Mastering AQA A-Level Physics: Comprehensive Thermal Physics Notes

Thermal Physics and Energy Transfer

Ever wondered why ice melts or why steam burns more than boiling water? Thermal equilibrium explains how energy naturally flows from hot objects to cold ones until they reach the same temperature. When objects are at the same temperature, no energy transfers between them.

Temperature scales matter here. The Kelvin scale starts at absolute zero $-273°C$, where all particle motion theoretically stops. Converting is simple: T(K) = T(°C) + 273.

Brownian motion proves that invisible air particles are constantly moving. When you observe smoke particles under a microscope, they move randomly because tiny air molecules are constantly colliding with them. This shows that all particles are in constant, random motion.

Internal energy is the total kinetic and potential energy of all particles in a substance. Higher temperatures mean particles vibrate more vigorously. During state changes (like melting), temperature stays constant whilst internal energy increases to break molecular bonds.

Quick Tip: Remember that specific heat capacity $c$ measures energy needed to raise 1kg by 1K without changing state, whilst specific latent heat (L) measures energy per kg needed to change state at constant temperature.

Ideal Gases and Particle Behaviour

Gas behaviour follows predictable patterns that you can calculate and measure. A mole contains 6.02 × 10²³ particles (Avogadro's constant), making it easy to work with massive numbers of gas particles.

The ideal gas equation PV = nRT connects pressure, volume, temperature and amount of gas. You can also write this as PV = NkT using individual particles instead of moles. These relationships give us three important gas laws that work when one variable stays constant.

Boyle's Law shows pressure inversely relates to volume at constant temperature. Charles' Law demonstrates volume increases proportionally with temperature at constant pressure. Both help you understand how gases respond to changing conditions.

The kinetic theory model assumes gas particles are tiny spheres in random motion with perfectly elastic collisions. This leads to the equation pV = ⅓Nm(Crms)², connecting observable properties like pressure with invisible particle motion.

Maxwell-Boltzmann distribution shows that most gas particles have moderate speeds, with few moving very slowly or very quickly. Higher temperatures shift the whole distribution towards faster speeds, explaining why gases expand when heated.

Key Insight: The average kinetic energy of gas particles depends only on temperature: Ek = ³⁄₂kT. This fundamental relationship links the microscopic world of particles to macroscopic temperature measurements.