Lesson video

Hello there, I'm Mr. Forbes, and welcome to this lesson from the "Energy of moving particles" unit.

This lesson is called "Thermal conduction and insulation." And in the lesson, we'll be looking at how we can control the flow of energy between hot and cold objects.

By the end of this lesson, you're going to be able to explain how energy is transferred by the processes of thermal conduction in metals and non-metals.

Here are the keywords that will help you get through the lesson.

The first of them is heating, and heating causes the particles in a substance to vibrate more rapidly.

And we can heat a biochemical reaction like using a Bunsen burner or heating electrically.

Thermal conduction is the process that transfers energy through a material due to the collisions between moving or vibrating particles.

Thermal insulation is basically the opposite of that.

It's a way of reducing the transfer of energy by thermal conduction.

Thermal conductors are substances that transfer energy by thermal conduction very quickly.

And thermal insulators are the opposite.

They transfer energy very slowly.

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

This lesson's in three parts, and in the first part, we're going to be discussing what temperature is and about the processes of thermal conduction in non-metals and in metals.

In the second part of the lesson, we'll talk about thermal equilibrium, which is when objects are at the same temperature, and what happens with the energy transfer in that situation.

And in the third part, we'll look at the example of insulating houses to reduce thermal energy transfer.

When you're ready, let's start by looking at temperature and thermal conduction.

When you heat up a solid material, the particles in it start to vibrate more quickly, more vigorously around fixed points.

So if I've got a cool material here, you can see my set of particles in a neat arrangement and I'll show the vibration with these little arrows.

If I heat that material up a little bit more, then the vibrations increase, they're very slightly larger, and if I heat it even further and they get very hot material and the particles are vibrating very vigorously indeed.

And the temperature of that material is increasing when I'm heating it.

So the temperature increased from, let us say, 20 degrees Celsius to 90 and then up to 150.

And what you should also notice is that the material itself is expanding because the particles in the material are taking up more space, as they're vibrating over larger distances.

But do you see that also that the particles themselves aren't changing size.

It's just that they're moving more and taking up more space.

If we touch that material with a thermometer, that will allow us to measure the temperature on the material.

What's happening is there's an energy transfer from the material to the thermometer and that will cause a change in the thermometer that we can see.

I've got some materials here, cool, hot, and very hot again.

And if I put a thermometer against the cool material, it'll show a reading of 20 degrees Celsius.

The liquid inside will rise up to a certain point on the thermometer and that will have a reading of 20 Celsius there.

If I put that against a hotter material, it'll rise further.

And a very hot material, it'll rise towards the top.

And what's happening is that the liquid inside the thermometer is expanding.

The particles are vibrating more because they've got more energy and they'll take up more space as the temperature of that liquid increases.

So the very hot thermometer is showing a higher reading.

Now if you touch that hot material with your finger, then there'd also be an energy transfer this time to you.

So if I took the hot material, there'd be an energy transfer into my finger.

I would sense that energy transfer and I'd know that material was hot.

If I touched the hotter material, which I shouldn't do, 150 degrees Celsius would certainly burn me, the higher temperature would have a faster energy transfer from the very hot material to my finger and that would cause damage to the skin.

So that would be a burn.

So don't touch those very hot materials.

Let's see if you understand that first concept.

So a hot metal plate on an iron can damage the cells in your skin.

Why does touching it for longer increase the damage? So pause the video, make your selection from those four options, and restart please.

Welcome back.

Hopefully you selected C.

Particles in the metal are vibrating against the skin for longer, and that's going to cause an increase in the energy transfer and that's going to cause an increase in damage to the skin.

But I warn you again, you should never touch something that hot.

Okay, a second check about that concept.

If you touch one of the metal blocks shown below and you've got three metal blocks there at different temperatures, which would transfer energy to your finger the fastest? So pause the video, make your selection and restart please.

Welcome back.

Hopefully you selected the 90 degrees Celsius block, block C.

Well done for choosing that.

So let's try and describe the process of thermal conduction in the material.

So we've got a section of material here and I've shown some of the particles in it as those little circles.

Now if I heat one end of the material, what's going to happen is those particles are going to start to vibrate more.

You're going to move around faster and faster.

So I've shown those in a slightly different colour there.

I can then see that those particles are going to collide with nearby particles.

Those second set of particles are very close to the first set and they're going to be pushed around by the vibrating particles and they're going to start vibrating as well.

