One of the forms in which energy can be present in a system is the random, internal kinetic energy of the particles (molecules or atoms) of a system, which can intuitively be called “thermal energy.” œermal energy is the portion of internal energy U that is responsible for a system temperature and has to be distinguished from the average, external movement of a system of particles as a whole, which can be called the “mechanical energy” of the system.For gases, thermal energy is closely related to the random velocity of the molecules and, in the case of multiatom molecules, the rotations and vibrations of the atoms within the molecules. œe thermal energy is an extensive property as it is dependent by the amount of matter the object contains. œe temperature is instead an intensive property, and its empirical deŸnition comes out from the zeroth law of thermodynamics. œe zeroth law of thermodynamics states that if two systems are each in equilibrium with a third system, they will be in equilibrium with each other. œey are said to have the same temperature. For gases, statistical mechanics shows the direct relation between the thermal energy stored in the system and the temperature . œis law, however, also applies to liquids and solids, although the quantitative relation between thermal energy (agitation of the particles) and temperature is less straightforward and not so easy to calculate as for gases. Temperature and thermal energy distribution can be viewed as the result of statistical processes, such as di¥usion.Di¥usion ensures that if ever there is a surplus of thermal energy (i.e., of fast molecules or electrons or a higher density of phonons) in some area, some of it will žow toward areas with a lower thermal energy density until thermal equilibrium has been established. œis žow of energy P (in W = J s−1) is called heat žow. Heat žow is therefore the transfer of (thermal) energy from one (part of a) system to another. Note that heat is not conserved, because a change in thermal energy of a system may be achieved by heat exchange with the environment but also by mechanical interaction, and heat can be used to do mechanical work (motor) just as well as heating up a system.Heat can be viewed as “thermal energy on the move.” Heat is energy transferred from one body to another due to thermal contact when the systems are at di¥erent temperatures, but whether it ends up as thermal, mechanical, or another form of energy depends on the circumstances.