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 recognizes three categories (mechanisms or modes) of such transfers:
- Heat[ing], which can occur via conduction, thermal electromagnetic radiation, and other mechanisms
- 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.
Although all the three categories of transfer play the same role in describing a process, they often have different linguistic constructions around them. Work, especially mechanical work, is often said to be "performed by" one system "on" the system whose point of view is "chosen" as in the previous section. Thus, performing a positive amount of work corresponds to an increase in the energy of the system "on" which the work was performed. Usage of heat varies, which can lead to confusion. Historically, heat was once considered a form of energy in itself that a system or object could possess, rather than describing a (simultaneous) change in (two) systems' energy due to the occurrence of a particular transfer process. This concept often persists in common usage and also affects the way in which the term is used in science. The term "work transfer" is not used, but the term "heat transfer" is common even though, strictly speaking, the word "transfer" is redundant since heat is already inherently a transfer. Other constructions include "energy transferred as heat", "energy transferred by heat", or energy transferred by a process called "heating" rather than heat. Terminology for mass transfer is less problematic because it can be used in the same ways as the word "transfer" itself.