Thermal energy

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.^[1] 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 possess "heat".^[2] 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.

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Internal energy

Internal energy – the sum of all microscopic forms of energy of a system. It is related to the molecular structure and the degree of molecular activity and may be viewed as the sum of kinetic and potential energies of the molecules; it consists of the following types of energies:^[3]

Definitions

System of N particles

According to the equipartition theorem, it is possible to define thermal energy. In a system of N molecules, each with f degrees of freedom, and if there are no other (non-quadratic) temperature-dependent forms of energy, then the total thermal energy of the system is:^[2]

To note, U_thermal is almost never the total energy of a system; for instance, there can be static energy that doesn't change with temperature, such as bond energy or rest energy (E=mc²).

Other definitions

Thermal energy per particle is also called the average translational kinetic energy possessed by free particles given by equipartition of energy.^[4]

Thermal energy is the difference between the internal energy of an object and the amount that it would have at absolute zero.^{[citation needed]} It includes the quantity of kinetic energy due to the motion of the internal particles of an object, and is increased by heating and reduced by cooling.

In a monatomic ideal gas, the thermal energy is exactly given by the kinetic energy of the constituent particles.^{[citation needed]}

See also

References

1. ^ Thermal energy - Britannica
2. ^ ^{a b} Schroeder, Daniel, R. (2000). Thermal Physics. New York: Addison Wesley Longman. ISBN 0201380277. 
3. ^ Cengel, Yungus, A.; Boles, Michael (2002). Thermodynamics - An Engineering Approach, 4th ed.. McGraw-Hill, 17-18. ISBN 0-07-238332-1. 
4. ^ Thermal energy – Hyperphysics