Thermal energy

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1. The energy of a particle in a system that moves relative to the system is the particle's kinetic energy ("motional energy"). 2. The energy of a particle in the system by virtue of its position in the system (displacement from other particles) is that particle's potential energy ("positional energy"). 3. At any moment any given particle may have kinetic energy, potential energy, or both. 4. At any moment the total internal energy (thermal energy) of a thermodynamic system is the sum of the kinetic energies and the potential energies of all the particles in the system at that moment. 5. The total internal energy (thermal energy) of a non-thermodynamically isolated system will change with time. The total internal energy (thermal energy) of a thermodynamically isolated system is constant.

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Thermal energy has no generally agreed definition and the term will not usually be found in most dictionaries of physics or science. In everyday usage, thermal energy may be regarded either as 1. a synonym for thermodynamic energy (itself a synonym for internal energy) or as 2. a synonym for heat.

1. Seen as the internal energy of a system, there are two components to thermal energy. One component is the internal potential energy of the system - the energy the system contains at any moment due to the relative placement within the system of all its constituent parts. The second component is the internal kinetic energy of the system - the energy the system contains at any moment due to the relative motion within the system of all its constituent parts.

There may be a constant interchange within the system of internal potential energy and internal kinetic energy. However, in any thermodynamically isolated system the total thermal energy (the sum of the internal potential energy and the internal kinetic energy) remains constant.

In this context, the thermal energy of an ideal gas is only the sum of the kinetic energies of the idealised, volumeless particles which interact only with the walls of any container and not with each other so lack potential energy.

2. In the opinion of Whelan and Hodgson, authors of the classical A-level Physics reference text used for many years in the UK, thermal energy is to be preferred to "heat" when the latter is used as a noun: unless used rigorously starting from its traditional thermodynamic definition, its better to use "heat" loosely only as a verb. For example, it would be better to state that "if system A is at a higher temperature than system B, then, unless the two systems are thermally isolated from each other, thermal energy will flow from system A to system B. That is, system A will heat system B until the two systems are at the same temperature".