Lesson video

Hello there, I'm Mr. Forbes, and welcome to this lesson from the heating and cooling unit.

The lesson's called Thermal Insulators, and in it we're gonna be looking at how materials can be used to reduce the flow of energy from one place to another.

By the end of this lesson, you're going to be able to explain why wrapping an object in a thermal insulator will keep it hot, using the idea of vibrating particles.

You are also going to be able to explain why wrapping a cool object in a thermal insulator will keep it cool as well.

Here are the key words and phrases that'll help you during the lesson.

First of all, thermal conduction.

And that's a process where a change in temperature due to particles passing their motion onto nearby particles due to the forces between them.

A thermal conductor is a substance where thermal conduction happens very quickly.

A thermal insulator is a substance where thermal conduction happens slowly.

And insulation is a layer of a thermal insulator around an object to help maintain its temperature.

The lesson's in three parts, and in the first part we're going to look at how thermal insulators will keep things warm when the object is warmer than the surroundings around it.

And the second part, you're going to carry out an experiment comparing thermal insulators to see which one's best.

And in the third part of the lesson we'll look at how thermal insulators will actually keep a cold object cool when it's placed in a warm place.

So let's start by looking at how things are kept warm.

Hot objects cool down because they're warming their surroundings, and eventually they'll cool down until they reach room temperature if you leave them for long enough.

So for example, I've got a hot object here and the particles in the air around the hot object are colliding with it.

And so as those particles touch this thermal conduction, the air particles get hotter, they travel faster, and the object itself cools down.

So over a period of time its temperature will fall something like this.

So you can see there's a gradual cooling, a gradual lowering of the temperature of the object until it reaches room temperature.

And you should know that the object doesn't cool to below room temperature.

It doesn't reach freezing point or something like that.

It will just cool down until it reaches that room temperature.

If we surround that object by an insulating layer, then we can reduce how quickly it cools down.

So you can see here I've got my hot object and I've wrapped material around it, and that wrapping material will affect how rapidly it cools because now the object isn't in contact with the air, it's in contact with that wrapping material.

So there's going to be vibrations passed on to that wrapping material instead of directly to the air.

So the hot object is now cooling by thermal conduction because it's in contact with the wrapping material, not the air itself.

Hot objects will cool down more quickly if they're in contact with a good thermal conductor.

So we've got an example of that here.

I've got a rod that's made out of metal at one end and wood at the other end.

And I'm gonna put a sheet of paper and wrap that around that so it's in contact with the wood and in contact with the metal as well.

And I'm gonna use a Bunsen burner with the lowest flame and heat up that paper directly.

As I'm doing that, I'm gonna rotate it round so that the whole of the paper gets heated by the Bunsen burner.

So what will happen is the paper will be heated by the Bunsen burner, and the paper in contact with the wood, which is a poor thermal conductor, it's a thermal insulator, will start to smoulder and become sooty or perhaps even burn, because that paper is staying hotter for longer.

The other paper, which is in contact with the metal, doesn't burn.

So the vibrations of the particles in the paper on that side in contact with the metal are being passed along the metal so the thermal conductor, the metal, is transferring energy away from that paper, it doesn't stay hot for particularly long.

Whereas, at the other end, the vibration of the particles of the paper aren't transferred to the wood because it's a thermal insulator, or they're transferred very slowly, so the paper warms up more on that side and starts to become charred or burn.

Okay, let's see if you understand that idea.

I've got a hot object and it's placed so that half of it is resting on a wood surface and half of it's resting on a metal surface, as you can see in my diagram here.

The different ends of the wood and the metal have been labelled in my diagram, W, X, Y and Z.

Which of the following statements are correct? So have a look at those three statements, decide which ones are correct, and then restart please.

Welcome back.

Hopefully, you selected option B.

Part W of the metal gets hotter than part Z of the wood.

Well done if you've got that.

Poor thermal conductors are good thermal insulators.

So metals are good conductors and therefore they must be poor thermal insulators as well.

Glass is intermediate, it's in between, it's an okay insulator and an okay conductor.

It's not particularly good either way.

Wood, plastics, water, wool, they're good insulators and poor thermal conductors.

And air is a very good thermal insulator, it's a very poor thermal conductor.

So thermal insulators and thermal conductors are kind of opposite to each other.

Many good thermal insulators contain trapped air inside them.

Air is a very poor thermal conductor, and as long as you don't let it move around too much it means that the material that's got trapped air inside it is a poor thermal conductor.

