Thermal energy
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The term "thermal energy" is often used ambiguously in physics and engineering.[1] It can denote several different physical concepts, including:
- Internal energy: The total energy contained within a body of matter or radiation.
- Heat: Energy transfer between two systems due to temperature difference.
- The characteristic energy kBT associated with a single microscopic degree of freedom, where T denotes temperature and kB denotes the Boltzmann constant.
Zemansky (1970) has argued that the term “thermal energy” is best avoided due to its ambiguity. He suggests using more precise terms like “internal energy” and “heat” to avoid confusion.[1] However, there is little agreement over this and the term is commonly used in textbooks.
Relation between heat and internal energy
[edit]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.[2][3][4] Heat refers to a quantity transferred between systems, not to a property of any one system, or "contained" within it.[5] 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
[edit]When the term 'thermal energy' refers to the internal energy of a body, one can think of chemical energy belonging to distinct molecules, and of global joint energy involved in the interactions between molecules and suchlike. Such thermal energy may be viewed as contributing 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".
Chemical internal energy
[edit]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, sometimes convenient to say that "the chemical potential energy has been converted into thermal energy". This is expressed in ordinary traditional language by talking of 'heat of reaction'.[citation needed]
Potential energy of internal interactions
[edit]In a body of 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 immediately apparent in the kinetic energies of molecules, as manifest in temperature. Such energies of interaction may be thought of as contributions to the global internal microscopic potential energies of the body.[citation needed]
Microscopic thermal energy
[edit]In a statistical mechanical account of an ideal gas, in which the molecules move independently between instantaneous collisions, the internal energy is just 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".[6]
In many statistical physics texts, "thermal energy" refers to , the product of the Boltzmann constant and the absolute temperature, also written as .[7][8][9][10]
Thermal current density
[edit]The term "thermal energy" is also applied to the energy carried by a heat flow,[11] although this can also simply be called heat or quantity of heat.[citation needed]
See also
[edit]- Geothermal energy
- Geothermal heating
- Geothermal power
- Heat transfer
- Ocean thermal energy conversion
- Orders of magnitude (temperature)
- Thermal energy storage
References
[edit]- ^ a b Zemansky, Mark W. (1970-09-01). "The Use and Misuse of the Word "Heat" in Physics Teaching". The Physics Teacher. 8 (6): 295–300. doi:10.1119/1.2351512. ISSN 0031-921X.
- ^ Bailyn, M. (1994). A Survey of Thermodynamics, American Institute of Physics Press, New York, ISBN 0-88318-797-3, p. 82.
- ^ Born, M. (1949). Natural Philosophy of Cause and Chance, Oxford University Press, London, p. 31.
- ^ Thomas W. Leland Jr., G. A. Mansoori (ed.), Basic Principles of Classical and Statistical Thermodynamics (PDF), archived (PDF) from the original on 2011-09-28, retrieved 2014-01-02
- ^ Robert F. Speyer (2012). Thermal Analysis of Materials. Materials Engineering. Marcel Dekker, Inc. p. 2. ISBN 978-0-8247-8963-3.
- ^ Kittel, Charles (2012). Elementary Statistical Physics. Courier Corporation. p. 60. ISBN 9780486138909.
- ^ Reichl, Linda E. (2016). A Modern Course in Statistical Physics. John Wiley and Sons. p. 154. ISBN 9783527690466.
- ^ Kardar, Mehran (2007). Statistical Physics of Particles. Cambridge University Press. p. 243. ISBN 9781139464871.
- ^ Feynman, Richard P. (2000). "The Computing Machines in the Future". Selected Papers of Richard Feynman: With Commentary. World Scientific. ISBN 9789810241315.
- ^ Feynman, Richard P. (2018). Statistical Mechanics: A Set of Lectures. CRC Press. p. 265. ISBN 9780429972669.
- ^ Ashcroft, Neil; Mermin, N. David (1976). Solid State Physics. Harcourt. p. 20. ISBN 0-03-083993-9.
We define the thermal current density 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.