Lesson video

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

This lesson is called, "Changes of Pressure" and in it, we'll be looking at the factors that affect the pressure of a gas.

By the end of this lesson, you're going to be able to explain how the temperature of a gas affects the pressure.

So by increasing or decreasing the temperature, what does that do to the pressure? Here are the keywords that will help you understand the lesson, and there's only two here.

Particle model, and that's the scientific model that we use to describe the behaviour of matter based upon the idea of tiny particles moving about and pressure, and pressure is caused by a force acting over an area.

And we're going to be calculating some of that using pressure equals force divided by area.

This lesson's in just two parts and in the first part we're going to be looking in more detail on the particle model of pressure and how that changes when you increase the temperature of a material.

And in the second part we're going to be looking at tyre pressure as an example of when the pressure changes inside a gas.

So when you're ready, let's begin by looking at the particle model of pressure.

Let's start with a quick recap of the particle model of fluids and the particles inside of fluid are in constant random motion.

What that means, is that the particles are moving very fast in random directions colliding with each other and the container that they're in.

So here's a sort of figure of that and you can imagine those as air molecules or air particles moving around quite fast.

They collide with each other and the surfaces that they come into contact with.

And those collisions produce very small forces that act over an area and produce pressure.

So there's billions of collisions per millimetre in a container every second.

If you imagine a balloon, that's going to contain many billions of particles of gas.

So I've got a balloon here, I'm just gonna put a few of the particles in there.

So there's a few of the particles and they're moving around in random directions at very high speeds and they're gonna be colliding with the inner surface of the balloon.

And each collision's gonna produce a small outwards force on that balloon and that outward force is acting over the inner surface of the balloon and it's producing a pressure.

So there's a pressure on the inner surface of the balloon, because of the gas particles inside it.

Now, the air around the balloon also contains many gas particles and they're moving around randomly as well.

So I've put a few of those on here and they're going to be colliding with the outer surface of the balloon.

And each time they collide, they produce a tiny force and that's going to create a pressure, because that force is gonna be acting over the area of the balloon, so I've got inwards pressure as well.

So when I've inflated the balloon, what I get is the inwards and the outwards pressures are balancing each other and the balloon doesn't expand or contract.

If I added more particles to the balloon, then the balloon would expand, and that's because of this.

If you add more air particles into a sealed container, you're going to have more gas inside.

So I've got a small mass of gas inside the container here and then I can add more gas to it.

So I've added some more gas particles.

And what that's going to do is there's going to be more collisions of those moving gas particles with the container walls, more collisions every second and more collisions every second means there's going to be an overall outward force increase.

More of those collisions happening everywhere, so I've got sort of double the collisions there.

There's going to be a greater outward force overall and that means there's going to be a greater outward pressure acting on it.

So adding more gas to the container is going to increase the pressure that gas is producing on the container.

Going back to the balloon example, I've got an inflated balloon here, and overall the outward forces and the inward forces balance each other.

So the balloon doesn't expand or contract, but adding more air is going to increase the number of particles inside the balloon and that's going to increase their outwards pressure.

There's gonna be more collisions of those particles with the inner surface of the balloon.

So what I get is this situation where there's a greater outwards force overall, a greater pressure pushing outwards and that's going to cause the balloon to expand.

It's gonna stretch the rubber of the balloon, so the balloons gonna become larger and as it becomes larger, then it's going to reduce the number of collisions per square millimetre on the inner surface of the balloon that's happening.

So eventually, the balloon stretches to a point where the outward forces and the inward forces are balanced again, the pressures are balanced and the balloon stops increasing its size.

So it stops expanding when we've got equalised pressures.

Let's check if you've understood the basics of that.

I've got three containers here.

All of them are the same volume and they're all at the same temperature.

So in which one does the air exert the highest pressure on the container surface? And what the difference is if they've got different masses of air inside.

So I'd like you to pause the video, make your decision, and then restart please.

Welcome back, hopefully you selected C.

That one contains one kilogramme of air or 1000 grammes of air and that means there's gonna be more air particles inside there, many more than the other two containers.

Those particles are gonna be colliding with the surface of the container on the inside and that's gonna create the greater pressure, because there's gonna be more collisions every second.

So well done if you selected that.

When you heat a gas, the average speed of the particles in that gas are going to increase.

So heating any gas will make those particles move faster on average.

So that's gonna have any effect on the pressure, because those particles are gonna be moving faster and hit the container walls with a greater force, and that's gonna increase the pressure.

So if I've got a gas at low temperature inside the container, those particles are moving quite slowly and they hit the container walls, then they are gonna produce forces, but those forces are going to be smaller.

Then if I get another gas and increase the temperature, those particles are gonna be moving faster.

So the faster particles are gonna hit the wall harder and that's going to create bigger forces, and the effect of that is that you're going to get more collisions and more force and that's going to increase the pressure.

