Lesson video

- Hello, I'm Ms. Horseman and I'll be your physics teacher today, you will need a pencil because later on we're going to do a sketch, let's get started.

Today's lesson is on convection, we're going to be able to explain the process of convection and give examples of convection currents, many of these examples happen in everyday life, this lesson is part of the energy topic within science.

First, let's take a look at some key words that we're going to need for the lesson, density is mass per unit of volume, a fluid is a liquid or a gas when particles hit one another, this is called a collision.

There are four learning cycles in today's lesson, convection in gases, convection in liquids, convection currents and examples of convection, we are gonna start by having a look at convection in gases.

Hot air balloons are really good examples of convection in gases, hot air balloons rise off the ground when the air inside the balloon is heated, now if you've ever seen a hot air balloon you'll know that people have to stand in the basket and this basket has a burner in it and that burner when it's controlled by the pilot of the balloon releases a really large flame.

This flame will transfer energy to the air particles, so the burner is what transfers energy to the air particles, this means the air particles gain energy and they move around more quickly.

When the particles move round more quickly this means they're going to collide more frequently with one another and they'll move further apart.

Think about it, if you are walking down a street and you bump into somebody you'd move apart from each other, this is what happens to the particles inside the balloon.

This means the air becomes less dense and that's because remember from the key words density is mass per unit volume, if there is less massive particles in a certain volume, the density will decrease.

So in this situation because the particles have more energy and are moving faster and are moving further away from one another this is going to reduce the massive particles in a certain volume, which means that the air in the balloon is less dense.

The heated air inside the balloon is less dense than the cooler air which is outside of the balloon, this causes the balloon to rise, why is this? Because hot air rises, so if you've ever seen a hot air balloon flying through the sky, it's because there is hot air in the balloon.

That hot air has had energy transferred to it which has made the particles move faster, which has made them collide more often, which has made the particles move further apart and resulted in the air becoming less dense.

It's less dense air in the balloon is less dense compared to the air outside the balloon and that's why the hot air balloon rises.

Let's check your knowledge of convection and gases so far, when the energy of particles is increased, how is the movement of the particles affected? That's right, the particles are going to move more quickly because they've had energy transferred to them, when particles in the air have more energy and move more quickly, how is the density of the air affected? Brilliant the air is going to become less dense and remember this is because there'll be less mass of air in a certain volume, really well done if you've got that one right.

Now it's your turn to practise your knowledge of convection and gases, choose one of the words in each set of brackets to describe what happens to particles in air when they are heated, pause the video now and press play when you're ready to take a look at the answers.

All right, let's have a look at the answers, when particles in air are heated, they gain energy, So remember the example from the hot air balloon, the burner will provide that energy to the air particles so they're gaining energy, this means the particles move faster because they've got more energy, the particles collide more frequently and move further apart.

The air becomes less dense and remember this is because there'll be less massive air in a certain volume, this means that the hot air rises really fantastic job if you've got all those words correct.

There's lots of different things to remember when we are heating up particles, so you've done a great job if you can now describe what happens to the particles in air when they're heated.

Now we're going to move on to looking at convection in liquids, liquids actually behave in a really similar way to gases when they are heated which is helpful for us because it means that convection is always a really similar process.

The energy that's transferred to the liquid particles causes the particles to move faster, which is the same as what happens in the gas, when these liquid particles move faster they go into collide with each other more frequently and move further apart, just like in a gas.

The liquid will then become less dense and right, an example of this would be in a kettle, the kettle has a heating element at the bottom which is similar to the burner that we saw in the hot air balloon.

The heating element is what transfers energy to the liquid particles that causes those particles to move faster, they'll collide more frequently and move further apart.

This liquid or in the instance of a kettle water would then become less dense and rise to the top of the kettle, so using your knowledge of what happens when particles are heated what do you think happens to cooler particles, pause video now and see if you can predict what might happen to the cooler particles in a liquid? Okay, let's see if your prediction was correct, the cooler particles in the liquid are going to be in a more dense area of liquid as they're away from the heat source, so that means that there is a higher mass of liquid in a certain volume.

