Thermal energy is the part of the total, internal energy of a thermodynamic system or sample of matter that results in the system's temperature. The internal energy, 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, such as the chemical energy stored in its molecular structure and electronic configuration, intermolecular interactions, and the nuclear energy that binds the sub-atomic particles of matter.
Microscopically, the thermal energy is 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. The thermal energy is equally partitioned between all available quadratic degrees of freedom of the particles. These degrees of freedom may include pure translational motion in fluids, normal modes of vibrations, such as intermolecular vibrations or crystal lattice vibrations, or rotational states. In general, the availability of any such degrees of freedom is a function of the energy in the system, and therefore depends on the temperature.
When two thermodynamic systems with different temperatures are brought into diathermic contact, they exchange energy in form of heat, which is a conversion of thermal energy from the system of higher temperature to the colder system. This heat may cause work to be performed on each system, for example, in form of volume or pressure changes. This work may be used in heat engines to convert thermal energy into mechanical energy. When two systems have reached a thermodynamic equilibrium, they have attained the same temperature and the net exchange of thermal energy ceases.
Thermal energy is distinct from heat. In the strict use in physics, heat is a characteristic only of a process, i.e. it is absorbed or produced as an energy exchange, but it is not a static property of matter. Matter does not contain heat, but thermal energy. Heat is thermal energy in the process of transfer or conversion across a boundary of one region of matter to another.
Thermal energy is the portion of internal energy that is responsible for a system's temperature. Microscopically, the thermal energy is identified with mechanical kinetic energy of the constituent particles or other forms of kinetic energy associated with quantum-mechanical microstates. The distinguishing difference between the terms kinetic energy and thermal energy is that thermal energy is the mean energy of disordered, i.e. random, motion of the particles or the oscillations in the system. The conversion of energy of ordered motion to thermal energy results from collisions.
In engineering and technology, and particularly in fields that deal with civil energy use and conservation in building construction, heating systems, and power generation, heat and thermal energy are often indiscriminately used interchangeably.
In thermodynamics, heat must always be defined as energy in exchange between two systems, or a single system and its surroundings. 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 is never a property of the system, nor is it contained within the boundary of the system.
In contrast to heat, thermal energy exists on both sides of a boundary. It 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. Statistically, thermal energy is always exchanged between systems, even when the temperatures on both sides is the same, i.e. the systems are in thermal equilibrium. However, at equilibrium, the net exchange of thermal energy is zero, and therefore there is no 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. 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.
Heat exchanged with a system may cause changes other than a change in thermal energy. For example, it may cause phase transitions, such as melting or evaporation, which are changes in the configuration of a material. Since such an energy exchange is not observable by a change in temperature, it is called a latent heat and represents a change in the potential energy of the system.
Rather than being itself the thermal energy involved in a transfer, heat is sometimes also understood as the process of transfer, i.e. it has the functioning as a verb.
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