Lesson video

Hello there, I'm Mr. Forbes and welcome to this lesson from the Particle Explanations of Density and Pressure unit.

This lesson's called Explaining Convection.

And in it, we're gonna look at the causes of convection currents and how they transfer energy throughout a fluid.

By the end of this lesson, you're going to be able to explain what a convection current is and describe how those convection currents can transfer energy throughout a fluid while in motion of that fluid.

And here are the key words for the lesson.

The first one is density, and that's the mass per unit volume of a substance.

And you should know that less dense substances will float on top of more dense ones.

The second is fluid, and that's the term we use for a gas or liquid because the particles can flow around.

The third is random motion.

And random motion describes motion where there's no overall direction.

Particles can move at different speeds and in different directions.

And finally, there's convection current, which is what we're looking at in this lesson, and that's the movement of fluids caused by differences in density causing hot parts of the substance to rise and cooler parts to sink.

You can return to this slide at any point in the lesson.

The lesson's in three parts.

And in the first part we're going to look at why some substances float on top of others.

In the second part, we're gonna look at how you can change the density of a material to cause it to rise up or fall down.

And in the third part we're gonna look at convection currents themselves where there are flows of fluid due to density changes.

So let's start by looking at floating substances.

Let's start by thinking about floating and sinking.

Less dense objects will float on top of more dense ones.

So if I've got a raft, and it's got a lower density of the wood in that raft, it's got a lower density than water, and so it's going to float on top of the water 'cause the density of the raft is less than the density of water.

But if I put a boulder in the water, made out of rock, then that boulder's going to sink in the water because it's made of a material that's much more dense than water.

So it's going to sink to the bottom.

It's not only solid materials that will float.

Fluids will float on top of other fluids in order of density.

So if I've got some water, which is a fluid, it's a liquid, and I pour another fluid on top, another liquid, vegetable oil, then the vegetable oil is less dense than water so it's gonna float on top of it and form a layer like that.

If I put a more dense liquid in, pour in something like glycerol, then the glycerol is more dense than water and will sink to the bottom.

So we get layers of different liquids, depending on their density.

Let's see if you can use the idea of density to decide which liquid will float on top of which.

So I've got four liquids and I pour them all into a measuring cylinder there, liquids W, X, Y, and Z.

And they form layers in that order.

What is the order of density based upon that diagram? So pause the video, make your decision and then restart please.

Welcome back.

Hopefully you selected answer D.

Z, Y, X and W.

As you can see, liquid Z's on the bottom.

That's got the highest density.

And the next highest density is Y.

And that floats on top of Z, but it sinks below X.

And the lowest density liquid was W.

That's on the top.

So well done if you've got that answer.

Air is a fluid and it's a mixture of molecules.

So it looks something like this where you've got nitrogen and oxygen molecules and they're approximately the same size as each other.

They're about the same mass, a nitrogen molecule and an oxygen molecule.

Because they've got similar mass, those two gases will mix freely and they won't form layers.

The oxygen isn't heavier than the nitrogen, so it doesn't sink beneath it.

So in any sample of air, you'll get a mixture of nitrogen and oxygen molecules.

They mix freely together.

If we introduce some helium into the air, the helium atoms are much smaller than the nitrogen and the oxygen molecules, as you can see in the diagram here.

And because they're much smaller, they've got a smaller amount of mass.

That makes helium less dense than air.

So helium being less dense than air, will tend to float on top of it.

So what will happen is the helium gas atoms will float upwards through the air and form a layer above the air.

So you can see here the helium molecules will move upwards.

So helium separates from the air and rises upwards.

We use that fact in helium balloons.

So I've got helium balloon here and the helium gas is less dense than the air.

So I fill the balloon with helium gas, but the rubber is more dense than the air so that would sink in air.

The average density of the balloon overall is less than the density of the air because we've got that very light helium gas counteracting the slightly more dense rubber.