So those nearby particles start to vibrate more and the temperature of that section of the material increases as well.

And so that process is going to continue.

I'm going to have more particles causing other nearby particles to vibrate.

And gradually, throughout that material, I'm going to get a gradual transfer of energy from one particle there as all the particles start to vibrate.

And that process continues along the full length of the material until after a while, all of the material has got the particles vibrating at a higher speed.

So the temperature of all the material has increased.

And there we have it.

Virtually all that material has now increased its temperature.

The energy has been transferred from one end to the other.

The rate of energy transfer through different materials can be different.

They're not all equally good thermal conductors, so they transfer the energy at different rates.

Good thermal conductors transfer energy very quickly.

So something like a metal, this is a copper pan, and that transfers energy very quickly indeed.

So that's a good thermal conductor.

But poor thermal conductors transfer energy very slowly, so much more slowly like a plastic.

So this plastic mug here, that transfers energy slowly.

It's a poor thermal conductor.

Very poor thermal conductors are known as thermal insulators.

So I've got an example here.

This is expanded polystyrene, and if you put something hot inside there, you won't feel much of that heat escaping through the material.

It's a very good thermal insulator.

And similarly, if you put something cold in there, it won't feel cold when you touch it on the outside either.

Thermal insulators still transfer some energy by conduction, but it's a much slower process.

If you touch a good thermal conductor like a metal object that's at room temperature, when you touch it, it feels cool.

And that is because it's transferring energy quickly away from your finger, which is slightly above room temperature.

And you'll notice that effect.

So it feels cool to you.

The opposite happens when you touch a thermal insulator.

It feels warm to the touch because it's not transferring energy away from your finger quickly.

And so you don't sense that loss of energy in your nerves.

So they feel different, even though they're at the same temperature.

Let's see if you get that idea.

I've got three blocks and they're left overnight in a cool room.

So all of them are cool.

Which will feel the coldest when you first touch it? The wood, the metal, or the plastic.

Pause the video, make your decision and restart please.

Welcome back.

Hopefully you selected B, the metal.

The metal object will feel cooler because it transfers energy away from your finger more quickly.

Well done if you got that.

There is a reason that why metals are good thermal conductors when you compare them to other materials like plastic or wood.

And that's because they've got a second process that allows energy to be transferred through them quickly.

So instead of just the vibrations of the particles themselves, the atoms, there's going to be another process and we'll describe that here.

So I've got a diagram here of a piece of metal and the particles in it.

But as you can see, I've added some extra particles and these represent electrons.

The metal's got a set of free electrons.

The outer electrons of each of the metal atoms has become free and is able to move around within the metal.

So I've got the metal ion that's left behind.

That's the charged centre of the atom.

And I've got some electrons here.

And when those electrons gain energy, if I heat that end of the metal, those electrons themselves will gain energy and it'll start to move faster.

And they're able then to travel through the metal between the ions very quickly and they can move from one part of the metal to a more distant part.

So one could move several atoms across and reach here and then that would collide with the ion that's there and transfer energy to that part of the metal.

So that part of the metal will be gaining energy and its temperature will be increasing.

So overall, that process allows energy to be transferred more quickly through a metal because the electrons can travel more freely through it and transfer that energy.

And another check for you here.

So I've got four spoons made from different materials.

You can see there's a copper one, a steel one, a plastic one, and a glass one, and all are placed in hot water at the same time.

Which of those statements is correct about what will happen? So pause the video, make your selection and restart please.

Welcome back.

Well, hopefully you selected all three.

The first one is the effect you'd feel.

The top of the spoons would be hotter.

The metal spoons quicker.

And that's because the outer electrons from the metal atoms can move quickly through the metal and the outer electrons from the metal atoms can transfer the energy therefore.

Well done if you selected all three.

Okay, it's time for the first task, and the first task is this.

I've got an artist who's created a sculpture made from wood and metal.

And as part of the process, they heat it in the oven, perhaps to make their glue dry or something like that.

All parts of the sculpture reach the same high temperature after a few minutes.

And then they've got three questions to answer about that.

So what I'd like you to do is to pause the video, answer those three questions, and then restart please.

Welcome back, and let's have a look at those.

Well, for the first one, explain why some parts of the sculpture feel hotter than others when the artist touches them.

And that's because metal's a good thermal conductor.

