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In thermodynamics, thermal energy refers to the internal energy present in a system by virtue of its temperature.〔(Thermal energy entry in Britannica Online )〕 The average translational kinetic energy possessed by free particles in a system of free particles in thermodynamic equilibrium (as measured in the frame of reference of the center of mass of that system) may also be referred to as the thermal energy per particle.〔(Thermal energy entry in Hyperphysics web site )〕 Microscopically, the thermal energy may include both the kinetic energy and potential energy of a system's constituent particles, which may be atoms, molecules, electrons, or particles. It originates from the individually random, or disordered, motion of particles in a large ensemble. In ideal monatomic gases, thermal energy is entirely kinetic energy. In other substances, in cases where some of thermal energy is stored in atomic vibration or by increased separation of particles having mutual forces of attraction, the thermal energy is equally partitioned between potential energy and kinetic energy. Thermal energy is thus equally partitioned between all available degrees of freedom of the particles. As noted, these degrees of freedom may include pure translational motion in gases, rotational motion, vibrational motion and associated potential energies. In general, due to quantum mechanical reasons, the availability of any such degrees of freedom is a function of the energy in the system, and therefore depends on the temperature (see heat capacity for discussion of this phenomenon). Macroscopically, the thermal energy of a system at a given temperature is proportional to its heat capacity. However, since the heat capacity differs according to whether or not constant volume or constant pressure is specified, or phase changes permitted, the heat capacity cannot be used to define thermal energy unless it is done in such a way as to ensure that only heat gain or loss (not work) makes any changes in the internal energy of the system. Usually, this means specifying the "constant volume heat capacity" of the system so that no work is done. Also the heat capacity of a system for such purposes must not include heat absorbed by any chemical reaction or process. == Differentiation from heat == Heat is the thermal energy transferred across a boundary of one region of matter to another. As a process variable, heat is a characteristic of a process, not a property of the system; it is not ''contained'' within the boundary of the system.〔 On the other hand, thermal energy is a property of a system, and exists on both sides of a boundary. Classically (see ideal gas), thermal energy is the statistical mean of the microscopic fluctuations of the kinetic energy of the systems' particles, and it is the source and the effect of the transfer of heat across a system boundary. According to the zeroth law of thermodynamics, heat is exchanged between thermodynamic systems in thermal contact only if their temperatures are different.〔For the purpose of distinction, a system is defined to be enclosed by a well-characterized boundary.〕 If heat traverses the boundary in direction ''into'' the system, the internal energy change is considered to be a positive quantity, while ''exiting'' the system, it is negative. Heat flows from the hotter to the colder system, decreasing the thermal energy of the hotter system, and increasing the thermal energy of the colder system. Then, when the two systems have reached thermodynamic equilibrium, they have the same temperature, and the net exchange of thermal energy vanishes and heat flow ceases. Even after they reach thermal equilibrium, thermal energy continues to be exchanged between systems, but the ''net'' exchange of thermal energy is zero, and therefore there is no heat. After the transfer, the energy transferred by heat is called by other terms, such as thermal energy or latent energy.〔 See box definition: "Heat transfer (or heat) is energy in transit due to a temperature difference." See page 14 for the definition of the thermal component of the thermodynamic internal energy.〕 Although heat often ends up as thermal energy after transfer, it may cause changes other than a change in temperature. For example, the energy may absorbed or released in phase transitions, such as melting or evaporation, which are the gain or loss of a form of potential energy called latent heat. Thermal energy may be increased in a system by other means than heat, for example when mechanical or electrical work is performed on the system. Heat flow may cause work to be performed on a system by compressing a system's volume, for example. A heat engine uses the movement of thermal energy (heat flow)to do mechanical work. No qualitative difference exists between the thermal energy added by other means. There is also no need in classical thermodynamics to characterize the thermal energy in terms of atomic or molecular behavior. A change in thermal energy induced in a system is the product of the change in entropy and the temperature of the system. Rather than being itself the thermal energy involved in a transfer, heat is sometimes also understood as the process of that transfer, i.e. ''heat'' functions as a verb. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Thermal energy」の詳細全文を読む スポンサード リンク
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