|
Latent heat is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process that is specified in some way. An example is latent heat of fusion for a phase change, melting, at a specified temperature and pressure. The term was introduced around 1762 by Scottish chemist Joseph Black. It is derived from the Latin ''latere'' (''to lie hidden''). Black used the term in the context of calorimetry where a heat transfer caused a volume change while the thermodynamic system's temperature was constant. In contrast to latent heat, sensible heat involves an energy transfer that results in a temperature change of the system. ==Usage== The terms ″sensible heat″ and ″latent heat″ are not special forms of energy; instead they measure two kinds of change in a material or in a thermodynamic system. ″Sensible heat″ measures change in a body's internal energy that may be ″sensed″ with a thermometer. ″Latent heat″ measures change in internal energy that seems hidden from a thermometer – the temperature reading doesn't change. Heat is energy in the process of transferring between a system and its surroundings, other than as work or by transfer of matter. Both sensible and latent heats are observed in many processes of transport of energy in nature. Latent heat is associated with the phase changes of atmospheric water vapor, mostly vaporization and condensation, whereas sensible heat is energy transferred that affects the temperature of the atmosphere. The original usage of the term, as introduced by Black, was applied to systems that were intentionally held at constant temperature. Such usage referred to ''latent heat of expansion'' and several other related latent heats. These latent heats are defined independently of the conceptual framework of thermodynamics.〔Bryan, G.H. (1907). ''Thermodynamics. An Introductory Treatise dealing mainly with First Principles and their Direct Applications'', B.G. Tuebner, Leipzig, pages 9, 20–22.〕 When a body is heated at constant temperature by thermal radiation in a microwave field for example, it may expand by an amount described by its ''latent heat with respect to volume'' or ''latent heat of expansion'', or increase its pressure by an amount described by its ''latent heat with respect to pressure''.〔Maxwell, J.C. (1872). ''Theory of Heat'', third edition, Longmans, Green, and Co., London, page 73.〕 Two common forms of latent heat are latent heat of fusion (melting) and latent heat of vaporization (boiling). These names describe the direction of energy flow when changing from one phase to the next: from solid to liquid, and liquid to gas. In both cases the change is endothermic, meaning that the system absorbs energy. If the change is exothermic, then energy is released. For example, when water evaporates, energy is transferred from a water molecule to an air molecule that contains less water vapor than its surroundings. Because energy is required for the water molecule to overcome the forces of attraction between water particles, the transition from water to vapor requires an input of energy and causes a temperature drop in the water molecule's surroundings. If the vapor then condenses to a liquid on a surface, then the vapor's latent energy absorbed during evaporation is released as the liquid's sensible heat onto the surface. The large value of the enthalpy of condensation of water vapor is the reason that steam is a far more effective heating medium than boiling water, and is more hazardous. 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「latent heat」の詳細全文を読む スポンサード リンク
|