Closed system
A closed system is a thermodynamic system beyond whose system boundaries no matter can pass. In contrast to closed or open systems, however, an exchange of energy with the environment is quite permissible. A closed system can be imagined as a dense container, which is deformable and has walls that are thermally conductive. The first law of thermodynamics states for closed systems:
In words, the change in internal energy is equal to the sum of work supplied (e.g., by compression) and heat supplied (e.g., by conduction). The second law is:
This means that the entropy in the closed system increases with the absorbed heat and the dissipated work.
Moreover, conservation of mass applies within the framework of classical physics:
However, in relativistic physics, the decrease of the energy content of the system automatically leads to the decrease of the mass of the system (see mass-energy equivalence).