So for example, wood has many pockets of trapped air inside it and that means that it's not a very good thermal conductor at all.

And wool, similarly, has lots of trapped air inside it so that's a good thermal insulator, a poor thermal conductor.

If you have a house and you want to keep it warm, then you may want to use double glazed windows instead of a single glazed or a single sheet of glass.

If I've got a single sheet of glass, I've got warm air inside the house and cold air outside the house, then the vibrations of the warm air are gonna be passed through the glass to the colder outside reasonably quickly, so the warm air is going to cool down.

However, if I've got two layers of glass with some trapped air in between, then the trapped air isn't going to pass those vibrations on very quickly at all.

And so the warm air inside the house is gonna stay warmer for longer than if I had just had that single pane of glass.

So the trapped air is preventing the transfer of those vibrations, of reducing the transfer anyway.

Okay, let's check if you understood that idea? Which of the following reasons for why double glazing provides better insulation than single glazing, is the correct one.

Pause the video, make your selection, and then restart please.

Welcome back.

Hopefully, you selected B.

Glass is a better conductor than trapped air.

So the trapped air is reducing the conduction.

Well done if you selected B.

Okay, it's time for the first task of the lesson.

And what I'd like you to do is to complete these three statements please, or these three sentences, by just using the words insulator or conductor.

So pause the video, complete the statements, and then restart when you're done.

Welcome back.

Hopefully, you selected these.

So wool is a good thermal insulator because its fibres trap air.

Wood is a poor thermal conductor because it has small air pockets inside it.

A hot object will stay warmer if it's wrapped in a thermal insulator rather than a thermal conductor.

Well done if you've got all of those.

And now it's time for the second part of the lesson, and in it you're going to carry out an experiment to compare different insulating materials.

So the basic setup for the experiment is shown here, and you're gonna test which materials provide the best insulation, which insulation performs best? So what I've got is a clamp on a stand to hold all the apparatus in position.

And then I've got a drinks can.

The drinks can is our container where we're going to put some hot water and we're gonna monitor how quickly that hot water cools down, 'cause that'll tell us how good different insulators are.

We're gonna place all that on top of a heat resistant mat so that we don't heat up the apparatus too much below.

We're going to then wrap some insulation around the can.

And we're gonna wrap different insulating materials around the can to test the different ones.

And to heat everything up, we're gonna add some hot water from a kettle into the can.

What's going to happen is over a period of time that hot water's gonna cool down.

And so the temperature reading on the thermometer we've placed in the hot water is going to fall over time.

And the slower that temperature falls, the better the insulation material must be.

So here's a picture of the can we're going to be using, and as you can see, what we've got is just a drinks can that's been emptied out and I've wrapped some insulation around it.

For accurate results and to make sure that I'm doing a fair test, I'm going to make sure I cut that insulation layer to the same size each time, and hold it firmly in place.

So I've cut that insulation so it's the same height as the can, so it wraps once completely all around it.

And you can see, I've got some cardboard insulation there.

And to hold that on, I'm going to use elastic bands.

That's the simplest way to hold that insulation in place.

To make my experiment a fair test, I need to keep everything the same except for the type of insulation I'm going to be using.

So I'm gonna consider each of those variables that could alter the results of my experiment.

Variables are called control variables because I need to control them and keep them the same for it to be a fair test.

So those variables can include: the material the can is made from, and I'm gonna use the same can each time so the material of the can is going to be the same each time.

How thick the layer of insulation is.

It wouldn't be fair to have a really thick layer of wool and compare it to a thin layer of cardboard.

That wouldn't be a fair test.

So I want to try and keep the thickness of the insulation the same in each of my tests.

I need to also control how much hot water I put in, the volume of hot water I put into the can, so I wanna put the same amount each time.

And the starting temperature of the measurements.

So I wanna start my experiment each time at the same starting temperature.

And finally, I want to control the time I measure the temperature over.

So how long I allow the liquid or the water to cool for.

Okay, let's see if you remember the control variables? When comparing insulating materials, which of the following must you keep the same for a fair test? So pause the video, make your decision on which of those, and it's several that need to be controlled, and then restart when you're done please.

Welcome back.

Well hopefully, you selected these.

The starting temperature for my measurements.

The time I measure the temperature change over.

The volume of hot water I put in the can.

The material that the can is made from.

Not E, the material you wrap around the can.

That's the thing I'm changing here to experiment.

And then how thick the layer of insulation is needs to be controlled as well.

So it's all of them except E.

Well done if you got that.

