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Heat Transfer Through Convection: Natural vs. Forced

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  • 2:21 Natural vs. Forced
  • 2:46 Example: Sea Breezes
  • 3:34 Example: Earth's Outer Core
  • 4:22 Calculations
  • 5:41 Calculation Example
  • 6:58 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this lesson, you will be able to explain what convection is, both natural and forced, give some examples of convection, and complete some basic calculations. A short quiz will follow.

What is Convection?

Back when I lived with my parents, my bedroom was the hottest room in the house. My room was in the loft, above the hot water heating system. Even in the coldest weather, when the family was shivering in the living room, I was toasty warm up above them. Maybe you've noticed that rooms upstairs are very often warmer than downstairs. But have you ever wondered why?

It's not just rooms. Heat seems to work this way a lot. Let's say your room is in the colder part of the house, and you decide to make some soup to help warm you up. If you have a saucepan of boiling soup, are you more likely to burn yourself with your hand above the saucepan or alongside it? If you've ever cooked anything on the stove top, you'll probably know that it's much hotter above.

So why do these things happen? It's all because of convection.

Convection is a process where hot stuff rises, and cold stuff sinks to take its place. That stuff must be a fluid, either a liquid or a gas. If there's a temperature difference between two places, one below and one above, convection is likely to happen, and continue in a cycle. Unlike the other two types of energy transfer, conduction and radiation, convection involves the material itself moving on a large scale.

But why does that happen? What exactly does it mean to heat something up? Temperature is the average movement energy of the molecules in a substance. The faster they move, the hotter the temperature. When molecules move faster, they tend to spread out a little and take up more space. And if the molecules are more spread out, that means the substance is less dense.

What happens when you mix a dense material with a not-so-dense material? Try putting a cork at the bottom of a bucket of water. You'll find that it bobs up to the surface. Less dense materials float on top of more dense materials. This is part of why oil and water don't always mix well when you're cooking a stew.

The same thing happens with convection. Okay, back to your lunch: the stove top will heat the bottom of the soup more than the top. As the soup near the bottom gets hot, it'll get less dense and rise to the top. Now it's near the surface, it cools again and sinks. This repeats over and over, forming continuous convection currents in the soup.

My childhood bedroom was hot because heat rises in convection currents, and so a lot of the house's heat went right up to my room.

Natural vs. Forced

The processes we've talked about so far are called natural convection. Natural convection is where processes just happen on their own whenever there's a temperature difference between two places. But there's another kind of convection called forced convection.

Forced convection occurs when you try to speed up the process of convection, by pushing the fluid along a bit. For example, you might push the air around with a fan. This is how fan ovens work.

Examples

There are many, many examples of convection in everyday life. But let's start with a nice one: relaxing on a lovely, warm beach. When you're lying on the beach, you get hot pretty fast. So you're always thankful to feel a nice, cool breeze. But have you ever noticed where those sea breezes come from? More often than not, they came from the sea, towards the land. But why is that?

Well, that's also because of convection. In the summertime, the air above land tends to be hotter than the air above the sea. The sea air is just harder to heat up. As the land air heats up, it rises like we've talked about, but this time it isn't the cooler air above that fills the gap. Instead, the cooler air out at sea gets sucked into the space the hot air left behind - sucked towards land. That's your nice, cool breeze.

We've talked about the sea air, but let's go deeper. Much deeper! Not under the sea, though they do say it's better down where it's wetter. Deeper than that. Let's go all the way down into the belly of the Earth itself. Deep below the surface, miles and miles below: in the outer core!

Convection happens here too. It's hotter the closer you get to the center of the Earth, and cooler further out... though, to be fair, it's insanely hot everywhere! Nevertheless, this temperature difference is enough for convection currents of liquid metals to flow even here. And thank goodness they do!

These liquid metals contain charged particles, and the movement of those charged particles gives the Earth its magnetic field. Without it, the Earth wouldn't be able to push dangerous radiation from the Sun out of the way. Without it, we humans wouldn't survive long at all. Not to mention Captain Cook would have had real trouble exploring the Pacific -- it's hard to navigate if your compass no longer works!

Calculations

One common calculation you might have to do involves figuring out the heat transferred by convection per second, otherwise known as the power, measured in watts. In algebra, the heat transferred per second is Q/t. So if we know how much time has passed, and we know how much energy was transferred in that time, we can divide one by the other to get the answer.

But finding the heat transferred, Q, might involve another step. When we add heat to a material, one of two things can happen: it can change the temperature, or it can change the state (otherwise known as phase) by melting or boiling.

If the heat is being used to change the temperature, we can use the Q = mc(delta)T equation. Plug in the mass being heated, m, measured in kilograms, the heat capacity of the material being heated, c, which is just a number that you should be given in the question, and the change in temperature, (delta)T. Multiply them together, and you get your heat transferred, Q.

But let's say the heat isn't being used to change the temperature of the material, but instead is being used to change the state, like melting or boiling. In that case, you can use the Q = mL equation to figure out the heat transferred. Here, all you need to know is the mass of the substance being heated and the latent heat. The latent heat is just a number that varies depending on the material, and what state change is happening. You should be given it in an exam question.

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