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 is defined as the sum of kinetic energy of random motion between the particles.

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 (although in strict definition heat is a transfer (flow) of thermal energy).

1. 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. One thing to remember is that [Internal energy] is bigger concept than a thermal energy. Internal energy can be defined as sum of [Kinetic] and [Potential] energy of molecule. In the sense that Thermal energy is a sum of Kinetic energy in the molecule, the Thermal energy and Kinetic energy is proportional to each other.

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, it is 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".