This means that the cooler particles are going to sink really well done if you manage to work that out, as you can see there from the diagram the hot particles have risen to the top of the kettle and the cooler particles will then sink towards the bottom to the heating element.

Let's check your knowledge of convection in liquids now, true or false cooler particles in a liquid sink because they are in an area of less dense liquid.

That's right, this is false can you justify your answer? Well done, it's because the cooler particles are going to be in an area of more dense liquid and that's why they're going to sink, great job.

This time you're going to practise applying your knowledge of convection in liquids to be able to describe what happens to the particles and the liquid when they are heated.

Pause the video now, write down a paragraph to describe this and then press play, when you're ready to go through the answers, let's check your answer.

When particles in liquid are heated, they gain energy, this means they move more quickly and move further apart, due to more frequent collisions with each other the density of this area of liquid decreases, so the hot particles rise, cooler particles are in a more dense area of liquid, so they sink, you might not written this paragraph word for word as this one here, but if you've spoken about gaining energy moving more quickly, moving further apart, more frequent collisions and density of the liquid decreasing causing the hot particles to rise and that is fantastic, you may also have added on information at the bottom about cooler particles sinking 'cause they're in a more dense area of liquid.

Really well done if you manage to remember all those factors, there's lots of things to consider when particles and a liquid are heated, so you've done a brilliant job to remember everything.

Now let's move on to looking at convection currents, convection is a type of energy transfer by heating, when the particles in fluids which are gases or liquids gain energy the fluid becomes less dense and rises cooler fluids sink, this is convection and as we've seen so far convection in gases and liquids is really similar.

Convection can only happen in gases and liquids and that's because they're fluids which means their particles can move, if we look at the diagrams the particle model of a gas shows that the particles are all free to move around.

In a liquid even though the particles are touching one another they're not in a fixed pattern, so again they can move around each other.

However, in a solid particles cannot move the solid particles are in fixed positions so convection can't occur in a solid, let's look at a convection current in water.

As the water is heated, the water particles gain energy, this means that they move faster and collide more frequently, this causes the water to become less dense and rise.

In the diagram we can see that where we've applied heat to the bottom of the water beaker the water has risen in that area, cooler particles that are away from the heat source sink because that area of water is going to be more dense when the hotter particles rise and the cooler particles sink and this repeats in a continued cycle, we call this a convection current.

As you can see from the diagram the hot particles rise and the cooler particles fall, the hot particles rise again, the cooler particles fall again, if this happens over and over we can call this a convection current.

A convection current will continue as long as energy is being supplied to the liquid or the gas, so in this instance here, the convection current in the water would keep going around and around as long as that heat source was still at the bottom of the beaker providing energy to those water particles.

Let's check your knowledge of convection currents, true or false convection can occur in a metal spoon.

Well done this is false, but can you justify your answer? Excellent, only liquids and gases, which we can call fluids, can transfer energy by convection and that's because their particles can move around in a solid the particles are in a fixed position as we saw in the diagram earlier on, this means the particles can't move around, so there's no way that convection currents could occur in a solid, what is the correct term for a continuing cycle of convection? Brilliant, it's called a convection current, when would a convection current stop? Well done, it's when energy stops being supplied, if energy's being supplied the convection current can continue, but when this stops there's no way for the particles to gain energy so they won't be able to move faster or collide more frequently or move further apart, so this would mean that the convection current would stop.

Now is your turn to have a go at a task related to convection currents, write these statements in the correct order to describe a convection current in a beaker of water, remember to think carefully about the correct order of all these events in order for a beaker of water to have a convection current in it, pause the video now and press play when you're ready to look at the answers.

Okay, let's check how you did, as the beaker is heated, energy is supplied to the water particles near the heat source, the particles gain energy they move faster and collide more frequently.

This reduces the density of the water which then rises upwards, cooler more dense water falls to replace the water that has risen.

The process of hotter particles rising and cooler particles sinking repeats in a convection current, great job if you put all of those in the correct order, there's lots of things to remember to make sure we get convection currents right, so you've done fantastically to get all that correct, well done.