Okay, now it's time for you to actually carry out the experiment.

I'd like you to pick three types of insulation and put them in order of how good each one is as a thermal insulator, and explain why you put them in order.

So basically, you're trying to figure it out without carrying out the experiment first.

Then I'd like you to carry out the investigation to check your prediction.

So for each insulator, we need to use a measuring cylinder to add 150 centimetres cubed of hot water in a kettle.

And then, you're going to allow that water to cool for a little bit until it reaches 80 degrees C.

Then measure the temperature change for five minutes.

So you've allowed it to get to a fixed starting temperature and you're going to allow it to cool for five minutes beyond that.

And then, once you've collected your data, I'd like you to compare your prediction and explanation to your actual results.

And then try to write out an improved explanation of why some insulators are better than others based upon the results you got.

So pause the video, follow all those instructions, and then restart when you're done.

Welcome back.

Hopefully, you've got some results that look a little bit like this.

So for your first predictions you might have said bubble wrap, corrugated cardboard, fabric, as your materials.

And you might have suggested that bubble wrap was the best because it's obviously got bubbles in it.

And corrugated card, you can see the air bubbles there in it as well.

So that might have been your sort of prediction.

Your collecting data should be something like this.

You'll notice that the starting temperature is the same for each one.

And then I've got different temperatures after five minutes there, so you can see some have cooled more than others.

The cardboard has cooled the most, the bubble wrap has cooled the least, and the fabric's in between.

And your improved explanation should be something like this where you are describing the trapped air as making that material a good thermal insulator.

So well done if you've got results anything like that.

Okay, now we're onto the third part of lesson, which is about keeping cool, and this is a fairly short section where we're going to try and explain why thermal insulators will allow objects to remain cool when they're placed in warmer locations.

So let's do that.

So what I have here is a cool box.

And a cool box is a box made of insulating material that you put cold objects inside and those objects remain cold for long periods of time, as long as you keep that box cold.

And we're going to try and explain how that works.

So what I have here is a model of that box.

I've got particles inside which are cold, they're not vibrating much, they're at low temperature, and the outside air particles, they're at higher temperature, they're moving around and colliding with the outside surface of the box.

So those particles collide with the insulating materials and cause those insulating materials to vibrate slightly more and the temperature to increase.

But those insulating materials don't transfer those vibrations quickly to the objects inside.

So what I've got is particles in the insulating material increasing in temperature, but they're not passing that increasing temperature into the centre of the box where the cold objects are.

So the cold objects will stay at low temperature for longer than they would if there was no insulating material there.

Okay, let's check if you understand that idea? I've got an ice cube here and I'm gonna wrap it in aluminium foil, and an identically cube in wool.

Which of those ice cubes would melt first? So pause the video, make your decision, and then restart please.

Okay, welcome back.

Hopefully, you selected B.

The ice cube in the aluminium foil would melt first because aluminium foil is a good thermal conductor and so the ice would melt very quickly 'cause the vibrations will be passed very quickly through the aluminium foil into the ice.

The wool is a thermal insulator and so those vibrations aren't passed on, so the temperature of the ice wouldn't increase as quickly.

Well done if you've got that.

Okay, now it's time for the final task of the lesson and it's about ice lollies melting.

We've got Izzy and Aisha taking ice lollies on a picnic, and to keep them frozen, Izzy decides to wrap hers in a woollen scarf, Aisha wraps hers in aluminium foil.

State which ice lolly will stay frozen for longer and explain why.

So pause the video, write out your explanations, and then restart please.

Okay, welcome back.

Let's have a look at the answers for those.

So for the first part, Izzy's ice lolly will stay frozen for longer when it's wrapped in a woollen scarf.

And that's because wood is a good thermal insulator 'cause it's fibres trap air, and air is a gas so it's a bad thermal conductor.

And that will mean it's harder for the warmer air of the surroundings to warm up the ice lolly on the other side of the wool, and so it's gonna stay colder for longer and not melt as quickly.

Well done if you've got an explanation something like that.

Now we've reached the final part of the lesson, so here's a quick summary of everything we've covered.

The better a thermal insulator is, the harder it is for the particles to pass their vibrations through the insulator.

A thermal insulator wrapped around a hot object is good at keeping the object warm because it prevents cooling.

A thermal insulator wrapped around a cold object is good at keeping the object cold because it helps prevent heating.

Materials that contain trapped air provide good insulation because air is a gas and gases are very poor thermal conductors.

Well done for reaching the end of the lesson.

I'll see you in the next one.