So the hotter the gas is inside the container, the more collisions there are per second, the harder the collisions are and therefore a greater pressure is produced by the hotter gas.

Let's check your understanding of that.

I've got three containers again here.

All of the containers have the same volume and they contain the same mass of gas.

So we've got an identical number of particles in each one.

In which one of those does the gas exert the highest pressure on a container surface? Pause the video, make your selection and restart.

Welcome back, well hopefully you selected option C there, the temperature of 30 degrees and that's the correct answer, because that's the highest temperature.

The particles are going be moving fastest in that, more collisions per second with the container walls and therefore a greater pressure.

You might have been fooled by option A, but remember, that's a minus 60 degrees Celsius and that's a cold temperature.

So those particles are moving slowest.

Well done if you picked C.

Another factor that will affect the pressure in the gas is the size of the container that the gas is contained, in it.

If you decrease the volume of the container, you're going to increase the pressure inside.

So imagine I've got some gas in a container here and they're in a large volume and they're going to be colliding with the sides and the surfaces, but they're going to do that less regularly, because they've got quite a large distance to travel once they've collided with one surface, bounce off it and hit a second surface.

So because the distances are quite large, the number of collisions per second is quite low.

In a smaller container, I've got much smaller distances, so the particles have a shorter distance to move before colliding, so they're going to be colliding with the container walls more frequently.

We're gonna get more collisions per second for the gas when it's in a small container than when it was in a large container, and that means there's more collisions per centimetre squared of the container every second and that gives a higher pressure.

So the smaller the container is, the higher the pressure if you've got an equal amount of gas in it.

And another check to see if you've understood that.

I've got some containers here and they all contain the same mass of gas at the same temperature, but they've got different volumes.

In which one is the gas exerting the highest pressure on the container? So I'd like you to pause the video, make your selection and restart please.

Welcome back, hopefully you selected option B.

That's got the smallest volume.

It's going to have the most collisions per square centimetre per second, and so it's gonna have the highest pressure.

Well done if you selected that.

And now it's time for the first task of the lesson, and I've got two questions for you here.

I'd like you to use the particle model to explain why each of the following cause a change of pressure inside the balloon.

So why does increasing the air temperature change the pressure? Why does pumping more air into the balloon increase the pressure and why does squashing the balloon, as shown in the figure there, increase the pressure inside that balloon? And in the second one, I'd like you to explain why a balloon expands when you add more air to it.

Okay, so pause the video, write out your answers and then restart please.

Welcome back, well here's the three factors and their effects.

If you increase the air temperature, the particles are moving faster, they'll collide with the balloon's inner surface more often and with a greater force and that will increase the pressure.

If you put more air into the balloon, there's more air particles, so there's going to be more collisions with the balloon's surface every second.

So more particles mean more collisions and finally, squashing the balloon decreases the volume of the balloon, so the particles travel less distance between the collisions, so there's more collisions with the balloon's inner surface every second.

Well done if you got those.

And here's your explanation of why adding more air to a balloon causes it to expand.

So we've got several points to mention there.

You're putting more air in it and so you're getting more particle collisions on the inner surface every second, the outwards pressure increases and that causes the rubber of the balloon to stretch and expand as the balloon's volume increases, that then increases the surface area and that decreases the number of particles hitting that surface per square centimetre per second, and so the pressure's going to drop and eventually, the balloon's going to stop expanding when the pressures are equal again.

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

Now it's time to move on to the second part of the lesson and in it, we're going to look at an example of pressure and the gas by looking at the pressure inside tyres.

The structure to a modern tyre is actually quite complex, but I'm not going to go into the details of it.

There's many different layers that are there to give strength and grip on the road.

So I've got a tyre, something like this, and as you can see, we've got rubber treads.

Their the thing that give grip on the road's surface, allowing the car to grip it tightly and speed up and slow down when you're braking, and also when you're steering you need grip.

So you need to produce forces against the road to allow you to change directions.

We're gotta concentrate on the inner part of the tyre and the inner part of the tyre is filled with pressurised air and that air is there to keep the tyre in a certain shape to allow it to grip the road properly.

I'm gonna use a simplified diagram for a tyre, something like this, where I've got the tread on the outside and I've got pressurised air on the inside.

You have to maintain tyres properly and one of the things you need to do, is to keep the pressure inside the tyre at the correct level.

If the pressure's too high or too low, then the tyre isn't going to be working properly.

We call a tyre that's got too high an air pressure inside is overinflated.

So if I've got a correctly inflated tyre, it will look something like this.

So you've got a gripping surface and you've got a bit of a curve, but if I overinflate the tyre, then it's going to distort the shape of that tyre and that creates too greater curve and you've got a smaller contact with the road.

So the overinflated tyre has got a smaller contact area with the road surface and that's gonna give you less grip when you're driving.

So if you drive with overinflated tyres, you're not gonna have the same amount of braking possibility, you are not gonna be able to accelerate as much and you're not going to be able to steer as much, 'cause you're not gripping the road.

