Temperature and Heat

Heat is best understood as energy IN TRANSIT. When two bodies at different temperatures are brought into contact, energy flows from the hotter to the colder one; this flow continues until no further net transfer occurs, a condition called thermal equilibrium. Temperature is the physical property that decides both the direction of this flow and when it stops -- heat always flows from a body at higher temperature to one at lower temperature, never the other way on its own, and once the two are at the same temperature the net exchange is zero even though, microscopically, energy keeps being exchanged both ways in equal amounts.

A glass of ice-cold water left on a table slowly warms up; a cup of hot tea on the same table slowly cools down -- in both cases heat flows between the object and its surroundings until they reach a common temperature (the state of thermal equilibrium). This happens whenever two bodies with different temperatures are in contact, because of that temperature difference.

At the particle level, matter in any state -- solid, liquid or gas -- is made of vibrating or moving particles (atoms, ions or molecules) that possess kinetic energy (from their motion) and, in condensed phases, potential energy (from the forces holding them near their neighbours). In a solid, particles vibrate about fixed mean positions; heating raises their vibrational kinetic energy, which we observe as a rise in temperature (except very near a phase change, where the energy instead goes into weakening the bonds between particles). Because the average kinetic energy of the particles is what temperature actually measures, a substance's TEMPERATURE reflects the degree of hotness -- the average kinetic energy per particle -- while its HEAT content depends additionally on how much of the substance there is. This is the crucial distinction between heat and temperature: temperature is intensive (a single number describing hotness, independent of amount), while heat is energy, an extensive quantity that depends on mass. Typical interatomic spacings illustrate the three states: about $10^{-10}\text{ m}$ in solids, roughly double that in liquids, and about $10^{-9}\text{ m}$ in gases at normal temperature and pressure. …