It's going to transfer energy very quickly to the artist's hand, and that will mean that the metal parts feel hotter than the wooden parts.

Even though they're at the same temperature, the metal will allow energy to be transferred quicker.

How does using padded gloves protect the artist when they pick up the hot sculpture? Well, the padding in the gloves is a thermal insulator.

It's a very poor thermal conductor, and so energy will be transferred very slowly through it and your hand won't be burnt.

Well done if you got those two.

And here's the answer to the third part, and this is a description about the energy processes.

And in it, we should be describing thermal conduction through a metal in two ways.

We've got the ions and the vibrations and those vibrations cause other particles to vibrate in the material and that's passed along slowly through the material.

And the second answer part is about outer electrons, which are free to move from one part of the metal to the other and transfer energy very quickly.

Well done if you've got answers something like this.

And now it's time for the second part of the lesson.

And we're going to look at something called thermal equilibrium, which is what happens when two objects are at the same temperature, and what energy transfers happen to cause that.

So let's have a look.

So if you place two objects which are at different temperatures in contact with each other, then there's going to be an energy transfer between those two objects.

Now let's have a look at a couple.

We've got an object here at high temperature and one at low temperature.

And I've drawn that as two different-colored particles.

And as you can see, the high-temperature object of the particles are a bit further apart.

So I'll show the vibrations of those particles as well.

The particles in the hotter object are vibrating more rapidly.

They're moving around more quickly than the particles in the cooler object.

And what's going to happen is at the point where they're touching each other, the faster-moving particles in the higher-temperature object, they're going to start to cause the slower-moving particles, the low-temperature object to vibrate more because they're against each other.

So those particles there I've just shown, they're going to start vibrating more rapidly.

And what that means is that the temperature of that part of the material is going to increase.

So what I've got is a temperature increase of the cold object as energy is transferred from the high-temperature object.

Let's see if you get that idea.

I've got two objects here, object X and object Y, and they're in direct contact with each other.

They're touching each other.

Which of those arrows shows the overall direction of energy transfer between those two objects? So pause the video, make your decision and restart please.

Welcome back.

And the energy transfer should be from object X to object Y.

So overall energy transfer would be downwards between those two objects.

So that would be answer D.

Well done if you selected that.

So what that means is if you get a hot object and place it on a cool surface, then it's going to gradually cool down as it transfers energy to that surface.

So let's say I've got a surface and I'm going to keep that at a constant temperature of 20 degrees and I put a hot object on it.

So it's a hot metal block.

It's 60 degrees Celsius there.

And what's going to happen is there's going to be an energy transfer from the block to the surface.

So an energy transfer in that direction.

And that means that the energy in the block is going to be reduced over time and it's going to gradually cool.

So that block is going to reduce in temperature over time as energy is lost from it to its surroundings or the surface there.

So the block's going to continue to cool until it reaches the same temperature of the surface.

So it starts at 60 and it gradually cools.

So it goes down to 50.

Then energy is still being transferred to the surface because the block is hotter than the surface.

So it cools down to 40 and then it goes up to 30 and so on.

And eventually, it'll reach 20.

And at that point, there's no overall energy transfer anymore because they both reached the same temperature.

The surface and the block are both at 20 degrees Celsius.

If we try the opposite and put a cold metal block on a hot surface, we get an energy transfer in the opposite direction.

So we've got a surface here at 60 degrees and a block at 20.

Energy's going to be transferred to the block overall.

So there's going to be an overall energy transfer into the block and its temperature is going to gradually rise.

So it's going to go up to 30, 40, 50, and eventually, it reaches the same temperature as the surface, so again it's 60.

And at that point, there's no overall energy transfer anymore.

They've both reached the same temperature.

Let's see if you get that idea.

So I've got two metal blocks here shown in different colours and they're in direct contact with each other.

Which diagram correctly shows the direction of the overall energy transfer when those blocks are touching? So pause the video, make your selection and restart please.

Welcome back.

You should have selected answer C.

The energy is transferred from the hotter block to the cooler block and that's the one that shows that.

Well done if you selected that.

When you have two objects at the same temperature touching each other, they're in thermal equilibrium.

So I've got two objects again here.

Got object A, it's at 60 degrees Celsius.

And it's in direct contact with another object I've made touch.

So that's object B and that's at 60 as well.

And what's happening there is, there is energy transfer between the two blocks, but it's equal in opposite directions.