The final learning cycle in today's lesson is looking at some examples of convection, convection currents occur in many systems in everyday life, can you think of any examples? A common example to think about would be when we're cooking in a kitchen, when a saucepan of water is placed on a hub convection occurs within the water and this would be the same if the source pan was full of any other liquid so it might be a source that you're using in cooking, a convection current would still occur within the liquid.

The metal saucepan itself conducts thermal energy, it's the water or the source inside that will have a convection current within it and there's a diagram there to represent how this convection current could look.

The heat is applied to the centre of the saucepan, this creates a less dense area of liquid in the middle which will then rise the cooler particles which are more dense sink and they'll become heated by the hub and this convection current will continue in the water as long as the saucepan is on the hot hub.

Another example of convection is one in the air, you might have seen birds appearing to be floating and not flopping their wings on a hot summer's day, this is because thermals which are columns of hot air that's become less dense and is rising occur.

Birds can soar on these thermals, which looks like they're floating, but they're actually using an example of convection, the thermal is hot air that's rising, which the bird can take advantage of and then rise up as well, it's a really clever way for them to not have to flap and save their energy.

Okay, let's check you really understand convection currents, where will a convection current occur, in a kettle of water being heated over a fire.

Well done, the convection current itself would occur in the water inside the kettle, the base of the kettle be heated by the fire which would transfer energy to those particles, they would move more quickly, which means that they're going to collide more frequently move further apart, that water would become less dense and then rise within the kettle.

The cooler, more dense particles would sink and this process would repeat for as long as the kettle is over the fire in the metal or handle of the kettle, this would be where conduction occurs, remember convection can't occur in a solid so it must be happening in the water inside the kettle well done, identify which two of these scenarios are examples of convection.

The first scenario that was an example of convection was soup being heated in a saucepan, the soup is a liquid and the saucepan will have energy being supplied to it by a hub underneath, so the soup would have less dense particles that rise and more dense particles that sink which is a convection current.

The second example of a convection current is a radiator heating air in a room, the radiator would transfer energy to the air particles, which would cause them to rise the cooler air in the opposite side of the room where the radiator isn't would then be more dense, it would sink and the radiator would then continue this cycle of a convection current as long as it's turned on.

Now it's your turn to practise drawing a convection current, complete this diagram by adding arrows and labels to show the convection current in the room, pause the video now and press play, when you are ready to see the sketch, remember to use a pencil so if you make a mistake, you can change easily.

Okay, let's take a look at how you did completing that diagram, the radiator will transfer energy to the air, this means that the air particles would move more quickly and collide more frequently with one another meaning they would move further apart, the density of this air will be reduced which causes the warm air to rise.

This warm air will then be at the top of the room because there was a cold window at the other side of the room, the air at this side of the room would be cooler and therefore more dense, we know that when air is more dense, it will sink so cooler air will fall towards the ground.

This cool air can then be heated by the hot radiator and the convection current can continue, the warm air will rise and the cool air will sink as long as the radiator is switched on, this convection current in the room would continue.

Well done if you manage to draw those arrows correctly, the key thing here to remember is that warm air will rise and cool air will sink and we end up with a convection current in the room.

Make sure you labelled your arrows to show which parts of the air were warm and what was happening to them and which parts of the air were cooler and what was happening to them as well, really great job if you managed to draw a diagram similar to this one.

Let's go over a summary of what we've learned in this lesson on convection, when energy is transferred to liquids or gases which we can call fluids, the particles rise as they're in an area of lower density.

Cooler particles in areas of higher density sink, when this process repeats this is called a convection current, so in a convection current particles have energy transferred to them.

They'll move faster, collide more frequently move further apart, that area of fluid will become less dense and rises and more dense areas of fluids will sink.

Convection currents occur in many everyday situations including water being heated in a kettle and a radiator heating a room, as long as energy is being supplied to the system a convection current will continue to flow, thanks for joining us on this lesson on convection, see you next time.