3.6 Pressure in iquids and gases

Pressure in
liquids and gases

MR. JAKE

LEARNING OBJECTIVES

Here’s what you’ll learn in this lesson:

• Explain that pressure is caused by the action of a force, exerted by a substance, on an area (pressure=force/ area)

• Use particle theory to explain pressure in gases and liquids
  

Success Criteria

• I understand what causes pressure in a liquid.

• I understand how pressure changes with depth in a liquid.

• I understand what causes pressure in a gas.

• I understand how altitude affects atmospheric pressure.

• I understand how the quantity of gas in a container affects the pressure.

• I understand how the temperature of a gas in a closed container affects the pressure.

Watch this video to explore more about pressure in liquids. It is important that you pause the video and take down notes.

Multiple Choice

In which directions do liquids exert pressure within a container?

• Only on the base

• Only on the walls

• On the base and walls

• Liquids do not exert pressure

Multiple Choice

Which layer of water in a container experiences the greatest pressure?

• Top layer

• Middle layer

• Bottom-most layer

• All layers have equal pressure

Multiple Choice

When making a hole near the top layer of water in a container, how does the water flow out?

• Straight down

• At an angle

• Along the walls

• Does not flow out Answer

Multiple Choice

Why does water squirt out more forcefully from holes near the bottom of a container?

• Higher water pressure at the bottom

• Lower water pressure at the bottom

• Equal pressure throughout

• Pressure depends only on container size

Pressure and
Depth in
Liquids

Pressure in Water

Divers have to be careful if they wantto dive to a great depth in the sea.

The weight of the water above presses down on them. The greater
the depth they go, the more water
there is above them pressing down.

So, what conclusion can you make about the pressure of water?

Pressure and depth in liquids

The pressure in a liquid increases with depth,
but why?

As you go deeper in a liquid, there is more liquid above you. The weight of this liquid, caused by gravity, pushes on the particles of the liquid.

Pressure and depth in liquids

When the particles of the liquid are pushed, the force on the particles increases.

As the particles in a liquid are moving randomly in all directions, then the pressure in the liquid is equal in all directions.

Atmospheric pressure is greatest at sea level. This is because, at sea level, we have the weight of the whole atmosphere above us, pressing down. It is as though we live at the bottom of an “ocean air”.

As the diver dives deep down to the depth of the ocean/water, the pressure increases because the particles of water pushes or the weight of the water presses on the diver.

Open Ended

Using the concept of pressure in liquids, explain why does the pressure increases as the diver dives deeper into the ocean or into the water?

Multiple Choice

Why does pressure increase as you go deeper in a liquid?

• The particles become smaller

• There is more liquid above exerting weight

• Gravity decreases with depth

• Liquid density decreases

Multiple Choice

What role does gravity play in the pressure of a liquid at greater depths?

• It reduces pressure

• It pushes liquid particles upward

• It causes the weight of the liquid above to increase pressure

• It only affects the surface

Multiple Choice

How do liquid particles contribute to pressure at a specific depth?

• They move randomly in all directions

• They only move downward

• They are stationary

• They repel each other

Multiple Choice

In which directions is pressure equal at a given depth in a liquid?

• Only upwards

• Only downwards

• In all directions

• Only sideways

Multiple Choice

Why do the particles in a liquid contribute to pressure in all directions?

• Because they are all stationary

• Because they move randomly

• Due to the force of gravity alone

• They only move downwar

Multiple Choice

Which of the following best explains why liquid pressure is different from atmospheric pressure?

• Liquid particles are stationary while gas particles move freely

• Liquid pressure depends on depth, while atmospheric pressure depends on altitude

• Liquid pressure depends on temperature

• Liquid pressure is unaffected by depth

Multiple Choice

If a diver goes 10 meters below the surface of a lake, why does the pressure on them increase?

• The diver’s weight increases at greater depths

• More water above adds weight, increasing pressure

• Gravity is stronger underwater

• Water density decreases with depth

Multiple Choice

Which factor directly affects the pressure on a submerged object in a liquid?

