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CHAPTER 4: Introduction to Systems Theory

# (b). Definitions of Systems and Models

As suggested in the previous section, a system is a assemblage of interrelated parts that work together by way of some driving process (see Figure 4b-1). Systems are often visualized or modeled as component blocks that have connections drawn between them. For example, the illustration below describes the interception of solar radiation by the Earth. In this system, the Earth and Sun, the parts or component blocks, are represented by two colored circles of different size. The process of solar emission and the interception of the Sun's emitted radiation by the Earth (the connection) is illustrated by the drawn lines.

 Figure 4b-1: Simple visual model of solar radiation being emitted from the Sun and intercepted by the Earth.

Most systems share the same common characteristics. These common characteristics include the following:

1. Systems have a structure that is defined by its parts and processes.
2. Systems are generalizations of reality.
3. Systems tend to function in the same way. This involves the inputs and outputs of material (energy and/or matter) that is then processed causing it to change in some way.
4. The various parts of a system have functional as well as structural relationships between each other.
5. The fact that functional relationships exist between the parts suggests the flow and transfer of some type of energy and/or matter.
6. Systems often exchange energy and/or matter beyond their defined boundary with the outside environment, and other systems, through various input and output processes.
7. Functional relationships can only occur because of the presence of a driving force.
8. The parts that make up a system show some degree of integration - in other words the parts work well together.

Within the boundary of a system we can find three kinds of properties:

Elements - are the kinds of parts (things or substances) that make up a system. These parts may be atoms or molecules, or larger bodies of matter like sand grains, rain drops, plants, animals, etc.
Attributes - are characteristics of the elements that may be perceived and measured. For example: quantity, size, color, volume, temperature, and mass.
Relationships - are the associations that occur between elements and attributes. These associations are based on cause and effect.

We can define the state of the system by determining the value of its properties (the elements, attributes, and/or relationships).

Scientists have examined and classified many types of systems. Some of the classified types include:

Isolated System - a system that has no interactions beyond its boundary layer. Many controlled laboratory experiments are this type of system.
Closed System - is a system that transfers energy, but not matter, across its boundary to the surrounding environment. Our planet is often viewed as a closed system.

Open System - is a system that transfers both matter and energy can cross its boundary to the surrounding environment. Most ecosystems are example of open systems.

Morphological System - this is a system where we understand the relationships between elements and their attributes in a vague sense based only on measured features or correlations. In other words, we understand the form or morphology a system has based on the connections between its elements. We do not understand exactly how the processes work to transfer energy and/or matter through the connections between the elements.

Cascading System - this is a system where we are primarily interested in the flow of energy and/or matter from one element to another and understand the processes that cause this movement. In a cascading system, we do not fully understand quantitative relationships that exist between elements related to the transfer of energy and/or matter.

Process-Response System - this is a system that integrates the characteristics of both morphological and cascading systems. In a process-response system, we can model the processes involved in the movement, storage, and transformation of energy and/or matter between system elements and we fully understand how the form of the system in terms of measured features and correlations.

Control System - a system that can be intelligently manipulated by the action of humans.
Ecosystem - is a system that models relationships and interactions between the various biotic and abiotic components making up a community or organisms and their surrounding physical environment.

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