So object A is transferring energy to object B and object B is transferring energy to object A.

And they're at the same rate.

It's the same amount of energy transfer per second in each direction.

So there's overall zero energy transfer there.

Those objects are in thermal equilibrium because there's no overall energy transfer between them.

Okay, let's see if you understand thermal equilibrium.

I've got again a set of metal blocks here.

Which of those diagrams shows a set that's in thermal equilibrium? Pause the video, make your selection and restart please.

Welcome back.

Hopefully you selected the second of those.

Both of those blocks are at the same temperature, 200 degrees Celsius.

Hopefully you weren't fooled by the example C.

They're at very different temperatures, one at plus 40 and one at minus 40.

So well done if you selected B.

It's at the same temperature between metal X and metal Y.

If you place objects in a room, eventually they're going to reach thermal equilibrium with each other and with the room that they're in.

So if I've got two objects here and I place them both in a room and room temperature is 25 degrees Celsius.

I've got a cold block and I've got a hot block.

What's going to happen is for the cold block, it's overall going to gain energy from the room.

There's going to be a net energy transfer into that block, and its temperature is gradually going to increase towards room temperature.

And for the hotter object, well, that's going to lose energy to its surroundings.

It's going to be dissipated, and the energy is going to be transferred outwards from that block and that's going to cool down.

Its temperature is going to fall.

And eventually, over a long enough period of time, both of those objects will reach the room temperature of 25 degrees Celsius and they'll be in thermal equilibrium.

Let's see if you can apply that idea to a situation.

I've got a customer who buys a hot pie and a very cold drink from the shop.

What happens to those objects when they're left at their desk at work? So I'd like you to select three of those answers please.

So pause the video, select the three correct ones, and then restart.

Welcome back.

Hopefully you selected these answers.

The pie's going to cool down over a period of time.

It's going to lose energy to the surroundings.

It's going to give out energy and that's going to be dissipated to the surroundings.

So that was answer A.

And the drink's going to warm up over time.

It's going to absorb energy from the surroundings.

And the pie and the drink are eventually going to reach the same temperature, the room temperature of whatever office it is.

So well done if you selected those three.

So now it's time for the second task.

And what I've got here is a pupil leaving three cubes in a freezer and that freezer's got a temperature of minus 10 degrees Celsius and they left it overnight.

They then place them on a wooden table in a room that has a temperature of 25 degrees Celsius.

And what I'd like you to do is to answer those three questions about that scenario please.

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

Welcome back.

Well, the answers are here.

For the first one, explain why the wooden cube would feel warmer than the other two blocks when touched.

That's because wood is a poor thermal conductor, so the energy is going to be transferred more slowly from your hand to that block.

Explain why the temperature of the cubes gradually rises to a maximum of 25.

Well, that's because energy is being transferred from the environment to those blocks and that means that their temperature is gradually going to increase.

Eventually, it's going to reach 25 degrees Celsius.

And at that point, the energy lost from the blocks and the energy gained by them is going to be equal to each other and they balance out.

So there's no overall energy transfer.

They're in thermal equilibrium with their surroundings.

Well done if you got those answers.

And for the third part, what would've happened if you'd have placed them on a metal table instead of a wooden one? Well, metal's a better thermal conductor than a wooden table.

So it's going to speed up the rate of energy transfer.

So energy would be transferred to the cubes more quickly and that means they'd warm up more quickly to reach that same maximum temperature of 25 degrees Celsius.

Well done if you got that.

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

And we're going to use our knowledge of thermal conductors and insulators to describe how we can insulate houses and reduce heating costs.

So on a cold day, the inside of your house is going to be warmer than the surroundings outside, and that means that there's going to be energy transferred from the inside of the house through the materials of the house to the outside.

We're going to have energy losses through the walls.

We're going to have energy losses through the ceiling.

And there are losses through the windows and even energy loss through the floor.

And that means that we're going to have to replace that energy.

We're going to have to keep the house warm by using some sort of central heating system or fire or something.

We need to replace those energy losses to keep the temperature of the house inside constant.

So to keep the bills down, we want to reduce those energy losses as much as possible.

So we build houses from thermal insulators, and some materials are better than others as thermal insulators.

And we want to reduce the energy loss from the house.

So we've got the outside part of the house.

Most of the houses are made from brick in the United Kingdom.

So brick is a thermal insulator.