• The object’s mass

• The surface area of the object

• The depth of the object in the liquid

• The object's shape

Multiple Choice

If a submarine descends deeper into the ocean, why does the pressure on it increase?

• The ocean temperature increases

• More water above the submarine exerts weight

• The submarine’s density decreases

• Ocean currents apply more force at depth

Multiple Choice

Why does pressure increase on the walls of a container as more water is added?

• The container expands, increasing pressure

• The added water exerts greater weight downward and outward

• Water molecules become more compact

• The temperature of the water rises

Multiple Choice

When a diver goes from 5 meters to 15 meters underwater, why does the pressure on them change?

• The diver becomes denser

• The water pressure from above increases with more depth

• The diver's temperature affects surrounding pressure

• Water density decreases at greater depths

Pressure in
Gas

Before you blow up a balloon, there is a small volume of air inside the balloon. The balloon is open at one end, so the pressure of the air inside is the same as the pressure of the air outside

As you blow air into the balloon, you are adding more gas particles. Particles in a gas move randomly and collide with the walls of the container.

Every time a gas particle collides with the wall of the container, the particle exerts a small force on the wall. The more particles there are in the gas, the more collisions happen with the walls, and so the force on the walls increases. As this force is exerted on an area, the force causes pressure. The pressure inside the balloon gets bigger as you blow in more air, pushing the walls of the balloon outwards

Multiple Choice

Before you blow up a balloon, why is the air pressure inside it the same as the outside?

• Because no air particles are inside

• The balloon is open, equalizing pressure with outside air

• The balloon has a vacuum inside

• The balloon walls exert outward pressure

Multiple Choice

What happens to the amount of gas particles inside the balloon as you blow into it?

• It decreases

• It remains the same

• It increases

• It fluctuates

Multiple Choice

How do gas particles move within a container?

• In a fixed pattern

• In straight lines only

• Randomly

• Downward only

Multiple Choice

How does adding more gas particles affect the frequency of particle-wall collisions in the balloon?

• Increases collision frequency

• Decreases collision frequency

• Has no effect on collisions

• Makes collisions random

Multiple Choice

How is pressure within the balloon created?

• By the balloon material

• By the collisions of gas particles with the walls

• By air outside the balloon

• By gravity pulling down on the balloon

Multiple Choice

Why does atmospheric pressure vary at different altitudes?

• Different altitudes have different gravitational forces

• The concentration and weight of air particles change with altitude

• Humidity increases with altitude

• Temperature decreases with altitude

As you go higher in the atmosphere, two variables that affect atmospheric pressure change:

• the number of particles in 1 m3 of air decreases, so the concentration of gas particles decreases

• the weight of air above your current position decreases.

Pressure and depth
in Gas

Atmospheric pressure at sea level is about 100 000 N/m2 .

This pressure is the equivalent of the weight of two large elephants pushing on every 1m² of surface!

We are not aware of the pressure from the atmosphere because we live in the atmosphere all the time.

Pressure in Gas

Why do climbers of the Mt.Everest have to wear a gas mask at the summit?

Open Ended

Why do climbers of the Mt.Everest have to wear a gas mask at the summit?

If you climb a high mountain, the air gets thinner (less dense) as you go higher. Its density decreases. The air is less dense at the top of a mountain, its pressure is less. We call the pressure of the air atmospheric pressure.

Pressure of the

Atmosphere

Multiple Choice

As altitude increases, what happens to the number of gas particles in 1 m³ of air?

(altitude means the height or distance above sea level)

• It increases

• It decreases

• It stays the same

• It fluctuates randomly

Multiple Choice

Which of the following statements is true about atmospheric pressure as altitude increases?

• It increases with altitude

• It decreases with altitude

• It remains constant

• It depends on humidityIt depends on humidity

Multiple Choice

Why does atmospheric pressure decrease at higher altitudes?

• Air density increases with altitude

• There are fewer gas particles and less weight of air above

• Gravity is stronger at higher altitudes

• Temperature is higher at higher altitudes

Multiple Choice

What is the approximate atmospheric pressure at sea level?

