Lesson video

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

This lesson's called Thermal Conductors.

And in it, we're going to be looking at different materials and how they transfer energy and how that's related to temperature.

By the end of this lesson, you're going to be able to explain the process of thermal conduction using the idea of vibrating particles within a material.

That transfer energy from one place to another.

Here are the keywords that will help you understand the lesson, and the first of them is thermal conduction.

And that's a process where particles pass their motion or the vibrations to each other by the forces and particle collisions.

The second is thermal conductor, and that's a substance in which thermal conduction can happen very quickly.

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

And forces of attraction are the forces that hold neighbouring particles together in solids and liquids, and those forces get weaker the further apart the particles are.

This lesson's in three parts.

And in the first part, we're going to look at what thermal conductors and thermal insulators are.

In the second part of the lesson, we're going to be comparing different thermal conductors to find out which one is the best and an experiment.

And in the third part of the lesson, we're going to be looking at how we can explain thermal conduction in terms of the forces between the particles inside the materials.

So, when you're ready, let's start by looking at what thermal conductors and thermal insulators are.

If you heat up a solid, then one part of material is gonna get hot very quickly, but it's gonna take some time for the other part of that solid material to heat up.

So, what I've got here is a metal rod, and what I'm going to do with it is heat up one part of it very strongly.

So, I've got a bump and burner position there and I've got a blue flame, so that part of the rod's gonna heat up very quickly 'cause I'm gonna be be providing it with a large amount of energy.

Along the rest of the length of the metal rod, I've placed some drawing pins and I've attached them with some candle wax, and that wax melts fairly easily when it gets warm.

So, I'm gonna heat it up and see what happens to those drawing pins as I heat up that end of the rod.

The part of the solid that's nearest of Bunsen burner flame is gonna heat up first.

So, the temperature of that region of the rod increases rapidly.

Nearby, the temperature of the rod increases as well.

It's gonna be a gradual increase in temperature along the length of the rod.

And the bit nearest at the Bunsen burner where I've attached the drawing pin, that's gonna heat up first, and the wax is going to melt, and that pin's gonna fall off.

A short time later, the next pin falls off.

Its temperature has increased so the wax has melted.

And finally, the furthest away pin's also gonna fall off.

So, what's happening is energy's being transferred along the road is a gradual temperature increase along the rod.

The closest parts heat up fastest and the further away parts heat up more slowly.

It takes some time for that wax to melt.

If the temperature along that rod is increasing very quickly, then we say that that material is a thermal conductor.

A thermal conductor is a material that transfers and then heat up very quickly.

So, I've got some examples of that.

We've got metals, and metals are very good thermal conductors.

They transfer energy along the length.

The temperature increases very rapidly as you heat them.

Opposing that we've got some materials that are poor thermal conductors, and things like plastic and paper and wood are poor thermal conductors.

Their temperature increases very slowly across their length if you heat one part of them.

Okay, a check for you now.

I'd like you to decide which of the following objects is made of a good thermal conductor.

So, have a look at the three diagrams, pause the video, make a selection, and then restart, please.

Welcome back.

Hopefully you set the metal fork.

Metals are good thermal conductors, plastics are poor thermal conductors, and wood is poor thermal conductor as well.

So, well done if you chose B, the metal fork.

So, we've described thermal conductors, but they're also things called thermal insulators, which are kind of the opposite.

If the temperature along the solid increases slowly, then that materials are thermal insulators.

So, good conductors are poor insulators, and good insulators are poor conductors.

So, for example, we've said that metals are good thermal conductors, but that means that they are poor thermal insulators.

Similarly, plastic, paper, and wood, they're good thermal insulators.

They don't allow energy to be transferred along in their land very quickly.

So, they're good thermal insulators, they're poor thermal conductors.

If we wrap a thermal insulator around a thermal conductor, it can prevent us from getting burned.

So, I've got an example here.

I've got a pan of tomato soup and I'm heating it strongly from below.

So, that pan is going to heat up very quickly.

It's gonna have a large temperature rise very quickly because it's a good thermal conductor.

So, a quick temperature increase for the metal parts of the pan, but I've got a wooden handle on this pan, and wood is a poor thermal conductor, it's a thermal insulator, and so it's going to have a slow temperature increase.

So, what will happen is when I turn on the flame, the pan gets hot very quickly, but the handle gets hot much more slowly.

So, it's safe to lift the pan by the handle because that wood isn't going to be too hot.

