Kinetic Theory
States of Matter
| State | Arrangement | Motion | Forces |
|---|---|---|---|
| Solid | Regular lattice, close-packed | Vibrate about fixed positions | Strong intermolecular forces |
| Liquid | Random, close-packed | Move/slide past each other | Moderate forces |
| Gas | Random, widely spaced | Move rapidly in all directions | Negligible forces |
Kinetic Theory of Gases
Gas particles are in constant, random motion. Temperature is a measure of the average kinetic energy of the particles. Higher temperature → faster particles → greater kinetic energy.
Absolute temperature is measured in kelvin (K): T(K) = T(°C) + 273. Absolute zero (0 K = −273 °C) is the temperature at which particles have zero kinetic energy.
Density
Density (ρ) = mass/volume: ρ = m/V (kg/m³). Water = 1 000 kg/m³; air ≈ 1.2 kg/m³.
Gases are much less dense than solids/liquids because the particles are far apart. Heating a gas at constant pressure causes it to expand, reducing density.
Pressure
Pressure = force per unit area: P = F/A (pascals, Pa = N/m²).
Gas pressure results from particles colliding with the walls of a container. More frequent or harder collisions → higher pressure.
Atmospheric pressure ≈ 100 000 Pa (100 kPa). Pressure increases with depth in a fluid: ΔP = ρgh.
Gas Laws (Fixed Mass of Gas)
Boyle's Law (constant temperature)
P₁V₁ = P₂V₂ — pressure and volume are inversely proportional at constant temperature. Decreasing volume → more frequent collisions with walls → higher pressure.
Charles' Law (constant pressure)
V₁/T₁ = V₂/T₂ — volume is proportional to absolute temperature at constant pressure (temperature in kelvin).
Pressure Law (constant volume)
P₁/T₁ = P₂/T₂ — pressure is proportional to absolute temperature at constant volume (temperature in kelvin). Heating at constant volume → particles move faster → more frequent, harder collisions → higher pressure.
Combined Gas Law
P₁V₁/T₁ = P₂V₂/T₂ (temperature in kelvin).
WJEC Required Practical: Boyle's Law
Use a Boyle's law apparatus (gas syringe or manometer) to vary volume of a fixed mass of gas at constant temperature. Record P and V; plot P vs 1/V — straight line through origin confirms inversely proportional relationship.
⚠Common mistakes
- Forgetting to convert to kelvin: gas law calculations always use kelvin. Adding 273 to Celsius.
- Charles' Law applies at constant pressure, not constant volume: if volume is fixed, use the pressure law.
- Density formula rearrangement: m = ρV; V = m/ρ — double-check which variable you need.
- Confusing pressure with force: pressure depends on both force and area.
AI-generated · claude-opus-4-7 · v3-wjec-physics