Cambridge IGCSE Physics 2 Thermal Physics Exam Questions

About Cambridge IGCSE Physics 2 Thermal Physics

Thermal physics in Cambridge IGCSE Physics 0625 is built on the kinetic particle model of matter. You need to describe solids, liquids and gases in terms of particles, explain gas pressure and the effect of temperature, and convert between the Celsius and Kelvin scales. The topic then covers thermal expansion, specific heat capacity, melting, boiling and evaporation, and the three ways thermal energy is transferred: conduction, convection and radiation. Extended candidates also use pV = constant and the specific heat capacity equation. It links to energy in topic 1 and to infrared in the electromagnetic spectrum. Examiners like particle-based explanations and practical questions on heating and cooling.

What you need to know

• 2.1 States and particle model Solids have particles in a fixed, regular arrangement vibrating about fixed positions; liquids have particles close together moving around each other; gases have particles far apart moving randomly at high speed. Temperature is related to the average kinetic energy of the particles.
• 2.1 Gas pressure and Brownian motion Gas pressure is caused by particles colliding with the walls of the container. Brownian motion is the random movement of microscopic particles (such as smoke) caused by collisions with fast-moving, much lighter gas molecules.
• 2.1 Absolute zero and gas laws Absolute zero, -273 °C or 0 K, is the lowest possible temperature, where particles have least kinetic energy. T (in K) = θ (in °C) + 273. Extended candidates use pV = constant for a fixed mass of gas at constant temperature.
• 2.2 Thermal expansion Solids, liquids and gases expand when heated; gases expand most and solids least. Examples include gaps in railway tracks and liquid-in-glass thermometers.
• 2.2 Specific heat capacity Increasing temperature increases the internal energy of an object. Extended candidates define specific heat capacity as the energy needed per unit mass per unit temperature increase and use c = ΔE / (mΔθ), with units J/(kg °C).
• 2.2 Melting, boiling and evaporation Melting and boiling take place at fixed temperatures without a temperature change. Evaporation is the escape of more energetic particles from a liquid surface, which cools the liquid. It is faster at higher temperature, larger surface area and with air movement over the surface.
• 2.3 Conduction and convection Good conductors are metals; poor conductors (insulators) include plastics, wood and air. Convection happens in fluids because heated fluid expands, becomes less dense and rises. Extended candidates explain conduction by lattice vibrations and, in metals, by delocalised (free) electrons.
• 2.3 Radiation Thermal radiation is infrared and needs no medium. Black surfaces are good absorbers and emitters; white and shiny surfaces are poor absorbers and good reflectors. Extended candidates know the rate of emission depends on surface temperature and area and explain the Earth's temperature balance.

Common mistakes in 2 Thermal Physics

• Saying particles get bigger when a substance expands. The particles move further apart; they do not change size.
• Forgetting that temperature stays constant during melting or boiling. The energy supplied breaks bonds between particles instead of raising the temperature.
• Converting to Kelvin by subtracting 273. Add 273 to a Celsius temperature to get kelvin.
• Saying heat rises in convection. It is the warmer, less dense fluid that rises.
• Using pV = constant when the temperature changes. It only applies to a fixed mass of gas at constant temperature.

Try three 2 Thermal Physics exam-style questions