Lesson video

Hello there, I'm Mr. Forbes and welcome to this lesson from particle explanations of density and pressure unit.

This lesson's called pressure in a fluid and in it, we're going to be looking at how the particles in a fluid cause that pressure.

By the end of this lesson, you're going to be able to explain what causes pressure in the fluid in terms of the movement of the particles within that fluid.

You're also going to be able to explain how that then causes pressure on a surface.

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

The first of them is atom, which is the building block of all matter.

In small pieces of matter have many trillions of atoms within them.

And particle, which is the general word we use to describe atoms or molecules that make up matter.

Third is fluid and that's a liquid or a gas.

And we call it fluids because they can flow.

So fluids have particles that can flow past each other.

And finally, pressure.

And that's caused by a force acting over an area on a surface.

And we can calculate it using the equation, pressure equals force divided by area.

You can return to this slide at any point during the lesson to help you with the keywords again.

The lesson's in three parts.

And in the first part, I'm going to explain what a fluid is in terms of the particles and how they behave.

In the second part of the lesson, I'm going to go on to describe how that movement of the particles causes pressure within the fluid.

And in the final part of the lesson, we'll see how all those particles moving around cause a pressure on a surface and how we can calculate that pressure.

So when you're ready, let's start by describing what fluids are.

Matter is made up of very small particles we call atoms. So all of the objects you see around you are composed at the basically level of atoms joined together in different combinations and arrangements.

The different types of atoms can be arranged in joining lots and lots of different ways.

Atoms themselves can mean different sizes depending on what element they're from.

Some are larger and more massive, they've got more matter in them than others.

In simple diagrams that we use throughout this lesson, we are going to be drawing particles or atoms as small circles like this and the size of those can vary.

We've got some small atoms and some larger ones.

An individual atom is incredibly small, far too small to ever see.

Even the tiniest volumes of matter contains many thousands of trillions of atoms. So if I've got a one millimetre cube of air, so a really tiny amount of air, I've got five times 10 to the 16 atoms. So one cubic millimetre of air contains over that many particles, 10,000,000,000,000,000 particles.

So it's a huge number even in a very small volume of air there.

If I move on to a liquid, that's gonna contain even more atoms in that small volume.

Over 2,000 times as many particles in a cubic millimetre of water in compared to air.

So a first, check for here.

Which of these will contend the least number of particles, one cubic metre of a solid, one cubic metre of a liquid, or one cubic metre of a gas? Pause the video, make a decision and restart please.

And welcome back.

Hopefully, you selected one cubic metre of a gas.

Gases have the least number of particles per unit volume or per cubic metre.

So well done if you selected that.

As you should already know, there are three states of matter, solid, liquids, and gases.

So we've got three different states.

I'm gonna start by looking at a solid.

And in a solid, I've got particles packed closely together in some sort of regular arrangement for this one.

There's no gaps between those particles.

So a solid can't be compressed.

You can't change the size of the particles themselves in a material, only the spacing between them.

And because a solid is already having the particles very close together, you can't compress it further.

The forces between all of those particles hold the solid in a fixed shape.

It doesn't flow around.

So solids have fixed shapes.

And the individual particles aren't stationary within a solid.

They vibrate a very short distances around those fixed positions.

So I've indicated that vibration with those small arrows there.

So those atoms would be sort of shaking about very, very slightly within those fixed positions.

Liquids have particles are closely packed together just like solids.

And it's also difficult to compress a liquid.

There's very little space between the particles, but the particles have a looser arrangement.

They're not in fixed positions.

In fact, the forces between them are weaker and that allows the particles to flow around past each other.

They're allowed to move.

So liquids don't have a fixed shape.

They can flow.

If I put a liquid into a container, it will flow to the bottom of the container and just fill the bottom part of that container.

The particles are moving around quite quickly, but they bump into each other a lot so they don't get very far.

So even though they're travelling at up to 1,000 metres per second.

In effect, they can only move a few millimetres per second before colliding with something and changing direction.

And a third state of matter is a gas, and the particles in a gas are very far apart from each other.

So if I tried to draw a gas, I could do something like this.

But actually the particles are much, much further apart than I'll be showing in this diagram here.

The forces between the particles are very small or even non-existent.

They can move very freely from one place to another.

And that allows a gas to flow in random directions very, very quickly.

It doesn't have a fixed shape.

And in fact, if I pour gas into a container, it'll rapidly fill the entirety of that container so it's flowing in every direction.

Those particles, again, are travelling very, very quickly indeed, but they collide with each other much less so they can go a further distance much faster.

So as you've seen, liquids and gases are both able to move around or flow, and they're both called fluids.

A fluid is something that can flow because the particles can rearrange themselves throughout it.

So something like milk, that's a fluid, it's a liquid and a fluid.

It can be poured from bottle to a glass.

