Temperature relates to the thermal energy held by an object or a sample of matter, which is the kinetic energy of the random motion of the particle constituents of matter. While the thermal energy of an object is proportional to the amount of matter it contains, temperature measures thermal energy in a manner that is independent of size; it is an intensive property, while thermal energy is an extensive property.
Differences in temperature between regions of matter are the driving force for heat, which is the transfer of thermal energy. Spontaneously, heat flows only from regions of higher temperature to regions of lower temperature. If no heat is transferred between two objects, the objects have the same temperature.
Temperature is one of the principal properties studied in the field of thermodynamics. The empirical definition of temperature arises from the conditions of thermodynamic equilibrium, expressed as the zeroth law of thermodynamics. When two systems are in thermal equilibrium, they have the same temperature, which is also a matter of common experience.
The extension of this principle as an equivalence relation between multiple systems fundamentally justifies the use of a thermometer and prescribes the principles of its construction to measure temperature. While the zeroth law permits the definition of a set of many empirical scales of temperature, the second law of thermodynamics selects the definition of a single preferred, absolute temperature function, whence called the thermodynamic temperature. This function is the variation of the internal energy with respect to changes in the entropy of a system. Its natural, intrinsic origin or null point is absolute zero at which the entropy of any system is at a minimum. Although this is the lowest absolute temperature described by the model, the third law of thermodynamics postulates that absolute zero cannot be attained by any physical system.
