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2.1.3Thermal Physics

Particle Model of Gases — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 8 of 45 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

2.1.3 Particle Model of Gases — IGCSE PHYSICS notes: Explain gas pressure with the particle model and relate it to temperature and volume, including Boyle's law (Extended).Gas particles are in constant, random, rapid motion and collide with each other and with the walls.Each coll…

  • What the symbols mean
  • Pumping up a tyre
  • Sealed syringe
  • Rising balloon
  • Compressibility of gases
  • Fixed mass

Part 1 of 7Gas pressure from the particle model

1
Gas particles are in constant, random, rapid motion and collide with each other and with the walls.
2
Each collision with a wall produces a tiny force.
3
Enormous numbers of collisions every second add up to a steady force, so the gas exerts a pressure.
4
The pressure acts equally in all directions and at right angles to any surface.
5
Gas particles are tiny compared with the gaps between them, so forces between them can be ignored except in collisions.
6
Pressure is caused by collisions with the container walls, not by particles pushing on one another or by their weight.

Part 2 of 7Pressure and temperature

A fixed mass of gas in a sealed container of constant volume is heated.
The particles gain kinetic energy and move faster.
They hit the walls harder and more often, so the pressure rises.
Examples
  • Examples a heated sealed aerosol can becomes dangerous and car tyre pressure rises on a hot day.
At constant volume, pressure is proportional to the Kelvin temperature.

Part 3 of 7Pressure and volume

1
Temperature and mass of gas stay constant.
2
Reducing the volume puts the same particles in a smaller space,
  • so they strike the walls more often at the same average speed and the pressure increases.
3
Increasing the volume makes the pressure fall.
4
Only the frequency of collisions changes, so the average kinetic energy stays the same.

Part 4 of 7Boyle's law (Extended)

Boyle's law
Boyle's law for a fixed mass of gas at constant temperature, pressure × volume is constant.
Equation
  • Equation p1V1 = p2V2,
    • or pV = constant.
      What the symbols mean: p1, V1 = pressure and volume before · p2, V2 = after; only for a fixed mass of gas at constant temperature
Any consistent units can be used, such as kPa and cm3, if the same unit appears on both sides.
Graphs:
  • p against V is a falling curve (a hyperbola) that never touches the axes.
  • p against 1/V is a straight line through the origin.
  • pV against p is a horizontal line.
The gas must be compressed slowly so it can lose heat and stay at constant temperature.

Part 5 of 7Using the model to explain

Pumping up a tyre
  • Pumping up a tyre more particles in the same volume give more wall collisions, so the pressure increases.
Sealed syringe
  • Sealed syringe halving the volume doubles the collision rate per area, so the pressure doubles at constant temperature.
Rising balloon
  • Rising balloon it expands because the outside pressure decreases.
Compressibility of gases
  • Compressibility of gases there is much space between the particles.

Part 6 of 7Investigating Boyle's law

1
Trap a column of air in a sealed glass tube above oil, connected to a hand pump and a pressure gauge.
2
Raise the pressure in steps.
3
Read the length of the trapped air column from a scale; the length is proportional to volume because the tube has constant width.
4
Wait after each change so the gas returns to room temperature.
5
Plot pressure against 1/volume to get a straight line through the origin, showing pV is constant.
  • Sources of error:
    • A leak changes the mass of gas.
    • Changing the pressure too quickly heats the gas.
    • Reading the scale at an angle causes parallax.

Part 7 of 7More from the specification

More from the specification
Kinetic contrast between gases and solids
  • Kinetic contrast between gases and solids gas molecules move freely in rapid random motion, unlike tightly bound vibrating particles in solids.
Particle Arrangement in States of Matter
  • In the kinetic particle model of matter, a solid substance has closely packed particles vibrating about fixed positions,
  • giving it a fixed shape and volume;
    • liquids have particles in close contact that flow past one another, taking the container's shape,
  • while gas molecules are far apart and move rapidly and randomly.
Brownian Motion and Molecular Evidence
  • Brownian motion provides direct experimental evidence for the kinetic model of matter;
    • microscopic smoke particles in air or pollen grains suspended in liquid water move in a jerky, erratic path due to continuous, uneven collisions with invisible, fast-moving fluid molecules.
Absolute Temperature and Kinetic Energy of Gases
  • The absolute temperature of a gas (measured in kelvin,
  • where 0 K represents absolute zero) is directly proportional to the average kinetic energy of its molecules;
    • at absolute zero, all translational molecular motion ceases.
Absolute temperature
  • Temperature measured in kelvin.
Inversely proportional
  • One quantity falls as the other rises in the same ratio.
Molecule
  • Tiny particle of the fluid, too small to be seen.
Liquid
  • State with a fixed volume that takes the shape of its container.
Solid
  • State with a fixed shape and a fixed volume.
Brownian motion
  • Random jiggling of small visible particles hit by fluid molecules.

Key terms

TermMeaning
Gas pressureForce per unit area from particle collisions with the walls.
Boyle's lawpV is constant for a fixed mass of gas at constant temperature.
Fixed massSame number of particles throughout.
CompressionReducing the volume of a gas.
HyperbolaCurved graph shape of pressure against volume.

Avoid these mistakes

1
Saying pressure is caused by the particles hitting each other — it is caused by collisions with the walls.
2
Using Boyle's law with changing temperature — the temperature must be constant.
3
Believing gas particles stop moving when compressed — they keep moving at the same average speed.
4
Forgetting that the mass of gas must remain constant.
5
Saying heating makes the particles expand — they only move faster.

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