A physical system is any defined portion of the physical world chosen for study under the laws of physics. The system can be as small as an atom or as large as a planet's atmosphere; everything outside it is called the environment and is treated only insofar as it affects the system. The act of selecting a system is a modelling decision: a researcher decides which parts of reality to include in detailed equations and which to treat as external influences, a process sometimes described as analysis.
Defining features and classification
Physical systems contain matter, energy, or both, and are described by state variables (such as position, temperature, momentum or quantum state). Systems are classified by how they exchange matter and energy with their environment: isolated systems exchange neither, closed systems exchange energy but not matter, and open systems exchange both. In quantum mechanics it is common to treat small groups of particles as isolated quantum systems until interactions are introduced.
History and conceptual development
The idea of isolating part of nature for study is fundamental to scientific method. As classical mechanics, thermodynamics and later quantum theory developed, physicists formalised system boundaries and conservation laws to predict evolution. Treating the entire universe as one system is possible in principle, but in practice scientists analyse subsystems to make problems tractable.
Typical examples used in instruction and research include macroscopic devices and structures — for example the Earth's atmosphere, a hydraulic pump, or an airplane wing — and microscopic or fundamental systems such as a crystal, an atomic nucleus, or elementary particles. Everyday objects like a glass of water are also modelled as systems to study heat flow, fluid dynamics and chemical exchange.
Why define a system? Doing so simplifies equations, clarifies which conservation laws apply, and isolates the mechanisms responsible for observed behaviour. Engineers design control strategies for pumps and wings by modelling them as open systems; chemists analyse reactors by tracking matter and energy across system boundaries; physicists studying quantum decoherence consider interactions between a small system and a large environment.
Unlike a purely conceptual construct such as a Turing machine, a physical system refers to tangible or measurable entities and their interactions, although the boundary between conceptual and physical models can be blurred when idealisations are used. Overall, the system concept is a central organising tool across physics and engineering, enabling focused study of complex natural and artificial phenomena.