IGCSE Physics revision · Thermal physics
The particle model of matter
No calculations here — this topic is marked entirely on wording. Every answer describes particles: their arrangement, their motion, and their energy. Students who write about 'heat rising' or 'molecules expanding' lose marks that particle language would have earned.
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What the syllabus demands
- —Describe the arrangement, separation and motion of particles in solids, liquids and gases
- —Explain the properties of each state using the particle model
- —Describe Brownian motion as evidence for moving particles
- —Explain gas pressure in terms of particle collisions with the container walls
- —Relate temperature to the average kinetic energy of particles
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Brownian motion
- The random movement of microscopic particles (such as smoke) caused by collisions with fast-moving, invisible air molecules — evidence that gases consist of moving particles.
- Gas pressure
- The force per unit area on the container walls caused by gas particles colliding with the walls and changing momentum.
- Absolute zero
- −273 °C, the temperature at which particles have their minimum possible kinetic energy.
The equations
More equations to practise: the IGCSE formula sheet.
Where the marks die
Common mistakes to check
- 01
Saying particles expand when heated. The particles stay the same size — they move faster and move further apart. Only the substance expands.
- 02
Explaining gas pressure without collisions. The mark scheme wants: particles collide with the walls, exert a force on the wall, and pressure = force per unit area.
- 03
Describing Brownian motion as smoke particles 'moving because they are hot'. They move because air molecules — too small to see — collide with them unevenly.
- 04
Explaining pressure rise on heating with 'particles push harder' only. Full marks: particles move faster, collide with the walls more often and with more force.
One worked example, done properly
Question
A sealed gas has volume 600 cm³ at a pressure of 100 kPa. It is compressed to 150 cm³ at constant temperature. Calculate the new pressure.
Method
- 1.Boyle's law: p₁V₁ = p₂V₂.
- 2.100 × 600 = p₂ × 150.
- 3.p₂ = 60,000 ÷ 150.
p₂ = 400 kPa