So overall the balloon will float upwards 'cause the average density of that balloon is less than the density of the air around it.

Now a check of your understanding so far.

Which property of helium gas makes it useful for airships? Pause the video, make your selection and restart please.

welcome back.

Hopefully you selected the first option.

We use helium gas in airships because it's less dense than air, and that'll allow the airships to rise up in the air.

Well done if you've got that.

Now, if we introduce some carbon dioxide gas into the air, carbon dioxide molecules are more massive than nitrogen and oxygen molecules.

That makes carbon dioxide gas more dense than air.

So the carbon dioxide gas will sink in air.

So if I release carbon dioxide into the air, it will sink downwards towards the ground, something like that.

We can use that property of carbon dioxide gas to help put out fires.

We use carbon dioxide in fire extinguishers, so when we release the gas from the fire extinguishers, the carbon dioxide gas will sink towards the ground and it'll push oxygen molecules outta the way.

And as you should know, fires require oxygen to burn.

So if I release carbon dioxide gas, that carbon dioxide gas will form a layer towards the ground and it will smother the fire.

It'll prevent oxygen reaching the fire.

Because the fire now has no oxygen, it will gradually go out.

And so we've used that property of carbon dioxide gas being more dense than air to help put out the fire.

Another check for you here.

Which properties of carbon dioxide gas make it useful for putting out fires close to the ground? So I'd like you to select two of those please.

Pause video, make your selections, and then restart.

Welcome back.

Hopefully you selected these two.

It's more dense than air and that makes it sink towards the ground where it's going to put out the fires.

And also it's important that it's not a flammable gas.

Well done if you've got those two.

Now we've reached the first task of the lesson and there's two questions for you here.

First of all, I'd like you to draw a diagram showing the four liquids in that top table and what they would look like if it were poured into the same beaker.

So as you can see, there's four liquids and their densities in that table.

And for question two, I've got balloon X and that's been filled with carbon dioxide gas and balloon Y, that's filled with hydrogen gas.

And I'd like you to describe and explain what will happen if I release both of those balloons into the air.

And to help you with that, there's another table showing the densities of those three gases.

So pause the video, answer those two questions and restart please.

Welcome back, and here's the answers to those.

Well first of all, there's the four liquids and they're in order of density.

From the bottom we've got mercury, which is the most dense, and floating on top of that is glycerol and floating on top of that is water.

And then at the top with the lowest density is kerosene.

Well done if you've got that all correct.

Your explanations about balloons should be something like this.

Balloon X is filled with carbon dioxide that's more dense than air, so it sinks to the ground.

And balloon Y is filled with hydrogen, which is less dense than the air, so it'll float upwards.

Well done if you've got that as well.

It's now time for the second part of the lesson.

And in it we're going to look at why heating air causes it to rise and cooling it causes it to fall.

So let's look at that.

There is a mixture of gases and the particles in that gas are in random motion.

They're moving around very quickly in random directions, at a range of different speeds.

Their average speed is about 500 metres per second, and occasionally they'll collide with each other and quite often they'll collide with surfaces as well.

As I said, they move in random directions, but there's a lot of space between the particles in air, so most of air is actually empty space with lots of small particles whizzing around at high speed in them.

And when they collide with things, they change directions.

And so when they collide with a surface, they'll bounce off it.

When they hit each other, they'll both bounce off each other and travel in different directions after that collision.

When you heat up a gas, what you're doing is actually increasing the speed of the particles within it.

So the average speed of the particle increases as the temperature of the gas increases.

So if I've got cool gas, the particles are moving quite quickly around, but if I heat that gas up and have hot gas, they're moving much faster now.

So heating up the gas will cause the particles to move faster.

At higher speeds, those particles are gonna collide with each other much more often and they're gonna push away against each other much more.

They're actually going to cause each other to move further apart on average.

So the gas is going to expand when it gets heated and that expanding gas is going to take up more space.

