Thermal physics
The particle model, gas pressure, thermal expansion, specific heat capacity, changes of state and the transfer of thermal energy.
📌 Key points
- Solids: particles close together in a regular pattern, vibrating about fixed positions. Liquids: close together but moving past each other. Gases: far apart, moving randomly at high speed.
- Temperature is linked to the average kinetic energy of the particles. Absolute zero (−273 °C or 0 K) is the lowest possible temperature. T (in K) = θ (in °C) + 273.
- Gas pressure is caused by particles hitting the walls. At constant volume, heating a gas increases its pressure; at constant temperature, pV = constant (Boyle’s law).
- Brownian motion: tiny visible particles (e.g. smoke) move randomly because they are hit by much smaller, fast-moving air or water molecules.
- For the same temperature rise, gases expand most and solids least. Gaps in railway tracks and bridges allow for expansion.
- Specific heat capacity c is the energy needed per unit mass per unit temperature rise: ΔE = mcΔθ.
- During melting and boiling the temperature stays constant while energy is being supplied.
- Evaporation happens at the surface at any temperature. The most energetic particles escape, so the liquid left behind cools. It is faster with a higher temperature, larger surface area and moving air.
- Conduction is best in solids, especially metals (free electrons). Convection happens in fluids because warm fluid is less dense and rises. Radiation (infrared) needs no medium.
- Dull black surfaces are good absorbers and emitters of infrared; shiny white or silver surfaces are poor absorbers and emitters (good reflectors).
📖 Key terms
- Absolute zero
- The lowest possible temperature, −273 °C (0 K), where particles have the least kinetic energy.
- Specific heat capacity
- The energy needed to raise the temperature of 1 kg of a substance by 1 °C.
- Internal energy
- The total kinetic and potential energy of all the particles in an object.
- Evaporation
- The escape of more energetic particles from the surface of a liquid.
- Conduction
- Thermal energy transfer through a material without the material moving.
- Convection
- Thermal energy transfer by the movement of a fluid caused by density differences.
- Thermal radiation
- Infrared electromagnetic radiation emitted by all objects; it needs no medium.
✏️ Worked example
How much energy is needed to heat 2.0 kg of water from 20 °C to 80 °C? (c = 4200 J/(kg °C))
- Temperature rise Δθ = 80 − 20 = 60 °C.
- ΔE = mcΔθ = 2.0 × 4200 × 60.
- Answer: ΔE = 504 000 J (504 kJ).
Use the temperature change Δθ, not the final temperature. In Boyle’s law questions, the pressures (and volumes) can be in any unit as long as each pair matches.
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