Thermal energy

Short description: Energy that is measured by temperature

Thermal radiation in visible light can be seen on this hot metalwork, due to blackbody radiation.

The "thermal energy" is used loosely in various contexts in physics and engineering, generally related to the kinetic energy of vibrating and colliding atoms in a substance. It can refer to several different well-defined physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, defined as a type of energy transfer (as is thermodynamic work); and the characteristic energy of a degree of freedom, [math]\displaystyle{ k_{\mathrm{B}}T }[/math], in a system that is described in terms of its microscopic particulate constituents (where [math]\displaystyle{ T }[/math] denotes temperature and [math]\displaystyle{ k_{\mathrm{B}} }[/math] denotes the Boltzmann constant).

## Contents

* 1 Relation to heat and internal energy
* 1.1 Macroscopic thermal energy
* 1.2 Microscopic thermal energy
* 2 Useless thermal energy
* 3 See also
* 4 References

## Relation to heat and internal energy

In thermodynamics, heat is energy transferred to or from a thermodynamic system by mechanisms other than thermodynamic work or transfer of matter, such as conduction, radiation, and friction.[1][2][3] Heat refers to a quantity transferred between systems, not to a property of any one system, or "contained" within it.[4] On the other hand, internal energy and enthalpy are properties 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.[citation needed]

### Macroscopic thermal energy

The internal energy of a body can change in a process in which chemical potential energy is converted into non-chemical energy. In such a process, the thermodynamic system can change its internal energy by doing work on its surroundings, or by gaining or losing energy as heat. It is not quite lucid to merely say that "the converted chemical potential energy has simply become internal energy". It is, however, convenient and more lucid to say that "the chemical potential energy has been converted into thermal energy". Such thermal energy may be viewed as a contributor to internal energy or to enthalpy, thinking of the contribution as a process without thinking that the contributed energy has become an identifiable component of the internal or enthalpic energies. The thermal energy is thus thought of as a "process entity" rather than as an "enduring physical entity". This is expressed in ordinary traditional language by talking of 'heat of reaction'.[citation needed]

The term "thermal energy" is also applied to the energy carried by a heat flow,[5] although this can also simply be called heat or quantity of heat.[citation needed]

### Microscopic thermal energy

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 [math]\displaystyle{ kT }[/math], the product of the Boltzmann constant and the absolute temperature, also written as [math]\displaystyle{ k_\text{B} T }[/math].[6][7]Cite error: Closing `</ref>` missing for `<ref>` tag 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.[citation needed]

## Useless thermal energy

If the minimum temperature of a system's environment is [math]\displaystyle{ T_\text{e} }[/math] and the system's entropy is [math]\displaystyle{ S }[/math], then a part of the system's internal energy amounting to [math]\displaystyle{ S \cdot T_\text{e} }[/math] cannot be converted into useful work. This is the difference between the internal energy and the Helmholtz free energy.[citation needed]

## See also

* Geothermal energy
* Geothermal heating
* Geothermal power
* Heat transfer
* Ocean thermal energy conversion
* Orders of magnitude (temperature)
* Thermal battery
* Thermal energy storage

## References

1. ↑ Bailyn, M. (1994). A Survey of Thermodynamics, American Institute of Physics Press, New York, ISBN:0-88318-797-3, p. 82.
2. ↑ Born, M. (1949). Natural Philosophy of Cause and Chance, Oxford University Press, London, p. 31.
3. ↑ Thomas W. Leland Jr., G. A. Mansoori, ed., Basic Principles of Classical and Statistical Thermodynamics, http://www.uic.edu/labs/trl/1.OnlineMaterials/BasicPrinciplesByTWLeland.pdf, retrieved 2014-01-02
4. ↑ Robert F. Speyer (2012). Thermal Analysis of Materials. Materials Engineering. Marcel Dekker, Inc.. p. 2. ISBN 978-0-8247-8963-3.
5. ↑ Ashcroft, Neil; Mermin, N. David (1976). Solid State Physics. Harcourt. pp. 20. ISBN 0-03-083993-9. "We define the thermal current density [math]\displaystyle{ {\bf j}^q }[/math] to be a vector parallel to the direction of heat flow, whose magnitude gives the thermal energy per unit time crossing a unit area perpendicular to the flow."
6. ↑ Reichl, Linda E. (2016). A Modern Course in Statistical Physics. John Wiley and Sons. pp. 154. ISBN 9783527690466.
7. ↑ Kardar, Mehran (2007). Statistical Physics of Particles. Cambridge University Press. pp. 243. ISBN 9781139464871.

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Energy

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Fundamental concepts|

* Energy
* Units
* Conservation of energy
* Energetics
* Energy transformation
* Energy condition
* Energy transition
* Energy level
* Energy system
* Mass
* Negative mass
* Mass–energy equivalence
* Power
* Thermodynamics
* Quantum thermodynamics
* Laws of thermodynamics
* Thermodynamic system
* Thermodynamic state
* Thermodynamic potential
* Thermodynamic free energy
* Irreversible process
* Thermal reservoir
* Heat transfer
* Heat capacity
* Volume (thermodynamics)
* Thermodynamic equilibrium
* Thermal equilibrium
* Thermodynamic temperature
* Isolated system
* Entropy
* Free entropy
* Entropic force
* Negentropy
* Work
* Exergy
* Enthalpy

Types|

* Kinetic
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Energy carriers|

* Radiation
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Primary energy|

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Energy system
components|

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* Integrated gasification combined cycle
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* Solar power tower
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* Wind power
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Use and
supply|

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