So the hotter a gas becomes the less dense it is because there's more space between the particles within it.

So hot gases are less dense than cool gases.

As I mentioned, the particles in air are moving around randomly in all directions.

So I've got some particles here above an unlit candle and they're moving around fairly quickly in all random directions.

But when I light the candle, that candle is going to heat the air directly above it.

So it's going to transfer energy to those particles and they're gonna start to move faster.

The temperature of the air is going to rise and those particles will move quickly.

But it doesn't make them all move in one direction.

It just makes them overall move faster in all the directions they're travelling.

So the particles speed up in all directions around the candle.

Let's see if you understand that concept.

I've got a candle here and it's heating air around it.

Which of the images shows how the particles above a candle would be moving once it's lit? Pause the video, make a decision and restart please.

welcome back.

Hopefully you selected the first one, answer A.

The particles are moving in random direction remember, so there's no overall direction in B, C and D.

There's a general direction away from the candle, and that's not true.

Their particles are just moving randomly, but they're moving a bit faster because they're at a higher temperature.

We can use the idea of changing density of the air by heating it and cooling it to allow a hot air balloon to rise and fall.

So we've got a hot air balloon here.

The balloon itself is actually heavier than the air obviously because it's made of materials more dense than air.

So we've got the balloon material itself, the cloth, that's more dense than air.

And we've got the basket below which is made of plastic or wood or something like that, which is again, more dense than air.

So if I just filled up the balloon full of normal air, at the same temperature as the air around it, the overall average density of the balloon would be greater than the average density of the air around it and the balloon wouldn't float at all, it would just lie on the ground.

We can make that balloon rise by decreasing its average density.

So I've got a diagram of a balloon here.

It's got cool air inside and it's cool air outside.

The air inside the balloon can be heated and so we can get a gas flame.

That gas flame will heat up the air and that hot air will rise into the balloon itself and that will make the particles move faster.

Those faster particles will be warmer air inside the balloon.

So now the air inside the balloon has warmed up and it's at a higher temperature.

That means that that air is going to expand and become less dense.

So the air inside the balloon now is less dense than the air outside of it.

At the same time, because it's expanded, it's going to be pushing some air out the hole in the bottom of the balloon.

So there's actually less air inside the balloon now and it's at a higher temperature and lower density.

So some air's been pushed out, the average density of the balloon has now decreased.

Now if I've decreased the average density of the balloon enough, that balloon is going to start floating upwards.

The average density of the balloon is lower than the density of the air around it.

And so it's gonna start floating upwards inside the atmosphere.

That balloon's going to rise up through the atmosphere until its average density matches the density of the surrounding air.

So it'll continue to rise upwards and the density of air gets lower as you rise up through the atmosphere.

So the balloon's gonna float upwards until it has reached a zone where the outside atmosphere is the same density as the air inside the balloon.

If the air inside the balloon is allowed to cool then, what's gonna happen is the air particles will slow down so their speed will decrease and gradually that air will cool and it will contract slightly.

So we've now got cool air inside the balloon and cool air outside.

So the balloon's density is now greater than the air around it, and it's going to start to sink downwards.

So the hot air balloon sinks downwards when we allow the air inside it to cool Okay, I'd like you to use that knowledge to explain why a hot air balloon will float.

A hot air balloon, it has a powerful heater to heat the air inside it.

Why does heating the air make the balloon float? I'd like you to select two of those four answers please in your explanation.

Pause the video, select the two, and then restart.

Welcome back.

Hopefully you selected, warm air is less dense then cool air so the warm air has expanded and become less dense.

And there are also fewer particles of air inside a warmer balloon than a cooler one.

So its weight is decreased as well.

Well done if you've got those two.

Right now, we've reached another task for the lesson, and what I'd like you to do is to answer these two questions.

The first of them, I'd like you to use single words to fill in the blanks for those statements please, about the particles and the fluid.