Another check for now.

Two pupils are discussing why a metal panel has a wooden handle.

Which of those pupils.

Are both of those pupils correct? Is is Izzy correct? Is Lucas correct? Or, both correct? So, pause the video, make your decision, and restart, please.

Okay, welcome back.

Hopefully you selected both are correct.

Izzy said, "It's because wood is a poor thermal conductor." And it is, wood is a poor thermal conductor.

And Lucas said, "It's because wood is a good thermal insulator." And that's true also 'cause a pore thermal conductor is a good thermal insulator.

So, well done if you selected C.

Okay, it's time for the first task now.

And what I've got is a series of sentences, and what I'd like you to do is to complete the ends of each of those sentences using the word insulator or conductor.

And, obviously, you're going to use each word more than once.

So, pause the video, read through the sentences, and fill in those gaps just using the word insulator or conductor, please, and then restart when you've completed them all.

Welcome back.

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

So, for the first one, Izzy is warming a soup.

She's warming soup in a pan.

All of it gets hot quickly because it is made of a good thermal conductor.

So, the whole panel gets hot quickly 'cause it's a good thermal conductor.

When she holds the handle of the pan, she doesn't burn herself.

This is because the handle is made of a good thermal insulator.

Izzy notices that the end of the wooden spoon she's using to stir the soup does not get hot.

And that's because wood is a poor thermal conductor.

Izzy eats the soup in a bowl by resting it on a newspaper on her knee.

This protects her knee because the paper's a good thermal insulator, again.

And finally, if she leaves her metal spoon in the soup, Izzy finds that it gets hot.

This is because metal is a good thermal conductor.

Well done if you got those.

Now, it's time to move on to the second part of the lesson.

And in it, you're going to carry out an experiment to try and compare different thermal conductors to see which one is the best.

Gases and liquids are generally poor thermal conductors.

They don't transfer energy very quickly.

The temperature doesn't increase across them very quickly at all.

That means they're good thermal insulators.

I've got two examples here.

I've got a candle.

And if I like that candle, then the nearby wax doesn't melt.

The wax towards the bottom of the candle doesn't melt.

Only the wax very close to the flame will melt.

And that's because air isn't allowing energy to be transferred to other parts of that candle at all.

It's a very poor thermal conductor.

Similarly, if I heat some water in a test tube like this, I heat the top part of the water, bottom part of the water doesn't get hot and the wax at the bottom outta place there won't melt because the top part of the water will get very hot 'cause of the buns and flame it.

It in fact, it boils, but water is a good thermal insulator and it doesn't transfer energy to the wax.

So, the wax doesn't melt.

I can rank substances based upon how well they transfer energy through them.

I can rank them by their thermal conductivity.

So, I've got some poor conductors, and poor conductors include plastics, paper, and wood, and they don't transfer energy very quickly at all.

And then something slightly better is glass.

There, glass will transfer energy fairly slowly through it, but it will do it.

So, we can categorise that as a reasonable thermal conductor.

Then, I've got good thermal conductors such as metals like steel and copper, and they'll transfer the energy very quickly.

And finally, I've got diamond, and diamond is the best thermal conductor of all.

So, if you've got any diamond, that will transfer energy and its temperature will rise very, very quickly.

So, to test thermal conductivity, what I'm going to do is heat some metal rods using a Bunsen flame and see which one of them gets hottest at the far end first.

So, what I've got here is the setup.

I've got Bunsen burner, and I'm gonna use it to heat some rods.

So, I've got four different metal rods here, and they may include things like copper or steel.

And what I'm gonna do is put a drawing pin on the end, attached with some candle wax, which will melt quickly if that end of the rod gets hot.

I'm gonna place it all on top of a tripod and heat the other end of the rod with a Bunsen burner.

And that Bunsen burner is gonna increase the temperature of that end of the rod very quickly.

I'm gonna be providing it with a lot of energy and it's temperature is going to increase rapidly.

And all of this is gonna be carried out on a heat-resistant mat to stop things getting too hot beneath it.

To make this a fair test, what I've got to use is the same flame on all of the rods.

And as you can see with my setup, I've got one Bunsen heating all four rods.

So, it's going to be exactly the same flame heating the full rods.

I've gotta make sure that the rods are the same length, so I'm sticking the drawing pins the same distance away from the flame.

Otherwise, that wouldn't be fair if one rod was longer than the others.

And I've gotta use the same size rods.

