In thermodynamics (and in nearly all other branches of natural science) the part on which
the focus of interest lies is called the system. The remaining is called the surrounding, i.e.
universe
= system + surrounding
The astonishing fact about thermodynamics is that all measurements of properties
and changes of the system are measured in the surrounding. The separation is important for many practical purposes: e.g. the
entropy of a system can decrease, but the entropy of the universe can only increase (2nd Law).
Several
kinds of systems exist:
Open systems: Exchange of matter and energy between system and surrounding exist.
Closed system: Exchange of energy between system and surrounding exist.
Isolated system: No exchange with surrounding exists.
Within the system ”phases” exist. A phase is a chemically homogeneous body on a macroscopic scale. No discontinuous changes of states are possible within a phase. Within a multi-phase material (composite) properties can change discontinuously at the phase boundary.
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© J. Carstensen (TD Kin I)