And then the second one I'd like you to think about when you're trying to escape from a house fire, why is it recommended that you crawl along the floor to escape? So pause the video, answer those two questions and restart please.

Welcome back.

Well the statement should read, the particles and the fluid are in rapid motion in random directions.

When a fluid is heated, the average speed of the particle increases and as the temperature of the fluid increases, its density decreases.

Well done if you've got those.

The explanation about why you should crawl along the floor is something like this.

The hot smoke-filled air is less dense, so that will rise to the top of the room.

So the hottest part of the air will be at the top and that will carry the smoke upwards.

Cooler air will be down toward the bottom, so near the ground.

So if you crawl along the ground, you're gonna be moving in cooler air and that will reduce the lung damage and there'll be less smoke there as well.

So well done if you've got an answer like that.

And now it's time for the third and final part of the lesson.

And in it we're going to look at convection currents, which are flows of a fluid caused by changes in density.

So let's do that.

As you've seen earlier in the lesson, when you heat a gas or a liquid, or fluid, it's going to expand.

The particles in it are going to move faster and they're gonna move slightly further apart as well.

And by moving further apart, that's going to change the density.

So if you imagine these as being particles in some cold water, you can see they're packed fairly closely together.

If I heat that water up, then the particles are gonna move just slightly further apart and they're going to also be moving around a bit faster.

If I heat only part of a fluid, just that part of the fluid's going to become less dense, so the rest of the fluid's gonna stay at the same density and that part's going to become more dense.

So if you imagine I've got a mixture of hot and cold water, the cold water's going to be more dense 'cause the particles are gonna be close together and the hot water's going to be less dense because the particles are slightly further apart.

So the lower density part of the fluid is going to float upwards.

It's gonna have a lower density and float upwards.

Just like all the other examples we've seen in the lessons and because it's moving, what we've got is a convection current.

A convection current is a flow of less dense material, rising upwards to float on top of more dense part of the same material.

We can demonstrate air convection current in this way.

We've got a small purple crystal and we're gonna place that inside a glass tube.

And then we fill that glass tube with water.

And as you can see, the purple crystal starts to dissolve and you get this purple sort of region in the water.

The next thing we do is we heat it from beneath.

So I heat gently from beneath where the crystal is.

What that's going to do, it's gonna heat the water surrounding the crystal so that part of the water is going to be hot.

So we'll have warm water in that area.

That warm water is going to expand and become less dense.

And because it's less dense, it's going to start to try and float upwards on the cooler water around it.

So you're gonna get a pattern like this where the purple colour is going to flow upwards.

It's in the warm water, so the warm water is rising up that part of the glass tube.

That's gonna continue to rise up that tube to the top.

And when it reaches the top, it's going to spread out along the top.

So that's the lower density water.

At the same time, cooler water is going to be sinking down the other side, so the cool water under there sinks and goes back towards the bottom of the tube.

And what I've got is a flow of water around the tube where it's warm, rising upwards, and cool, sinking downwards on the opposite side.

And what I've got is a convection current.

So that convection current shows the flow of water around the tube due to it becoming less dense and then more dense on the opposite side.

We can watch a video of that process here.

Convection currents will allow energy to be transferred throughout all of our fluid, and evening out the temperature as the fluid flows.

So I've got some water in a container here, in a pan, and if I heat that water strongly at the bottom there, what's gonna happen is the water just above that heating point is going to expand and become less dense.

Now it's less dense than the rest of the water in the pan so what's gonna happen is it's going to float towards the surface so it travels upwards gradually like this, reaching the surface.

When it reaches the surface, it spreads out across the surface.

So I'm gonna get hot water rising to the top and then spreading across the top.

As it spreads along the top, it's going to start cooling.

So as it cools, it's also going to be more dense.

So towards the sides of the pan that water becomes a bit cooler and it starts to sink back down again.

And it sinks towards the bottom of the pan where it's heated once again.

