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PHYSICS · Stage 8 · Chapter 6: Forces, Motion and Pressure

Physics 6.17 Pressure in gases — Particle Collisions and Atmospheric Pressure

5 min read 6 parts Lesson 17 of 19

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Physics 6.17 Pressure in gases — Particle Collisions and Atmospheric Pressure — Alhussan International Academy — Physics (Stages 7–9) notes: Explain gas pressure using particles and describe the factors that affect gas pressure and atmospheric pressure.Particle model — The partic…

  • Cause of pressure
  • Adding more gas
  • Smaller volume
  • Higher temperature
  • Oxygen tanks
  • Reason for compression

Part 1 of 6Gas pressure and particles

  • Particle model — The particles in a gas are far apart, moving fast in random directions.
  • Cause of pressure — Gas particles bump into each other and into the walls of their container.
    • The collisions with the walls produce gas pressure.
    • All gases, including gases inside you, exert a pressure on the walls of their container.
  • Balloon — Air particles hit the inside and outside of the balloon, and more collisions on the inside make it bigger.

Part 2 of 6Factors affecting gas pressure

  • Adding more gas — More particles mean more collisions with the walls, so the pressure increases.
    • In a container of fixed shape the pressure rises as air is pumped in.
    • Too much gas can make a balloon burst.
  • Smaller volume — The same particles in a smaller volume collide with the walls more often, so the pressure is higher.
  • Higher temperature — Particles move faster, so there are more and harder collisions and the pressure is higher.

Part 3 of 6Atmospheric pressure

1Atmospheric pressure1 point
= the pressure of the air around us, caused by air particles colliding with surfaces.
  • We do not feel it because it is balanced by the pressure of gases and liquids in our bodies pushing outwards.
2Height3 points
— Atmospheric pressure at sea level is bigger than high up a mountain.
  • Gravity pulls air particles towards the Earth, so the air near the surface is compressed.
  • Closer particles mean more collisions and a bigger force.
  • Higher up there is less air above, so fewer particles and fewer collisions.
3Effects
— Marshmallows contain air pockets that expand when the air around them is pumped out, because outside pressure falls while inside pressure stays the same.

Part 4 of 6Compressed gases

  • Oxygen tanksFirefighters and people in places with low oxygen use tanks of oxygen compressed into a small volume.
  • Reason for compressionOtherwise they could not carry enough oxygen.
  • High pressureThe pressure inside is very high, so the tanks are made of metal that can withstand it.
  • ReleaseGas flows out of the valve because its pressure is higher than the atmospheric pressure outside.

Part 5 of 6Comparing gases and liquids

PropertyGasLiquid
Particle spacingFar apartTouching
CompressibleYes, easilyAlmost not at all
Cause of pressureCollisions with wallsCollisions and the weight of liquid above

Part 6 of 6Everyday gas pressure

  • Bicycle tyrePumping adds more air particles into a fixed volume, so collisions with the inner wall increase.
  • Sealed crisp packetAt altitude the outside pressure falls but the trapped gas pressure stays the same, so the packet swells.
  • Drinking strawSucking lowers the pressure inside the straw, and the greater atmospheric pressure pushes liquid up.

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