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:

{\displaystyle \mathrm {d} U=\delta W+\delta Q}

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:

{\displaystyle \mathrm {d} S={\frac {\delta Q}{T}}+{\frac {\delta W_{\mathrm {diss.} }}{T}}}

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:

{\displaystyle m=\mathrm {const.} }

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).


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