Gas Pressure

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.

In a solid, particles are close together in a regular arrangement and vibrate about fixed positions. In a liquid they are still close but disordered, and they move around one another. In a gas they are far apart and move rapidly and randomly. Raising the temperature makes the particles move faster; at −273 °C — absolute zero — they have their least kinetic energy. Gas particles collide with the walls of their container, and those collisions are what produce gas pressure. The random jiggling of microscopic particles suspended in a fluid, known as Brownian motion, is direct evidence that the fluid is made of particles in constant random motion.