And what I mean by that is the sort of same thickness or diameter of rods.

'Cause again, a thicker or thin rod might affect the results.

And so I've got all four of the rods touching the flame at the end, and I've got the drawing pins with the wax attaching them at the other end, and they're all the same distance away from that flame.

Let's check if you understand that idea.

When comparing metals to see which is a better thermal conductor, which part of the rods should be touching to make it a fair test? Is it A, the whole length of the rods, B, the ends of the rods where the drawing pins are, or C, the ends of the rods that are heated by the flame? Pause the video, make a decision, and then restart, please.

Welcome back.

Hopefully you selected C, the ends of the rods heated by the flame.

Well done if you've got that.

If you're carrying out an experiment like this, and then it's best to try and take some repeat measurements and find a mean average for times taken.

So, we are going to be measuring the time it takes for the pins to fall off.

And we've got a rod here that's zinc.

So, what I've done here is I've carried out the experiment three times, and I've measured the time it takes for the pin to fall.

And the first one was 40 seconds, and the second one, 44 seconds, the third one, 42 seconds.

And what I need to do is to add those things together and then divide by three to get a mean.

So, what I do is I add those three values 40, 44, and 42.

That gives a total of 126.

And then, I'm gonna divide by the number of values.

And there were three separate values there.

So, we divide the 126 by 3, and that gives me a mean value of 42 seconds.

Okay, let's see if you can calculate the mean value.

I've got a table here, and you can see I've got my zinc results there.

What I've got lead results as well.

And what I'd like you to do is to calculate the meantime for the lead rod, please.

So, in the results table below, what numbers should go in that empty box? Pause the video, make a selection, and restart, please.

Welcome back.

Hopefully you selected B, 50.

If you add the values together, 50 plus 49 plus 51 is 150, then divide by 3, 'cause it's three values, that gives 50.

Well done if you've got that.

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

I'd like you to use the apparatus below to compare some thermal conductors.

I'd like you to set up the equipment as shown in the diagram and you can see all the labels there.

We've got steel, aluminium, copper, and brass rods, and we've got the drawing pins on the end, heat resistant mat, tripod, and Bunsen.

I want you to start a timer when you start heating the rods and record the time it takes for the pin to fall off at the end of each rod.

So, make sure you observe those rods carefully and repeat two more times to find a mean average.

So, pause the video, carry out that experiment, and then restart when you're done.

Welcome back.

Hopefully your experiment went well.

These are my sets of results.

And as you can see, I've got different rods, and I've calculated the meantime for each.

And you can see, all of the smallest meantime is the copper rod.

That was only 30 seconds.

And the longest meantime was the steel rod, with 45 seconds.

Your results might be different than this, depending on which materials you used.

So, the best conductor would've been the copper in my results because that's got the smallest meantime.

And now, it's time to move on to the final part of the lesson.

And in it, we're going to describe the forces acting between particles and how that relates to thermal conduction.

So, let's go on with that.

So, in a solid, the particles are held in position by the forces that are acting on them.

And so in a solid, the particles don't move around too far, they just vibrating position because there's forces holding together quite tightly.

You can see I've got a model here of a vibrating particle, and it's just oscillating back and forth left and right.

There's kind of swinging.

So, I'm using a a ball on the end of a piece of string to represent particle and as you can see, my particle diagram on the left, I've got a particle there and I've got other particles surrounding it.

And between each of those particles are quite strong forces of attraction holding those particles in place, so they can't move around too freely.

All they can do is vibrate around a sort of fixed point in the centre there.

When you heat a solid, the particles in it vibrate more quickly.

And I've got an example of that process here.

I've got a solid and I'm gonna heat it with a candle flame.

So, if you look at the leftmost diagram first, I've got a flame and I'm placing that beneath the solid and the particles in the flame are at very, very high temperature and they're moving very, very quickly.

So, what's going to happen is those very fast moving particles are gonna collide with the vibrating particles in the solid, and the particles in the solid are then going to vibrate more.

So, you can see that in my sort of model animation here.

I've got a yellow ball moving in at high speed, and it collides with the vibrating particle in the solid, and it makes it vibrate about more.

So, it's temperature has increased, it's vibrating more, it's moving more rapidly.

It's still in a fixed place but it vibrating more.

So, I can improve my model slightly by showing you four particles here.

So, I've got four particles in the solid, and I'm representing the forces between them by the strings.

