you can’t really control what the air does as it moves around outside. But you can reduce the effects of the last type of heat transfer: radiation. Radiation is the transfer of energy in the form of electromagnetic waves – and in this context, we’re talking about any electromagnetic waves, not just the cancer-causing kind.

You feel this radiation every time you walk outside and bask in the sun or warm up by a fire. Unlike conduction and convection, radiation can happen without any contact between the heat source and the object. That’s why heat energy from the sun is able to reach the earth.

Reflective coatings can help reduce the heat transfer from radiation a little since they absorb fewer electromagnetic waves. But when it comes to radiation, where your house is will be much more important than what it’s made of. Since the sun will likely be the biggest radiative source, you’ll want to build the house somewhere it won’t get too much direct sunlight.

So, there are a lot of factors to consider when you’re trying to minimize heat transfer. But with a shady spot, walls with lots of thermal resistance, and some double-paned windows, you should have a pretty comfortable place to hang out on your tropical island. So today we learned all about heat transfer and the different mechanisms behind it.

We started with conduction and learned about the thermal conductivity of materials. Then we moved on to convection and boundary layers, learning that convection can be both forced and natural. Finally, we covered radiation and how it differs from the other two methods in that it doesn’t need an intervening medium to transfer heat energy.

Next time we’ll talk all about how we can use what we just learned about heat transfer, and apply that to heat exchangers. Crash Course Engineering is produced in association with PBS Digital Studios. You can head over to their channel to check out a playlist of their latest amazing shows, like It’s Okay to Be Smart, Above the Noise, and Global Weirding with Katharine Hayhoe.

Crash Course is a Complexly production and this episode was filmed in the Doctor Cheryl C. Kinney Studio with the help of these wonderful people. And our amazing graphics team is Thought Cafe.