But something like toffee like this, it's solid.

It doesn't flow.

If you tried to pour that whole container, just chunks of it would fall out.

So fluids can flow.

A check for you here.

Which of these are fluids at room temperature please? So pause the video, make your decisions, and then restart when you're done.

Welcome back.

We're going through them in turn.

Olive oil can be poured.

It's a fluid.

You can pour that out container, but wood isn't.

So that's not a fluid.

Air is a fluid.

It flows around at room temperature.

And diamonds are not fluid at room temperature.

They are a fixed shape.

Mercury is a fluid, it's a liquid at room temperature.

Carbon dioxide is a gas at room temperature and that can flow.

Iron at room temperature is a hard solid, and honey, well that flows quite slowly, but it still flows, so it still counts as a fluid.

Well done if you got those.

Okay, now sample for the first task.

So what I'd like you to do is to take a single A4 sheet of paper and summarise the properties of solids, liquids, and gases.

And there's a list of what your summary should include.

The description of the structure, the properties of each state of matter, an explanation of how those properties are linked to the particle behaviour, and a definition of a fluid.

So pause the video, complete the sheet of paper, and then restart when you're done, please.

Welcome back.

Well, hopefully, your summary should include this sort of information.

So I've got descriptions of what matters made of and some information about the number of them.

And then I have information about solids, liquids, and gases, and how the particles are arranged and how they move around.

And then a definition of fluids, liquids, and gases.

Fluids, the particles are free to move so they can flow.

So well done if you've got a summary something like that.

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

And in it, we're going to be looking at fluid pressure, pressure caused by a fluid within itself.

And explain that in terms of what the particles in that fluid are doing.

So let's do that.

The cause of the pressure within a fluid is the particles interacting with each other and putting forces on each other when they collide.

So if look at gas particles here, we've got particles that are quite far apart and we've got particles a bit closer together when that gas is compressed a little, but they're still moving around randomly and they'll collide with each other.

When the particles are closer together, they're going to collide with each other more often.

And it's those collisions between things that produce forces and those forces produce the pressure.

So when the particles are closer together, there's going to be a higher pressure because they're gonna collide more often, and more collisions mean a greater pressure.

You can see that effect with a gas when you've got a sealed syringe.

So I've got a syringe here and I've sealed the end.

I've melted the plastic or put some sort of seal on it and I've got some gas trapped inside it.

The gas is compressed when I push that plunger inwards like that.

So if I try to squash the gas, I'm gonna compress the gas, and that's going to push the gas particles closer together.

So they're gonna get closer together and collide with each other more often.

Because they're in a smaller space, that gas pressure is then gonna increase because there's gonna be more particle collisions with each other.

So I'm gonna increase the pressure when I push the plunger in.

It becomes harder and harder to actually compress the gas that I push it in there and because the pressure builds up, so I have to push hard on that plunger to compress the gas when it gets smaller.

And you can see that with this simple demonstration where I'm trying to squash the gas, and as I squash it, it becomes harder and harder to push on that.

If we understand how the particles within air substance are behaving, then we can explain why the pressure increases.

So if I've got particles that are far apart here inside that syringe, and you can see all those particles are drawn, those small dots there, I'm gonna get few collisions between them.

So those particles, because they're very spaced out, aren't gonna collide with each other as much, and we're going to get a low pressure because there's a small number of collisions.

But as I squash that slightly, I'm gonna force those particles into a smaller volume and they're gonna hit each other more often and more collisions is gonna create a higher pressure.

'cause the pressure is actually caused by those particles collision.

If I push it further again, they're gonna be quite tightly packed.

There's gonna be more collisions again.

I've got the most collisions now, so I'm gonna get the highest pressure when I've compressed that gas.

So the particles in the gas are moving very rapidly about, they're spread out evenly throughout the gas and they're gonna be moving very, very quickly.

They're the same number of collisions in all different directions within the gas.

So the pressure is the same everywhere.

The pressure within a gas is constant when it's inside a container or I should say it's the same everywhere inside the container.

But if I reduce the size of that container and compress the gas, then I'm gonna get more collisions, but there's gonna be more collisions everywhere, and that means the pressure has increased everywhere in gas.

So the pressure is higher, but it's still equal everywhere throughout the gas.

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

I've got the same number of gas particles inside each of these three containers.

In which container is the pressure going to be greatest? So pause the video, make your decision, and restart please.

Welcome back.

Well, the answer to that was the middle one.

That's got the smallest volume, therefore it's gonna have the highest pressure.

So the smaller the volume, the higher the pressure for a fixed amount of gas.

Now if we consider a liquid instead of a gas, the pressure inside that liquid is due to the weight of the liquid above a certain point within it.

So the pressure in open water, so we're deep under water here, is caused by the weight of the water acting above a specific point.

