In a loose sense, thermal energy is a term used to describe the energy content of a system related to heating effects, e.g. temperature increase or decrease.


In thermodynamics, thermal energy is the internal energy present in a system in a state of thermodynamic equilibrium by virtue of its temperature.


The term is not widely used, however, in a rigorous sense, owing to the result that the phrase "thermal (heat) energy" is counter-intuitive. That is, "thermal energy" can only be defined as any spontaneous flow of energy (energy in transit) from one object to another, caused by a difference in temperature between two objects; thus, an object cannot possess "heat". This is explained by the second law of thermodynamics. Hence, by extrapolation, it is difficult to define quantities of heat energy (thermal energy).

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