Internal Energy

Topic 2 of Cambridge IGCSE Physics 0625 contains 45 printed statements: 24 Core, taught to all candidates, and 21 Supplement, taught in addition to Core for candidates following the Extended route. The Core statements cover the kinetic particle model and states of matter, Brownian motion as evidence for that model, gas pressure and the qualitative gas relationships, the kelvin scale, thermal expansion and its applications, internal energy, changes of state and evaporation, and thermal energy transfer by conduction, convection and infrared radiation with basic everyday applications. The Supplement statements add inter-particle forces, force per unit area in gases, \(pV = \text{constant}\), the particle explanation of expansion, average kinetic energy and specific heat capacity with its two experiments, the boiling–evaporation comparison and evaporative cooling of an object, the mechanisms and limits of conduction, the radiation balance including the Earth's temperature, emitter and absorber experiments, and complex multi-route applications.

Heating a substance normally increases the average separation of its particles, so the substance expands; Extended candidates also compare how much each state expands. A rise in temperature increases an object's internal energy. When a pure substance melts or boils, the energy supplied separates the particles against the forces between them instead of speeding them up, so the temperature stays constant during the change; condensation and solidification are the reverse, with particles coming closer together and energy being given out. Evaporation is different again: it happens only at the surface, at any temperature below the boiling point, and it cools the liquid left behind.