So, you can see in my little animation, I've got four particles attached together by those strings.

And when the gas particle, the particle moving at high speed collide with them, you can see all four of the particles starts to move or vibrate more because there're attached together.

The forces between them cause all of the particles to vibrate when one of them is hit.

Obviously, my diagram, I've got exaggerated spaces between the particles, but one particle, a high temperature, colliding with my solid will cause all of the particles to gradually start to vibrate.

That process will happen even if we've got many, many particles.

So, I've got here a a piece of metal, and I'm heating one end of it, and I'm representing the particles that are vibrating more quickly by putting yellow circles around them.

But what's gonna happen is when I hit one end, the particles are gonna start to vibrate, and they'll cause particles nearby to vibrate, which will cause other particles nearby to vibrate.

And gradually, those vibrations will be passed along the full length of the solid.

So, something like this.

So, the whole solid is gradually going to warm up because the vibrations are passed along.

And eventually, the temperature at the far end of the metal is gonna increase enough to melt the wax and the pin's gonna fall off, something like that.

So, I've described to you thermal conduction.

Which of the following statements best describes thermal conduction in solids? So, pause the video, read through those three options, select the correct one, and restart, please.

Welcome back.

Hopefully you select A, particles passing on vibrations.

I describe processes involving vibrations being passed on from particle to particle.

Well done if you got that.

The stronger the forces of attraction in the solid, the easier it is for those particles to pass vibrations between each other.

You saw earlier that diamond was the best thermal conductor, and that's because it's got the strongest forces between its particles.

So, in my table here, we had plastics, paper, and wood.

They've got the weakest forces between the particles, so they're poor thermal conductors.

And diamond is the best.

It's got the strongest forces.

So far, we've only talked about solids and how they perform thermal conduction.

The particles in the liquid are a bit further apart than the particles in the solid, and that means they're generally poorer thermal conductors.

They can't pass those vibrations on as easily.

So, I've got a sort of model of a liquid here.

And as you can see, the particles are a little bit further apart than the particles in a solid.

And so the vibrations can't be passed on as quickly.

The forces of attraction are weaker.

And so they're poorer thermal conductors than solids in general.

In a gas, we've got a different situation.

Again, there are no forces of attraction between the gas particles, and gases are very poor thermal conductors.

In fact, they can only transfer energy between particles when the particles actually directly collide with each other.

Something like this, or got particle, moving in, it collides with the other one.

And only at that point where they collide when they be able to pass on vibrations or pass on energy through collisions.

So, let's go back a bit and see if you remember why is diamond the best thermal conductor? So, pause the video, make your selection from those three options, and restart, please.

Welcome back.

Hopefully you selected the first option.

It's got very strong forces of attraction between its particles.

And the strength of those forces is what allows it to be a good thermal conductor.

Well done if you select A.

Okay, it's time for the final task of the lesson, and it's this, Izzy set up model of particles in a solid by hanging two balls on a string like that.

So, you can see there's two balls represented by the blue circles.

They're hung from string and they're quite far apart along the metal rod.

And I'd like you to answer the three questions about that model, please.

So, pause the video, read through those three questions and try and answer them, and then restart when you're done.

Welcome back.

Hopefully your answers look something like this.

So, for the first one, like particles in a solid, the balls can vibrate but they can't move past each other.

They're kind of held in fixed positions by the string there.

For the second question, the balls can pass on vibrations.

She needs to connect the string between those two because they're too far apart and they're not influencing each other.

She's not modelled the forces between those two particles there.

And for the last one, if they want, she wanted to try and model a gas, then she would have to remove the forces.

So, she'd have to remove the string that's holding them in position.

And the only thing you could do then to cause energy transfer between them is to roll the balls towards each other so they'll collide with each other.

So, well done if you've got answers, something like that.

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

Heating a solid causes its particles to vibrate more quickly, and those vibrations are passed on to neighbouring particles by the forces of attraction between those particles.

That process is called thermal conduction.

And the temperature increases as the vibrations are passed on from the hot part of the solid to the colder part of the solid.

Solid metals are good thermal conductors and they're poor thermal insulators.

Wood and plastic are poor thermal conductors and they're good thermal insulators.

Liquids are poor thermal conductors 'cause they have weaker forces of attraction between particles, so the vibrations are not passed on as easily.

And gases are very poor thermal conductors because there are no forces of attraction between the particles.

Well done for reaching the end of the lesson.

I'll see you in the next one.