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In the physical sciences, an energy transfer or 'energy exchange' from one system to another is said to occur when an amount of energy crosses the boundary between them, thus increasing the energy content of one system while decreasing the energy content of the other system by the same amount. The transfer is characterized by the quantity of energy transferred, which can be specified in energy units such as the joule (J), in combination with the direction of the transfer, which can be specified as ''in'' (''to'') or ''out of'' (''from'') one system or the other. The transfer occurs in a process which changes the state of each system. Thermodynamics recognises three categories (mechanisms or modes) of such transfers: *Heat(), which can occur via conduction, thermal electromagnetic radiation, and other machines *Work (thermodynamics), for example, electric power transmission is a (non-mechanical) thermodynamic work process that can transfer energy from a power generation station to your home, where a pump can use it to perform mechanical work on water to lift it out of your basement. *Mass transfer, processes in which material moves from one system to the other, carrying its energy with it Note that these mechanisms or modes refer to the kind of ''transfer'' or ''process'' acting on the energy, not the kinds (or forms, types, etc.) of ''energy'' that's being transferred. The increase or decrease in energy of each system can be in any of several forms of energy. == Units and sign +/- of an energy transfer == Each system is said to be in its initial state at the start of the transfer and its final state after the transfer process has completed. To quantify the transfer precisely, we can choose to view the transfer from either of the two systems' point of view; then the amount of the transfer is given by the chosen system's final energy minus its initial energy. As a ''difference'' between two amounts of energy, the transfer amount itself has energy units (the SI unit being the joule (J)), and is positive (>0) if the chosen system's energy increased (representing a transfer of a magnitude of energy in or to it), or negative (<0) if the chosen system's energy decreased (representing a transfer of that magnitude from or out of the chosen system) during the energy transfer process. In practice, the system whose energy is increasing is often (but not always) chosen, in order for the transfer amount to be positive. 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「Energy transfer」の詳細全文を読む スポンサード リンク
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