Temperature and Particle Motion

Objective: Interpret temperature with particles and absolute scale.

Temperature is a property related to the average kinetic energy of particles in a simple kinetic model, though molecular rotation, vibration, interactions, and collective behavior require care in detailed systems. Kelvin is the SI temperature unit, with T(K) = T(°C) + 273.15. Two objects at the same temperature can contain different total internal energies because mass, composition, and state differ. Absolute zero is 0 K, the lower limit of the thermodynamic temperature scale, not a claim that all quantum motion literally stops. Celsius temperature differences equal kelvin temperature differences. A thermometer measures a property that changes with temperature and must be calibrated.

Objective: Distinguish stored energy from energy transfer due to temperature difference.

Internal energy includes microscopic kinetic and potential energy associated with particles and their interactions in a system. Heating is energy transfer caused by a temperature difference; it can raise temperature, change phase, or both, depending on the process. A hot metal spoon placed in cooler water typically transfers energy toward the water until thermal equilibrium is approached; the spoon’s internal energy can decrease while the water’s increases. Heat is not a material that resides in an object. Equal temperatures indicate no net thermal transfer between bodies in thermal contact under ordinary equilibrium conditions, while microscopic exchanges continue. A temperature reading alone does not give total internal energy.

Objective: Use particle reasoning to explain dimension changes.

Many materials expand as temperature rises because the average spacing of their constituent particles changes, but the response depends on material, structure, and phase. In a simple solid model, linear change is approximately ΔL = αL₀ΔT over a limited temperature range, where α is an expansion coefficient. A 2 m rod with α = 1.0 × 10⁻⁵ K⁻¹ and a 50 K rise lengthens by 0.001 m, or 1 mm, in this model. Expansion joints accommodate size changes in structures. Water near freezing has unusual density behavior, so a universal claim that all substances always expand upon heating is inaccurate. Temperature-induced volume changes do not create new matter.