section_path: Energy transfer/Closed systems
section_name: Closed systems
paragraph_text: Closed systems
  
  Energy transfer can be considered for the special case of systems which are closed to transfers of matter. The portion of the energy which is transferred by conservative forces over a distance is measured as the work the source system does on the receiving system. The portion of the energy which does not do work during the transfer is called heat. Energy can be transferred between systems in a variety of ways. Examples include the transmission of electromagnetic energy via photons, physical collisions which transfer kinetic energy, tidal interactions, and the conductive transfer of thermal energy.
  
  Energy is strictly conserved and is also locally conserved wherever it can be defined. In thermodynamics, for closed systems, the process of energy transfer is described by the first law:
  
  where E {\displaystyle E} is the amount of energy transferred, W {\displaystyle W} represents the work done on or by the system, and Q {\displaystyle Q} represents the heat flow into or out of the system. As a simplification, the heat term, Q {\displaystyle Q}, can sometimes be ignored, especially for fast processes involving gases, which are poor conductors of heat, or when the thermal efficiency of the transfer is high. For such adiabatic processes,
  
  This simplified equation is the one used to define the joule, for example.