And what we've got there is a convection current, a cycle where the warm water is rising to the top, spreading out and sinking down to the sides, and then getting heated again.

Overall that process is going to mix the water up and gradually all the water is going to get hotter and hotter.

Okay, let's see if you can identify the early stages in the formation of a convection current.

I put purple crystal in a beaker of water, it starts to dissolve and then I gently heat it beneath that crystal.

Which of those images shows the initial stages of the movement of the water? Pause the video, make your selection, and restart please.

Welcome back.

Hopefully you selected C.

What's happening there is the water is rising upwards because it's less dense, reaching the top and spreading out, and then it's going to start to sink on the other side.

Well done if you selected that one.

Convection currents don't just form in liquids.

They form in all fluids, so they form when you're heating air.

So I've got an empty room here with a radiator on one side, and the radiator heats the air that's in contact with it.

So the air touches the radiator, it warms up, its temperature increases, and so its density will decrease.

The air's going to expand near the radiator, and that means that that warm air is going to rise up, above the cooler air in the room and move towards the ceiling.

It's gonna reach the ceiling and spread out across the ceiling there.

And when it reaches the far end of the room, it's gradually cooling as it travels, and so it starts to sink back down.

It contracts and it sinks downwards because it's more dense than the air around it, and it travels towards the floor.

And what happens then is we've formed a convection current again.

We've got warm air rising on one part of the room and cool air sinking on the other part of the room.

And we've got this cycle and gradually that air will mix together and the whole room will warm.

So let's see if you understand convection current and air.

Imagine a large cold examination hall and it's got a single radiator in it, on a cold day, and that's near one wall.

What's the correct order in which the parts of the hall are heated by the convection current? As you can see in the diagram, I've got the radiator mounted there in red, and I've got four different parts of the hall.

I've got W, which is the far wall, X, which is the ceiling, Y, which is the floor and Z, which is the point above the radiator.

So pause the video, make your selection of the correct order and restart please.

Welcome back.

Hopefully you selected Z, X, W and Y.

The hot air is gonna rise to Z, spread across the ceiling to X.

Keep on moving across the ceiling, gradually cool and fall along point W, sinks to the floor and then travels to Y and drifts back towards the radiator where the whole convection current cycle starts again.

And eventually the whole hall will warm up.

Well done if you've got that.

And now it's the final task of the lesson.

And what I'd like you to do is to read carefully through the statements listed there and they're statements that describe how the water gets heated in an electric kettle when it's turned on.

And as you can see, you've got a kettle there in the diagram.

It's got heating element on the bottom.

That heating element is what heats the water.

So what I'd like you to do is to put those statements in the correct order, please.

So pause the video, put the statements in the correct order and then restart when you're ready.

Welcome back, and here's the correct order.

We've got an electric current making the heating element very hot.

The temperature of the water close to the element increases and the particles move around more quickly.

The density of the water near the element decreases.

The hotter, less dense water rises to the top of the kettle.

The water at a lower temperature sinks down in the kettle.

A convection current is then formed in the kettle, mixing the hotter and cooler water.

And eventually all of the water has been heated by the element and the kettle will boil.

Well done if you've got that.

We've reached the end of the lesson now and here's a summary of everything we've covered.

Particles in a fluid are in constant random motion.

So all the particles are moving around at high speeds, colliding with each other, travelling in random directions.

Less dense substances float on top of more dense ones.

So if you mix fluids, the more dense one will sink to the bottom and the less dense one will float to the top.

A fluid will expand when it's heated because its particles move faster and become further apart.

It becomes less dense when it's hotter.

Air inside a hot air balloon is heated to lower its density and that allows the hot air balloon to rise above the more dense air surrounding it.

So it will float upwards.

And finally, convection currents are formed when parts of a fluid are heated and rise above the cooler, more dense parts.

And you can see that pattern in the diagram on the right there.

Well done for reaching the end of the lesson.

I'll see you in the next one.