Chapter 19: Thermal Physics

The kinetic model of matter provides a foundation for understanding how particles in solids, liquids, and gases behave differently based on their arrangement and motion. When thermal energy is supplied to a substance within a single phase, molecular kinetic energy increases and temperature rises correspondingly. However, during phase transitions such as melting or boiling, temperature remains constant despite continued energy input, as the supplied energy breaks intermolecular bonds and increases electrical potential energy rather than kinetic energy. Evaporation represents a special case where the most energetic liquid molecules escape at the surface, causing the remaining liquid to cool. Internal energy is defined as the combined random kinetic and potential energies of all atoms and molecules within a system, which can be increased through heating or mechanical work. The first law of thermodynamics expresses energy conservation through the relationship between change in internal energy, heat supplied, and work performed on a system, with work at constant pressure calculated from pressure multiplied by volume change. Temperature serves as the fundamental property that determines the direction of thermal energy flow, always proceeding from higher to lower temperature regions until thermal equilibrium is achieved. The Kelvin scale provides an absolute temperature measurement independent of any particular substance's properties, beginning at absolute zero where internal energy is minimal. Practical temperature measurement relies on thermometers that detect changes in physical properties such as liquid volume, electrical resistance, or thermoelectric effects, with different designs suited to various applications based on thermal response time and sensitivity requirements. Quantifying thermal energy transfers requires understanding specific heat capacity, which represents the energy needed to raise a unit mass by one temperature unit, and specific latent heat, which quantifies energy required for phase changes at constant temperature, with latent heat of vaporization typically exceeding that of fusion due to the greater molecular separation involved in gas formation.