Physical system
A material system, physical system or concrete system is an existing in space-time physical object, or a collection of such objects that can be distinguished in a well defined way of his environment as a whole. Typical examples are the solar system, an atom, a crystal or a fluid. Technical systems (for example, clock ), or biological systems, such as a cell, are physical systems. As a rule, the universe is seen as a physical system, even though it has no environment. Mere concepts, such as a canonical ensemble, however, are not among the physical systems.
Each physical system is completely determined by its composition, its environment, its structure and its internal system effective mechanisms. The properties of physical systems can be approximately described by idealized physical or mathematical models. With the control and regulation of physical systems, the control technology is concerned.
The systematic development of general concepts of physical systems is the subject of the philosophy of physics and ontology. In science and engineering, the term is ubiquitous, being used here often either as an undefined reason the term or refers only to a discipline - specific subset of physical systems. For example, the thermodynamics concerned with thermodynamic systems. The general representation used in this paper based on the ZUSM model ( ZUSM = composition, environment, structure, mechanism ) is based on the system concept of the physicist and philosopher Mario Bunge, but also related system concepts from other authors are considered.
Composition
The composition of the system is the set of all of its components. In open systems (see below) may change with time, the composition. Systems that are not composed of other objects are referred to as simple systems. Examples of simple systems are electrons, quarks and other elementary particles. Most physical systems are composed of other objects, one then speaks of complex systems or wholes. Each physical system is a subsystem of a larger physical system. An exception is the universe, which is not a subsystem of a larger system.
For determining the composition of different criteria proposed in the literature. Bunge differs depending on the thickness of intrinsic bonds between two classes of composition: the aggregation and combination. The aggregation is a loose joint storage of physical objects, such as the juxtaposition of sand grains in a sand pile. The combination, however, is the result of stronger bonds between the components of the system. Such systems also referred to as cohesive wholes are often characterized by a more or less pronounced resistance. In Bunge's theory of physical systems only the latter cohesive systems are physical systems. Objects that have no or only weak bonds with the system components, not part of the system, but to the environment (see also next section). Other authors define the composition does not bonds but geometrically as the content of - depending on the specific question of arbitrarily selectable - space volume. In control theory, the system composition is often defined by functional relationships as the amount of objects, together meet the specific technical purpose.
Environment and system boundary
Each physical system - with the exception of the Universe - exists in a system environment, from which it is separated by its system boundary. The environment is thus defined as the set of all physical objects outside the system. In the description of a system not the whole area involved in the rule. Only the objects in the environment, ie the non-system forming group of the universe, considered that have a relevant impact on the system.
The system boundary is defined in Bunge's ontology as the set of system components that are directly linked with items from the environment. Typical examples are the cell wall of a cell, the surface of a water droplet, the boundary layer of a fluid or the inner wall of a pipe. In an alternative geometric system definition, the system boundary is, however, the surface of the volume of space, which is based on the system definition. In this case the system boundary does not necessarily coincide with the position of material objects.
Open, closed and isolated systems
Physical systems are due to unavoidable physical interactions, such as the gravitational or thermal radiation, never completely isolated from their surroundings. The composition or the state of a physical system, may be altered by the transfer of matter and heat. The mass or energy transport can be inhibited by natural or artificial barriers in whole or in part. Depending on the type of insulation is a distinction between open systems, closed systems and closed systems. Open systems can exchange matter and energy with their environment. In closed systems no matter, but rather an energy exchange with the environment is possible. In a closed system refers to a system that exchanges neither matter nor energy with its environment. The universe has no limit and no system environment, it is therefore neither an open nor a closed system.
In thermodynamics and statistical physics open, closed and isolated systems are described by a grand canonical, canonical and microcanonical ensembles. Ensembles are quantities physically possible systems, they are therefore no real physical systems, but abstract concepts.
Structure
The totality of all relations of a system with one another and with the components of the environment, forms its structure. The relations between the parts of a system are called internal structure or Endostruktur. The relations between the system components and objects from the environment form the external structure or exostructure. Bunge distinguishes further between binding relations ( links) and non-bonding relations. A bonding relationship between two objects is in front of x and y when the condition of y changes when the relationship is x. Otherwise, the relation is non- binding. Typical examples of links are the fundamental forces of physics. Non -binding relations are, for example, spatial or temporal relations. For example, the mere fact that the two objects at a distance of one meter, have no effect on the two objects.
Mechanisms
The explained in the last three sections of composition concepts, environment and structure describe only snapshots of systems. Although the state of real material systems can be approximately stationary for certain time periods may, sooner or later it comes, however, always be changes. In addition to their formation and destruction have material systems usually also more characteristic dynamic processes. Typical examples are the function of a clock, or the photosynthesis of chloroplasts. The characteristic processes of a system are called its mechanisms or functions. The mechanisms of a system may, but need not be causal in nature. Bunge distinguishes between state changes by self-motion (example: inertial motion ) by causal processes (example: collision of two billiard balls ) or by random events (example: radioactive decay of an atomic nucleus ).
Properties of composite systems
The thermodynamics differs in composition between intensive and extensive variables. The former do not change with the amount of substance, the latter are proportional to the amount of substance. Very often, however, material systems have also qualitatively new properties that might be different radically from the properties of its components. This occasionally occurs qualitatively new features in the composite system is also called emergence. In particular, in quantum mechanics, there are numerous effects that result from the entanglement, which, arising for quantum systems specific form of the composition. Examples are the decoherence of macroscopic quantum systems, the formation of electronic states in atoms, molecules or solids as well as the occurrence of non-local correlations in experiments on Bell 's inequality.