Topic 4 of 8 · Form 3
Thermal physics
Solids, liquids and gases: arrangement and motion of particles; Brownian motion as evidence for moving particles; gas pressure explained by molecular collisions with container walls.
Notes for this topic 1 file, in order
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Kinetic particle model
Solids, liquids and gases: arrangement and motion of particles; Brownian motion as evidence for moving particles; gas pressure explained by molecular collisions with container walls.
Gas behaviour
At constant temperature, pV = constant (Boyle's law). Temperature in kelvin: T(K) = θ(°C) + 273.
Worked example: A gas at 100 kPa occupies 3.0 litres. It is compressed to 1.2 litres at constant temperature. Find the new pressure.
p₂ = 100 × 3.0 ÷ 1.2 = 250 kPa
Thermal properties
Thermal expansion and its uses and dangers; specific heat capacity c (ΔE = mcΔθ); melting, boiling and evaporation; specific latent heat (E = mL); evaporation causes cooling.
Worked example: How much energy raises 2 kg of water by 15 °C (c = 4 200 J/(kg °C))?
2 × 4 200 × 15 = 126 000 J
Transfer of thermal energy
Conduction (free electrons and lattice vibrations in metals), convection (density changes in fluids), and radiation (infrared; black surfaces are good absorbers and emitters).
Check yourself
- Why does evaporation cool a liquid?
- Convert −23 °C to kelvin.
- Why are cooking pots often made of metal with plastic handles?
Show answers
- The most energetic molecules escape, lowering the average kinetic energy of those left.
- 250 K
- Metal conducts heat to the food; plastic is a poor conductor, so handles stay cool.
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