Solids, liquids and gases
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 minMass, weight and density
Mass is how much matter something contains, in kilograms, and it's the same everywhere. Weight is the gravitational force on that mass, , in newtons, and it changes from planet to planet. Density is mass per unit volume; objects less dense than a liquid float in it.
≈ 30 minPressure
Pressure is force per unit area, , measured in pascals ( Pa N/m²). The same force on a smaller area gives a bigger pressure. In a liquid, pressure increases with depth and density: .
≈ 25 min