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Unit 3: Forces, motion and pressure

Pressure in liquids and gases — HIA International - Grade 7 Revision Notes & Topical Questions

5 min read 9 parts Lesson 7 of 8 3 quizzes

📚 HIA International - Grade 7 Hub / Topical Revision
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Lesson summary

3.6 Pressure in liquids and gases — HIA INTERNATIONAL notes: Explain what causes pressure in liquids and gases and how depth, altitude and temperature change it.Increases with depth — water spurts out with more force from lower holes in a container.Reason — there is more liquid a…

  • Submarines
  • More particles
  • Faster particles
  • Pressure cooker
  • Atmosphere
  • Not noticed

Part 1 of 7Pressure in liquids

Increases with depth
  • Increases with depth water spurts out with more force from lower holes in a container.
Reason
  • Reason there is more liquid above, and its weight pushes on the liquid particles.
Direction
  • Direction particles move randomly in all directions, so pressure in a liquid acts equally in all directions.
Dam
  • Dam the wall is wider at the bottom where the pressure is greatest.
Submarines
  • Submarines must be very strong to withstand high pressure at great depth.

Part 2 of 7Pressure in gases

Pressure in gases
Cause
  • Cause gas particles move randomly and collide with the walls of the container, each collision exerting a small force.
More particles
  • More particles more collisions, so higher pressure.
Balloon
  • Balloon blowing in more air increases the pressure and pushes the walls outwards.
Tyre
  • Tyre low pressure cannot support the weight of the vehicle, and adding air increases the pressure.

Part 3 of 7Temperature and gas pressure

Temperature and gas pressure
Heating
  • Heating gas particles move faster in a closed container.
Faster particles
  • Faster particles collide harder and more often, so pressure increases.
Cooling
  • Cooling pressure decreases.
Pressure cooker
  • Pressure cooker a sealed lid traps steam, so the pressure and temperature rise to cook food faster.

Part 4 of 7Atmospheric pressure

Atmosphere
Atmosphere the layer of gases around the Earth that makes up the air.
Sea level
  • Sea level the atmosphere is deepest, so atmospheric pressure is highest at about 100 000 Pa.
As altitude increases.
  • The number of air particles in a given volume falls.
  • The weight of air above falls.
Not noticed
  • Not noticed we live in the atmosphere all the time.
Crushing
  • Crushing a metal container crushes when air is pumped out because the pressure outside stays high while the pressure inside falls to nearly zero.

Part 5 of 7Vacuum

  • Vacuum
    • a space with no particles, so there are no collisions and the pressure is zero.

Part 6 of 7Hydraulic systems

Hydraulic systems
Transmission
  • Transmission liquids cannot be compressed, so pressure is passed through them.
Use
  • Use hydraulic brakes in cars.

Part 7 of 7Investigating pressure

1
Make three small holes of equal size at different heights in a bottle.
2
Cover the holes with tape and fill the bottle with water.
3
Remove the tape and watch the water jets.
4
Warm the open bottle, seal it with the lid, then put it in a cold place.
5
Observe the bottle collapse as the cold air pressure inside falls.
  • Result
    • the lowest hole sprays furthest, and the cooled bottle is squashed because the air particles inside move more slowly and hit the walls less.

Key terms

TermMeaning
PressureForce divided by area.
Atmospheric pressureThe pressure caused by the weight of the air above.
AltitudeHeight above sea level.
Sea levelThe average height of the sea surface.
CollideTo hit another particle or surface.
DepthHow far below the surface.
VacuumA space containing no particles.
ContainerA closed object that holds a gas or liquid.

Avoid these mistakes

1
Saying liquid pressure depends on the shape of the container — it depends on depth.
2
Saying gas pressure comes from particles pushing each other — it comes from collisions with the walls.
3
Saying a vacuum has high pressure — it has zero pressure.
4
Saying atmospheric pressure is greatest at high altitude — it is greatest at sea level.

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