in visible light can be seen on this hot metalwork.
Thermal energy refers to several distinct physical concepts, such as the internal energy
of a system; heat
or sensible heat
, which are defined as types of energy transfer (as is work
); or for the characteristic energy of a degree of freedom
in a thermal system
is the Boltzmann constant
Relation to heat and internal energy
, heat is energy
in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work
or transfer of matter.
Heat refers to a quantity transferred between systems, not to a property of any one system, or 'contained' within it.
On the other hand, internal energy is a property of a single system. Heat and work depend on the way in which an energy transfer occurred, whereas internal energy is a property of the state of a system
and can thus be understood without knowing how the energy got there.
In a statistical mechanical account of an ideal gas
, in which the molecules move independently between instantaneous collisions, the internal energy is the sum total of the gas's independent particles' kinetic energies, and it is this kinetic motion that is the source and the effect of the transfer of heat across a system's boundary. For a gas that does not have particle interactions except for instantaneous collisions, the term 'thermal energy' is effectively synonymous with 'internal energy
'. In many statistical physics
texts, "thermal energy" refers to
, the product of Boltzmann's constant
and the absolute temperature
, also written as
. In a material, especially in condensed matter, such as a liquid or a solid, in which the constituent particles, such as molecules or ions, interact strongly with one another, the energies of such interactions contribute strongly to the internal energy of the body, but are not simply apparent in the temperature.
The term 'thermal energy' is also applied to the energy carried by a heat flow, although this can also simply be called heat or quantity of heat.
In an 1847 lecture titled "On Matter, Living Force, and Heat", James Prescott Joule
characterised various terms that are closely related to thermal energy and heat. He identified the terms latent heat
and sensible heat
as forms of heat each affecting distinct physical phenomena, namely the potential and kinetic energy of particles, respectively.
He described latent energy as the energy of interaction in a given configuration of particles, i.e. a form of potential energy
, and the sensible heat as an energy affecting temperature measured by the thermometer due to the thermal energy, which he called the living force.
Useless thermal energy
If the minimum temperature of a system's environment is
and the system's entropy is
, then a part of the system's internal energy amounting to
cannot be converted into useful work. This is the difference between the internal energy and the Helmholtz free energy
* Heat transfer
* Ocean thermal energy conversion
* Orders of magnitude (temperature)
* Thermal energy storage
Category: Thermodynamic properties
Category: Forms of energy