It helps reduce energy loss because energy is transferred very slowly through brick.

And if you want to reduce that loss even further, we can have two layers of brick, and we can put foam in between those layers of brick as well.

So because now we've got two layers of brick, it's twice as thick, the energy losses are going to be reduced.

There's going to be a slower transfer of energy through that.

And the foam itself helps to insulate.

That reduces energy loss as well.

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

I've got three different walls here.

Through which of these walls is the rate of energy loss going to be greatest? And I've got temperatures there as well.

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

Welcome back.

Hopefully you selected C.

That's the thinnest brick wall.

You can see it's thinner than the bricks used in part A.

So that's going to have the greatest rate of energy loss.

So well done if you selected that.

When you're heating a house, you're going to produce a lot of hot air and that hot air is going to float towards the top of the house.

It's going to be less dense than the cooler air.

So it's going to gradually rise to the top of the house, reaching your loft.

And that means that you're going to get a lot of energy loss through the top of your house potentially.

So we've got a roof made of tiles and they're made from poor conductors.

Poor thermal conductors are thermal insulators.

But they're typically much thinner than the brick walls because the roof has to be lighter than the bricks.

So we can't build a brick roof.

So we've got slates and other materials similar to that.

But there still could be quite a lot of energy loss because those materials are thin.

So we can put fluffy materials that trap air and prevent the hot air rising to the loft or through to the roof.

And that will reduce the amount of hot air that gets into the loft and that will reduce energy loss from the top of the house.

Of course, we want our houses to be light so we have windows in them and they allow light in.

And glass is a good thermal insulator.

Energy does not travel very quickly through glass.

It doesn't conduct very well at all.

But window glass is very thin.

So it's only a few millimetres thick in most places.

So energy would be transferred through that quite quickly, much more quickly than it would be through the thick walls.

So to reduce energy lost by the windows, we can have two layers of windows.

We can have double-glazed windows.

So what we've done is effectively doubled the thickness of the glass there.

And the air inside or between those two layers is also a good insulator and that prevents energy lost through the windows as well.

So we've got those two panes of glass with an air gap and that reduces energy loss through the windows quite substantially.

Energy can also be lost in other ways from a house.

So we could have energy loss through the bottom of the house through thermal conduction into the cooler ground beneath the house.

And one way of reducing that is to use thick carpets or underlay.

Those thick carpets will have trapped air and they'll prevent conduction through them.

They're very poor thermal conductors.

Another way we can lose energy is through draughts.

And draughts might be left through cracks beneath doors or around windows or something like that.

And there'll be convection currents and they'll transfer energy from the house.

So the warm air will escape from the house to the outside and we'll have to replace that warm air with a heater.

That energy loss can be reduced by having something like a draught excluder, some sort of foam or layer around those doors and windows.

Okay, let's check if you understood those ways of reducing energy loss.

Which of the following methods will not reduce energy lost from a house during winter? So pause the video, make your selection and restart please.

Welcome back.

Hopefully you selected the bottom one.

Setting the heating system to a higher temperature will actually increase energy loss.

There'll be a greater temperature difference between the inside and the outside of your house.

So all the other three would reduce your energy lost from the house.

So well done if you selected D.

Okay, time for the final task of the lesson.

And I've got a homeowner and they want to reduce the bills due to heating, and they live in an old house, over winter.

I want you to suggest three ways of reducing energy lost from their house and explain how each of them works.

So pause the video, make your descriptions and restart please.

And welcome back.

And here's my list.

I've got several there.

I'm not going to go through them all.

You should have just chose three and tried to describe.

So well done if you got three of those.

Okay, we're at the end of the lesson now and here's a quick summary.

If you heat a solid substance, its particles will vibrate more quickly and its temperature will increase.

Energy can be transferred by thermal conduction processes.

At higher temperatures, the particles vibrate more quickly and they interact with nearby particles, causing them to vibrate more quickly too.

And that process happens throughout the material, gradually causing it all to warm up.

Thermal conductors can transfer energy quickly and thermal insulators transfer energy much more slowly.

Metals are excellent thermal conductors because they've got those extra free electrons that can transfer energy quickly from one part of the metal to another part further away.

Objects at the same temperature as each other are in thermal equilibrium and there's no overall energy transfer between those objects.

And houses are insulated to reduce energy lost by conduction to the outside of the environment.

Well done for reaching the end of the lesson.

I'll see you in the next one.