In thermal physics, thermal energy is
the energy portion of a system that increases with its
temperature. In a loose sense, "thermal energy"
is a term often 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, heat 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
posess "heat". This is explained by the second
law of thermodynamics. Hence, by extrapolation, it is
difficult to define quantities of heat energy (thermal
energy). In isolated cases, however, a few definitions
do exist.