We've got all of this water pushing down on the diver and that's what's producing the high pressure on them.

Water's a dense material, so we get much higher pressure under the water than we get underneath a column of gas, for example, like the atmosphere.

So we've got a very high pressure acting when you're deep on the water.

But the water is also a fluid.

The particles are moving around.

So just like a gas, the pressure at a particular depth is the same everywhere.

So the pressure all over that diver would be the same on the head though.

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

I've got a diver hiding from a shark in a cave and the diver and the shark are at the same depth within water and the diver's inside the cave.

I've got two points marked X and Y and they're the same depth.

So which statement about those points is correct? So pause the video, make your selection, and restart please.

Welcome back.

Hopefully, you selected C.

The pressure X and Y is the same in the same depth.

It doesn't matter that the diver is inside the cave, it's just the depth on the water that's important here.

Well done if you selected that.

Okay, let's see if you understood why the explanation about pressure is the same.

So which of the following statements explains why the pressure is the same point X and Y? So pause the video, make a selection from those four options, and restart please.

Welcome back.

The answer is B.

The pressure depends only on the depth beneath the water's surface.

And they're both the same depth though X and Y.

Well done if you selected that.

Now let's try and explain why the pressure in a liquid acts equally in all directions by thinking about the individual particles.

All of the particles are moving randomly in all different directions.

So they're all moving past each other in different directions, constantly colliding with each other and changing directions as well.

Now the force on one particle is always going be equal and opposite to the force on another particle.

This was Newton's third law of motion if you remember that from other lessons.

So if we look at an individual particle, let's say this particle A here, that's going to put a force on the particle adjacent to it, it's next to it in that direction.

And by Newton's law, we've got a force in the other direction as well.

So those forces are equal and opposite.

And if we look at another particle, let's say this particle here, particle C, that's put a downwards force in the particle beneath it and the particle beneath it, which we'll call particle D, is gonna produce an upwards force.

So on average, all the forces and all of the particles are going to be the same in all directions because throughout those constant collisions, there's gonna be many forces and many directions and they'll all even out to give forces between the particles that they have equal in all directions.

A liquid is not compressible, you can't squash a liquid very easily at all because the particles are already very close together.

So if I put a liquid inside a syringe and try and squash it, applying a force will apply a pressure throughout the liquid, but it will not reduce the volume liquid.

I'm not gonna be able to compress it by very much at all.

So I've got an example where I try to squash a liquid in a syringe and it's not going to compress in the least.

So the forces between all the particles in liquid increase, I'm gonna increase the pressure inside the liquid, but I'm not going to compress it and reduce its volume.

What that means is the pressure in open water is the same everywhere at equal depths.

So I've got a diagram here of some open water with some fish in it, and I'm gonna have low pressure near the surface of the water and high pressure deeper within the water.

And that's because if you look at that purple dash line and the two fish there, it's gonna be the same weight of water above them pushing downwards to produce the pressure.

While at the deeper fish, there's gonna be a greater weight to water above them that's gonna produce a higher pressure acting on that fish.

So the pressure increases with the depth of the water, but it's the same at the same height.

Let's check your understanding of that one.

The pressure is high at the bottom of some deep pools, and I've got three pools there, A, B, and C.

In which pool is the pressure greatest at the bottom, A, B, or C or is it the same for all three? So pause the video, make your selection a restart please.

Welcome back, and it's the same for all three.

They're at the same depth, so the pressure's going to be the same in all of those.

Well done if you've got that.

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

And what I'd like you to do is to explain why gas is easier to compress inside a sealed container than a liquid is, and do that in terms of particles please.

And then for question two, I've got a submarine travelling upwards from deep within the ocean.

I'd like you to explain why the pressure on the submarine is very high when it's far beneath the surface and while there's still a pressure acting on it when it reaches the surface of the water.

So pause the video, answer those two questions, and then restart please.

Welcome back.

Well, here's the reason why the gas is easier to compress than a liquid.

Should have an explanation about the spacings of the particles.

So the gas particles are very spaced out so they can be pushed together to make a smaller or lesser volume.

And the particles and a liquid have very little space, so you can't squash those closer together.

Well done if you've got that.

And your answer to part two should be something like this.

The submarine is being hit by moving water particles which cause the pressure, and there's a large weight of water above the submarine, so the pressure's very high.

And there's still a pressure when there's submarines at the surface because it's being hit by air molecules.

It's still beneath several tens of kilometres of earth.

So there's gonna be pressure acting on it because of the moving air particles.

Well done if you've got those.

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

And this is all about pressure on the surface and then it will be doing some calculations of pressure as well.

So you may need a calculator.

Let's go on with it.

So as you've seen in an earlier part of the lesson, the particles in a fluid are in constant motion.

So if I show you the particles in a gas here inside the container, then those particles are moving around and have shown that by those small arrows there.

