Thermal physics

Structure of matter and kinetic theory

Matter consists of microscopic particles in continual motion; absolute temperature is linked to the mean translational kinetic energy of gas molecules.

Kinetic theory models a gas as a large number of randomly moving molecules. Collisions with container walls create pressure; absolute temperature measures mean translational kinetic energy.

Pressure and temperature

In the model, pressure depends on number density and mean kinetic energy. At fixed volume, increasing absolute temperature makes molecules move faster, collide harder, and raises pressure.

Quick check

Kinetic theory links microscopic motion to macroscopic quantities. In a dilute gas, molecules move randomly, with brief collisions separated by longer free flights; impacts on the container wall transfer momentum and produce pressure. A rise in absolute temperature means a higher mean translational kinetic energy, not that every molecule has the same speed. At a given temperature, lighter molecules have a greater typical speed than heavier ones. The ideal-gas model neglects molecular size and long-range attraction, so it works best at low density, moderate pressure, and away from condensation. Solids and liquids also have particulate structure, but their particles vibrate around sites or remain close together; these pictures qualitatively explain thermal expansion and diffusion. This is a statistical model: it predicts averages over enormous numbers of particles rather than tracking each molecule individually.

In an elastic collision with a container wall, a molecule's momentum component normal to the wall changes, and the wall receives the opposite impulse. Many such impacts per unit area and time produce pressure. Pressure therefore does not come from molecules continuously pushing the wall; it emerges from a vast number of microscopic collisions.