Thermal physics
Kinetic particle model and gases
Everything is made of tiny particles in constant motion. In solids they vibrate in fixed positions; in liquids they move past each other; in gases they move fast and far apart. Gas pressure comes from particles hitting the walls. Temperature in kelvin starts at absolute zero, and for a fixed mass of gas at constant temperature is constant.
≈ 30 minThermal expansion and specific heat capacity
Heating makes particles move faster and push further apart, so most materials expand: gases most, liquids less, solids least. Heating also raises the internal energy. The energy needed for a temperature rise depends on the mass and the material: , where is the specific heat capacity.
≈ 30 minMelting, boiling and evaporation
When a substance melts or boils, the temperature stays constant even though energy is still going in: the energy is used to break the forces between particles, not to speed them up. Evaporation happens at any temperature, from the surface only; the fastest particles escape, so the liquid left behind cools.
≈ 25 minTransfer of thermal energy
Thermal energy moves from hotter to colder places in three ways. Conduction passes vibrations between particles (and free electrons in metals). Convection carries energy as warm, less dense fluid rises. Radiation is infrared waves, which need no material at all. Dull black surfaces are the best emitters and absorbers; shiny surfaces are the best reflectors.
≈ 30 min