But they're gonna collide with the walls of the container as well as they move.

And when they hit the walls of the container, they're gonna bounce off those walls, but they're also gonna produce a force on those walls.

So those collisions are gonna produce the pressure on the container walls because they're gonna produce forces acting on the walls.

Each of the collisions only produces a very tiny force.

Remember, each atom is very, very small indeed.

So the forces are gonna be very small, but there are many, many particles inside there.

So there's billions of collisions with each part of that surface every second.

So every square millimetre of that surface is going to be hit by hundreds of millions of particles every second, and that's gonna produce a larger overall force.

The result of all of those collisions acting over that area is causing that surface pressure and we can calculate that pressure using the pressure equation, pressure is forced divided by area, or if we write down in symbols, p = F / A.

And we can define those symbols here.

So force, F is measured in newtons.

Area, A is measured in square metres or perhaps square centimetres sometimes.

And pressure is measured in newtons per square metre or newtons per square centimetre.

Let's try out a pressure calculation based upon forces and areas.

So I've got a book resting on a table as a weight of five newtons and a surface area of 200 centimetre squared in contact of the table.

What's the pressure on the table? Well, to solve that, I write out the equation, pressure is force divided by area.

I substitute in the values, five newtons and 200 centimetre squared, and I calculate the answer to that.

So the pressure is 0.

025.

And as I used newtons and centimetre squared, the unit is newtons per centimetre squared.

So now I'd like you to try and calculate the pressure based upon the same book with the same weight of five newtons, but it's resting on its edge with a different area.

So calculate the pressure on the table, please.

Pause the video, do the calculation, and restart.

Welcome back.

Well, we'll use symbols.

So the pressure is the force divided by the area.

You fill in the value of the force with still five newtons, but this time the area is 20 centimetres squared.

So we'll get a pressure of 0.

25 newtons per centimetre squared.

Well done if you've got that.

We can use those ideas to calculate the pressure called by the liquid at the bottom surface of any container it's in 'cause we have the weight and that's the force acting and we have the area of the bottom of the container, so that will allow us to get the pressure.

So let's have a look at example.

I've got some liquid in a container here.

I've got a beaker containing water that weighs 20 newtons and the surface area of the bottom is 40 centimetres squared.

I'm gonna calculate the pressure at the bottom of that container in newtons centimetre squared.

What I do is write out the expression for pressure.

I substitute in the values.

And remember the force was the weight of the liquid, it's 20 newtons, and I've got the area there, 40 centimetre squared.

That gives me a pressure of 0.

5 newtons per centimetre squared.

Now you can try a simple calculation like that as well.

So a barrel of oil has a base area of 0.

20 metre squared and the oil inside is a weight of 3,000 newtons.

What's the pressure caused by the oil on the bottom of the barrel? And you've got three possible answers there.

So pause the video, work out the pressure, and then restart please.

Welcome back.

You should have selected the bottom one, 15,000 newtons per metre squared.

Here's the calculation.

We've got the force of 3000 newtons divided by the pressure of not 0.

20 metre squared, and that gives a pressure of 15,000 newtons per metre squared.

Well done if you've got that.

And now it's time for the final task of the lesson.

I've got our pupil that's got a cylindrical beaker and we've got the area of the bottom surface 50 centimetre squared and they slowly fill the beaker at a constant rate until it's full of water with a weight of 25 newtons.

I'd like you to describe what happens to the pressure of the bottom surface of the beaker as it's gradually filled.

Calculate the pressure due to the water when the beaker is full, and then discuss what would happen to the pressure if the pupil gradually filled the beaker with a less dense liquid.

So a liquid with lower density than water.

So pause the video, find your answers to those three questions, and then restart please.

Welcome back and hopefully your answers are something like this.

The pressure would gradually increase because the weight of the water is gradually increasing, so it would be a nice, smooth increase in pressure.

The pressure when the beakers full can be calculated like that.

We've got a force of 25 newtons divided by the area of 50 centimetre squared, giving us a pressure of not 0.

5 newtons centimetre squared.

And what would happen if you used a less dense liquid is, the pressure would still increase gradually, but at a lower rate and the maximum pressure at the bottom of the beaker would also be lower.

Well done if you've got those answers.

Okay, we've reached the end of the lesson and here's a summary of what we've learned.

Liquids and gases are fluids that can flow because their particles are free to move about.

A gas flows much more quickly than a liquid 'cause its particles move at a higher speed and it can be compressed because its particles have large gaps between them.

A liquid cannot be compressed easily and it flows more slowly.

Pressures and fluids are caused by collisions of the particles with each other and with the surfaces of things that they touch.

And pressure on the surface can be calculated using pressure equals force divided by area or p = F / A.

Welcome done for reaching the end of the lesson.

I'll see you in the next one.