So it's gonna be harder to control a car with a tyre that's overinflated.

There's also problems if your tyre is under-inflated.

If the air pressure is too low and tyre is under-inflated, so we've got a correctly inflated tyre again, under-inflated tyres are going to sag into the road and have a greater contact area, but the tyre itself is going to be too flexible.

It's going to be not stiff enough and that's going to cause the tyre to wear out more quickly as it rubs against the road and it's going to damage the tyre over time.

So you're going to have a damaged tyre much more quickly if you've actually under-inflated it.

And that becomes dangerous, because if the tyre ruptures, then the tyre will be, we say the tire's blown, and that means that all of a sudden you've got no grip on that part and you can crash.

So a under-inflated tyre can basically blow and rupture and you can have a crash because you've not inflated it properly.

Okay, let's check your understanding of tyres.

Which of those images represents a tyre with the lowest internal pressure? So which one has got the lowest pressure inside it? Pause the video, make your selection and restart please.

Welcome back, well, it's option C.

That one is under-inflated.

It's not got enough air and the air pressure is too low.

You've got an overinflated tyre for B and a correctly inflated tyre is picture A.

Well done if you selected C.

As I've mentioned, you need to regularly check the tyre pressures in order to make sure that they match these pressures that the manufacturers have suggested.

If you've got a typical car tyre, their pressure may be 220 kilonewtons per metre squared, so that's a typical car tyre.

However, if you're driving a lorry, you might will need to inflate those tyres to a pressure of 800 kilometres per metre squared, so nearly four times as great for the lorry.

And the lorry needs that because the lorry's much heavier and the tyre needs to support a much greater weight.

So you've got a tyre with higher pressure so it can support a greater weight of lorry.

You've also then got to have thicker material to make the tyres from so they don't rupture, so lorry tyres are much thicker than car tyres.

When you're inflating a tyre, you've got to be aware of the temperature.

Gas pressure increases with temperature and if you're driving around, so let's say you you wanted to drive to your garage and inflate tyres and you drove around for a while before that, then as you're driving around, the tire's going to rub against the ground.

The frictional forces that you use to make the car move are going to cause a heating effect and that heating effect's gonna warm up the tyre and it's going to increase the temperature of the air inside the tyre.

So when you start off, the temperature is quite low inside the tyre, but as you drive more and more, the temperature increases and that's gonna affect the pressure of the tyre.

So if I've got a cool tyre, then the pressure is going to be lower than a hot tyre after I've been driving for a while.

So it's always important that you check the pressure of your tyres when they're cool, because the manufacturers state the pressure they should be when the tyres are actually cool.

If you add air to the tyre when it's hot, then you are actually not going to be adding enough air, because the pressure's already too high inside the tyre.

Let's see if you understand a bit about tyres.

Which of these tyres is supporting a vehicle with the greatest weight? So I've got three different tyres there and they've all got different pressures inside them.

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

And welcome back, the answer to that one was C.

The pressure's greatest, so if you wanna support a greater weight, you need a greater pressure.

So that tyre, they've got the highest pressure and it's also got the thickest rubber on it as well.

Well done if you spotted that.

Okay, it's time for the final check of the lesson, and they're both about tyre pressure obviously.

So the first question is, a driver correctly fills their tyres with air on a warm autumn day.

I'd like you to use the particle model for pressure to explain why the tyre pressure is likely to be too low if a period of very cold weather follows that warm day.

And the second one, suggest why an incorrectly inflated lorry tyre can be very dangerous.

So pause the video, work out the answers to those two questions and restart please.

And welcome back, and here's the answer to the first.

And when the temperature is lower, then the particles in the air inside the tyre are gonna be moving slowly and causing fewer collisions to the tyre wall, and that's gonna reduce the pressure, meaning that the tire's gonna be under under-inflated when cold weather arrives.

So you'll be driving around with under-inflated tyres.

Well done if you've got that.

And here's the answer to the second one.

Basically, the tyre could wear out too quickly.

It can be more easily damaged and it could even explode.

If that happens, then the lorry may lose control.

You'll have pieces of tyre flying off, they may damage other vehicles.

You might see exploded tyres on the edge of motorways.

There's bits of rubber around there, because this does happen fairly regularly.

If you overinflate the tyre, then the grip is going to be too low and so the lorry might not be able to control itself properly.

It's going to be harder to control.

So well done if you've got those answers.

And we've reached the end of the lesson.

So here's a quick summary of everything we've covered.

The pressure of a gas in a container can be increased by doing these three things, increasing the mass of gas in the container, increasing the temperature of that gas in that container, or reducing the volume of the gas by compressing it, making the container smaller.

And we can use the particle model to explain those changes in pressure.

Car tyres need to be inflated to the correct pressure to be safe and work effectively.

Overinflated and under-inflated tyres can be dangerous.

Well done for reaching the end of the lesson.

I'll see you in the next one.