Convection

Transcript

0:00Introduction

Hello everyone! Okay, look around your home for a moment. Where is the air conditioner fitted? High up on the wall, near the ceiling, right? And the room heater in winter? Down on the floor. Why opposite places? Is it just habit? No, there is real physics behind it. In this chapter, heat moves in three ways: conduction, convection and radiation. Today, convection. Concept questions on it do come up in JEE, so let's begin.

0:33Convection: heat carried by a moving fluid

So, what is convection? Here is the line from the course: heat carried by the bulk motion of a fluid. Bulk motion means large portions of the fluid actually flow from one place to another. And a fluid is anything that can flow: a liquid, or a gas. Water in a pan, air in a room, both are fluids. With me? Now tell me. In convection, what is it that actually travels from the hot place to the cold place? The fluid itself! Yes, exactly! The hot fluid moves, and it takes its heat along with it, like a delivery van carrying a parcel. That is the whole idea of convection. Clear?

1:13Natural convection: warm rises, cool sinks

Now, how does a fluid start moving all by itself? Take some air, and heat it. Does it become more dense, or less dense? Less dense! Correct! Warm air expands, so the same mass takes up more room. On screen, rho, the Greek letter, stands for density, and the double arrow means so. Lower density, so it rises, okay? Remember upthrust from the fluids chapter? The lighter fluid is pushed up by the heavier fluid around it. And the cooler fluid? It is denser, so it sinks, and slides in to take the place of the warm fluid that went up. Right? Now that cool fluid gets heated, rises, and more cool fluid comes in. Round and round: a loop. This loop is called a convection current. And when density differences alone drive it, we call it natural convection. Got it?

2:07Why we heat a pan from below

Let us use this right away, on a solved example from the course. We always heat a pan of water from below, and here is why. The water at the bottom gets heated, becomes less dense, and rises. Cooler water sinks to the bottom, gets heated, and rises too. These convection currents carry heat through the whole pan. Okay? Now flip it. Suppose we heat the pan from the top instead. What goes wrong? The warm water is already on top, and it is less dense. So where would it go? Nowhere! The warm layer just stays on top, and no convection current forms. With me? So the heat has to creep downwards by conduction alone, and that is slow. So, heat the fluid from below, and let convection do the work. Clear?

2:58Natural or forced convection

Now, everything so far was natural convection. The fluid moves on its own, because of density differences. Okay? But we can also push the fluid ourselves. When a fan or a pump moves the fluid, it is called forced convection. Think of a table fan on a hot day: the moving air carries heat away from you. Right? The course gives one more example: the cooling system of a car. In a car, a pump pushes the coolant around the hot engine, and the moving coolant carries the heat away. Forced convection again. Got it?

3:33Trap: convection in a satellite

Now here is a lovely question, also solved in the course. Can convection happen inside a satellite in orbit? Many students say yes, of course, there is air inside. Hmm, wait. In orbit, the satellite and everything in it are in free fall, so there is no effective gravity. Okay? Now, natural convection needs buoyancy, the upward push on the lighter fluid. And buoyancy needs effective gravity. No effective gravity, so no buoyancy, so warm air does not rise at all. No natural convection! With me? So, the answer is simply no convection of any kind, right? Oh no, careful! That is the trap. Only natural convection stops. Switch on a fan, and the fan pushes the air around just fine. So, in a satellite, only forced convection is possible. Write that down. Clear?

4:29Sea breeze: convection on the coast

Now, one more example from the course, this time from nature. Stand on a beach on a sunny afternoon. The same sun shines on the land and on the sea. Which one heats up faster? The land! Correct! Land has a lower specific heat than the sea, so the same heat raises its temperature more. Okay? So the air above the land gets warmer, becomes less dense, and rises. Just like the warm water in our pan. And cooler, denser air from over the sea flows in to take its place. That cool wind blowing in from the sea is the sea breeze. Pure natural convection. Right? The same idea, on a much bigger scale, drives the trade winds. The course lists them as another example of convection. Okay? And at night, the wind turns around and blows from the land to the sea. That is the land breeze. Working out the reason is your first practice problem on this page, so that one is for you.

5:29Example 1: why a heater is low and an AC is high

Now, remember our puzzle from the start? Let's finish it. Why is a room heater placed near the floor, but an air conditioner near the ceiling? Pause the video, and try it first. Done? Okay, let's check. First, the heater, sitting near the floor. The air next to it gets warm, becomes less dense, and rises. Cooler air from the top of the room sinks down towards the heater, gets warmed, and rises too. Okay? Now the air conditioner, near the ceiling. Exactly the opposite. It cools the air at the top. That cooled air is denser, so it sinks. And warm air from below rises up to the air conditioner, to be cooled in its turn. Right? So both positions set up convection currents that mix the whole room. Mystery solved: it is not habit, it is physics! And notice, it is the same reason we heat a pan from below. Perfect!

6:27Example 2: why a tall chimney draws air better

Now a slightly harder one. Why does a tall chimney draw air better than a short one? Hint: think about the column of gas inside the chimney. Pause the video, and try it. Done? Alright. Hot gas rises up a chimney anyway, short or tall. So the height should not matter, right? No, no, wait! Height matters a lot. Look inside the chimney: there is a whole column of hot, less dense gas, standing over the fire. Okay? Outside, the air is cold and denser. Remember from the fluids chapter: the pressure due to a column of fluid grows with its height. So compare the light, hot column inside with the cold, heavier air outside. The taller the hot column, the larger the pressure difference between them. Got it? And this pressure difference pushes cold air in at the bottom. That inward flow of air is called the draught. Taller chimney, larger pressure difference, stronger draught. So a tall chimney draws air better. That's it!

7:34Quick revision: natural vs forced convection, free fall

So, quick revision. Convection is heat carried by the bulk motion of a fluid. Natural convection: warmer, less dense fluid rises, and cooler fluid sinks. Forced convection: a fan or a pump moves the fluid, and in free fall, that is the only kind possible. Now go and try the two practice problems on this page, including that land breeze. You will find them easy. All the best!