• 50,000 N/m²

• 75,000 N/m²

• 100,000 N/m²

• 150,000 N/m²

Multiple Choice

Atmospheric pressure at sea level is roughly equivalent to the weight of:

• One large elephant on every square meter

• Two large elephants on every square meter

• Ten small birds on every square meter

• A small dog on every square meter

Multiple Choice

Why are humans generally unaware of atmospheric pressure?

• The atmosphere exerts no pressure on humans

• We are used to it as we live in the atmosphere constantly

• It only affects objects at high altitudes

• The human body resists atmospheric pressure

Multiple Choice

How does the weight of the air above change as altitude increases?

• It increases

• It decreases

• It stays constant

• It depends on temperature

Multiple Choice

What happens to the concentration of gas particles in the atmosphere with altitude?

• It increases

• It decreases

• It remains the same

• It varies randomly

Multiple Choice

Which force is responsible for the pressure exerted by the atmosphere at sea level?

• The weight of the air above

• The temperature of the atmosphere

• The speed of wind currents

• The rotation of the Earth

Multiple Choice

Why does atmospheric pressure vary at different altitudes?

• Different altitudes have different gravitational forces

• The concentration and weight of air particles change with altitude

• Humidity increases with altitude

• Temperature decreases with altitude

The effect of atmospheric pressure can be shown by pumping the air out of a metal container.

Before the air is pumped out, the pressure on the inside of the container is equal to the pressure on the outside.

Effects of atmospheric pressure

When the air is pumped out, the pressure inside the container becomes close to zero.

The pressure on the outside does not change. The container is crushed by the pressure of the air outside the container. The picture shows a conttainer crushed by atmospheric pressure.

Multiple Choice

Before the air is pumped out of the container, why does it not get crushed?

• The pressure outside is higher than inside

• The pressure inside equals the pressure outside

• The container is very strong

• Atmospheric pressure only affects open containers

Multiple Choice

What happens to the pressure inside the container when air is pumped out?

• It increases slightly

• It decreases close to zero

• It remains the same

• It becomes higher than outside pressure

Multiple Choice

Why does the container get crushed after the air is pumped out?

• Internal pressure increases

• Outside pressure exceeds the pressure inside

• Air rushes back into the container

• The container has no structural support

Multiple Choice

When the air is pumped out, what does the remaining atmospheric pressure do to the container?

• It stretches it

• It compresses it

• It cools it

• It causes it to expand

Multiple Choice

Which concept does the crushed container best demonstrate?

• The effects of reduced internal pressure

• The force of gravity

• How gases expand when heated

• Conservation of energy

As the temperature of a gas increases, the speed of the particles in the gas increases. When particles are travelling faster, their collisions exert more force. This means that "increasing the temperature of a gas will increase the pressure"
of the gas.

Pressure and Temperature in Gases

Multiple Choice

What happens to the pressure in a sealed container of gas if the temperature increases?

• Pressure decreases

• Pressure increases

• Pressure stays the same

• Pressure fluctuates randomly

Multiple Choice

If you heat a gas in a balloon, what will likely happen to the balloon’s size?

• The balloon will expand

• The balloon will shrink

• The balloon will remain the same size

• The balloon will deflate

Multiple Choice

Why does increasing the temperature of a gas increase its pressure?

• Particles slow down, exerting more force

• Particles move faster, colliding more forcefully

• Particles stay the same speed but grow in size

• The volume of the container increases

Multiple Choice

In a closed metal container, what would happen to gas pressure if the container is left in the sun?

• The pressure will decrease as it cools

• The pressure will increase as it heats

• Pressure won’t change

• Pressure decreases initially, then increases

Multiple Choice

If a gas’s temperature is doubled in a fixed-volume container, what effect does this have on particle collisions?

• Collisions decrease in frequency

• Collisions become weaker

• Collisions become stronger and more frequent

• Collisions remain constant

Success Criteria

• I understand what causes pressure in a liquid.

• I understand how pressure changes with depth in a liquid.

• I understand what causes pressure in a gas.

• I understand how altitude affects atmospheric pressure.

• I understand how the quantity of gas in a container affects the pressure.

• I understand how the temperature of a gas in a closed container affects the pressure.

Do you have any questions?

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Thanks!