1 result for ‘Thermal energy’
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system temperature. This quantity may be difficult to determine or even meaningless unless the system has attained its temperature only through heating, and not been subjected to work input or output, or any other energy-changing processes. The internal energy of a system, also often called the thermodynamic energy, includes other forms of energy in a thermodynamic system in addition to thermal energy, namely forms of potential energy that do not influence temperature, such as the chemical energy stored in its molecular structure and electronic configuration, intermolecular interactions associated with phase changes that do not influence temperature (i.e. , latent energy), and the nuclear binding energy that binds the sub-atomic particles of matter. Microscopically, the thermal energy is partly the kinetic energy of a system's constituent particles, which may be atoms, molecules, electrons, or particles in plasmas. 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, this vibrational part of the thermal energy is stored equally partitioned between potential energy of atomic vibration, and kinetic energy of atomic vibration. Thermal energy is thus equally partitioned between all available quadratic degrees of freedom of the particles. As noted, these degrees of freedom may include pure translational motion in gases, in rotational states, and as potential and kinetic energy in normal modes of vibrations in intermolecular or crystal lattice vibrations. 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 phenomena). Macroscopically, the thermal energy of a system at a given temperature is related proportionally 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 define thermal energy unless it is done in such a way as to insure that only heat gain or loss (not work) make any changes in the internal energy of the system. Usually, this means constant volume heat capacity so that no work is done, and also the heat capacity of a system for such purposes must not include heat absorbed by any chemical reaction or process. Thermal energy is not a state function, or a property of a system, since the total thermal energy needed to warm a system to a given temperature depends on the path taken to attain the temperature, unless all forms of work and chemical potential change in the system are zero or neglectable. Thus, thermal energy is process-dependent except in systems in which processes to change internal energy other than heating, can be neglected. Nevertheless, when this is true, thermal energy and heat capacity may be a useful concept in the study of heat transfer in solids and liquids, in engineering and other disciplines. Heat, in the strict use in physics, is characteristic only of a process, i.e. it is absorbed or produced as an energy exchange, always as a result of a temperature difference. Heat is thermal energy in the process of transfer or conversion across a boundary of one region of matter to another, as a result of a temperature difference. In engineering, the terms "heat" and "heat transfer" are thus used nearly interchangeably (heat transfer is the rate of heat flow in time, or the heat power), since heat is always understood to be in the process of transfer. The energy transferred by heat is called by other terms (such as thermal energy or latent energy) when this energy is no longer in net transfer, and has become static. Thus, heat is not a static property of matter. Matter does not contain heat, but rather thermal energy, and even the thermal energy is subject to transformations into and out of other types of energy, and so can be considered to be "conserved" only when these processes are small. When two thermodynamic systems with different temperatures are brought into diathermic contact, they spontaneously exchange energy as heat, which is a transfer of thermal energy from the system of higher temperature to the colder system. Heat may cause work to be performed on a system, for example, in form of volume or pressure changes. This work may be used in heat engines to convert thermal energy into other forms of energy. In geothermal power plants it is used for the generation of electricity. When two systems have reached a thermodynamic equilibrium, they have attained the same temperature and the net exchange of thermal energy vanishes, and heat flow ceases.