Infrared Radiation

Thermal physics runs along a single chain of reasoning. Particles are arranged and move in a way that depends on the forces between them; heating makes them move faster, which raises temperature; faster particles collide harder and more often, which creates gas pressure and drives expansion; energy supplied at a change of state pulls particles apart instead of speeding them up, so the temperature holds still; and thermal energy travels from hot to cold by conduction, convection and infrared radiation.

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.

Thermal energy always travels from a hotter region to a cooler one, by three distinct routes. Conduction passes energy through a material without the material itself moving, and is fastest in metals. Convection occurs only in fluids: the heated fluid expands, its density falls, it rises, cooler denser fluid sinks to replace it, and a convection current carries energy by bulk movement of the fluid itself. Infrared radiation is emitted and absorbed by every object and needs no medium at all; dull black surfaces are the best emitters and absorbers, while shiny light surfaces are the best reflectors.