Pressure in gases and liquids
Pressure in liquids
Look at the picture of a dam. The wall of this dam is wider at the bottom than it is at the top.
Behind the wall of the dam, the water is almost to the top of the wall.
You can see water coming out from two levels.
The water coming out close to the top is coming out with less force.
The water coming out from further down is coming out with more force.
This is because pressure in the water increases with depth.
The wall is wider at the bottom to make the wall stronger where the pressure from the water is greatest.
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.
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.
Pressure in gases
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. The container is the balloon.
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.
Look at the tyre in this picture.
The air inside the tyre is at low pressure. The pressure is not enough to support the weight of the vehicle.
If more air is put inside the tyre, the pressure will increase. More collisions will happen with the inside walls of the tyre, pushing the tyre outward and supporting the vehicle.
Pressure and depth in gases
As with liquids, the pressure in a gas increases with depth.
Most people on Earth live at low altitude. Altitude is height above sea level.
The Earth’s atmosphere, which we call air, is made from gases. The atmosphere extends to a height of about 400 km above sea level.
Sea level is, therefore, where the atmosphere is at its deepest. So, at sea level, atmospheric pressure is highest.
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
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 1 m2 of surface! We are not aware of the pressure from the atmosphere because we live in the atmosphere all the time.
Effects of atmospheric pressure
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.
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 container crushed by atmospheric pressure.
Pressure and temperature in gases
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.
High pressure can be used to cook food.
The picture shows a type of cooking pot called a pressure cooker.
Water and food are placed inside the pressure cooker. The pressure cooker has a sealed lid. When the water boils, the steam cannot escape so the pressure of the gas inside increases. This increases the temperature.
On top of the pressure cooker, there is a weight to control the pressure and valves that allow steam to escape, once the required pressure has been reached.
Questions
1a. Submarines are ships which are designed to go underwater.
Explain why submarines that go to the deepest parts of the oceans must be very strong.
Show Answer
Because the deeper you go, the higher the water pressure becomes. The hull of the submarine must be very strong to resist this high external pressure and avoid collapsing.
1b. Explain why the wall of a dam is thicker at the bottom than at the top.
Show Answer
Water pressure increases with depth. The bottom of the dam experiences the highest pressure, so it needs to be thicker to withstand the force of the water.
2a. The sketch graph shows how the pressure in a liquid changes with depth in the liquid.
Explain the pattern shown in the graph.
Show Answer
As depth increases, the pressure increases in a linear pattern. This is because the weight of the liquid above adds more force on the particles below.
2b. When the depth in the liquid doubles, the pressure in the liquid ______.
Show Answer
Doubles
3. A fish is in water. The water exerts pressure on the fish.
Which of these causes pressure on the fish? Write one letter.
- A The weight of water beside the fish.
- B The weight of water all around the fish.
- C The weight of water above the fish.
- D The weight of water below the fish.
Show Answer
C — The weight of water above the fish causes the pressure.
4a. Explain what causes the pressure inside the ball.
Show Answer
The pressure is caused by air particles colliding with the inner walls of the ball.
4b. Explain how putting more air in the ball will change the pressure inside the ball.
Show Answer
More particles means more collisions with the walls, which increases the pressure inside the ball.
4c. Explain why the pressure inside the ball is higher on a hot day.
Show Answer
Higher temperature increases the speed of particles, causing more forceful collisions with the wall, which increases pressure.
5a. Explain why the pressure in a vacuum is zero.
Show Answer
Because there are no particles inside the vacuum to collide with the walls and exert pressure.
5b. The metal container collapses when there is a vacuum inside. Explain why.
Show Answer
The air pressure outside the container is greater than inside, which causes the container to collapse inward.
Think Like a Scientist
Observing the effects of pressure
Part 1: Pressure and depth in liquids
In this investigation, you will observe the effect of pressure increasing with depth in water.
You will carry out the investigation by yourself.
- 1- or 2-litre empty plastic bottle
- adhesive tape
- something to make small holes in the side of the bottle
- large tray or sink to collect water
- Make three small holes in the side of the bottle at different heights. Try to make the holes the same size.
- Predict what will happen when the bottle is filled with water.
- Use one piece of adhesive tape to cover all the holes.
- Place the bottle in the tray or the sink.
- Fill the bottle with water, but do not put the lid on the bottle.
- Pull off the adhesive tape to open the holes.
- Watch what happens.
Show Answer
Yes, the observation matched the prediction. Water squirted out more strongly from the lower holes.
Show Answer
(Student diagram expected. It should show longer water streams from the lower holes and shorter from higher ones.)
Show Answer
Water pressure increases with depth, so the lowest hole experiences the highest pressure, making water shoot out further.
Think Like a Scientist
Observing the effects of pressure
Part 2: Pressure and temperature in gases
In this activity, you will observe the effect of changing temperature on the pressure in a gas.
You will carry out the investigation by yourself.
- empty plastic bottle (0.5–2 litre) with lid
- access to a warm place
- access to a cold place, such as a refrigerator
- Take the lid off the bottle.
- Leave the bottle and the lid in a warm place for 5–10 minutes.
- Put the lid tightly on the bottle without squeezing the bottle. Do this while the bottle is still in the warm place.
- Put the bottle into a cold place for 15–20 minutes.
- Observe what has happened to the bottle.
Show Answer
(Student diagram expected. It should show an expanded bottle before and a squashed bottle after cooling.)
Show Answer
The air pressure decreased as the temperature dropped, causing the bottle to collapse inward.
Show Answer
Cooling the air reduces the kinetic energy of the particles, so they collide with the inside walls less often and with less force, reducing pressure.
Show Answer
The air pressure inside the bottle would increase, and the bottle may expand or become more rigid. The particles move faster and collide more with the container walls.