Types of Systems

Types of systems

Systems can be classified as open, closed, or isolated. Open systems allow energy and mass to pass across the system boundary. A closed system allows energy but not mass across its system boundary. An isolated system allows neither mass or energy to pass across the system boundary.

Open Systems

Figure ES.9 The ocean is an open system

The ocean is an example of an open system. The ocean is a component of the hydrosphere and the ocean surface represents the interface between the hydrosphere and the atmosphere that lies above. Solar radiation passes through the atmosphere and is absorbed by the ocean. The absorbed energy evaporates water from the ocean. As the water vapor (mass) enters the atmosphere it carries with it the heat used to evaporate the water (called latent heat). When the water vapor enters the air it raises the air's humidity. If the humidity is high enough, condensation occurs, latent heat is released, and clouds are created. Continued condensation creates precipitation (mass) that falls back into the ocean. Hence, energy and heat (solar radiation, latent heat) as well as mass (water vapor and precipitation) passes across the boundary between the atmosphere and hydrosphere.

Closed Systems

Figure ES.9 The Earth as a closed system
(Click image to enlarge)

The Earth system as a whole is a closed system. The boundary of the Earth system is the outer edge of the atmosphere. Virtually no mass is exchanged between the Earth system and the rest of the universe (except for an occasional meteorite). However, energy in the form of solar radiation passes from the Sun, through the atmosphere to the surface. The Earth in turn emits radiation back out to space across the system boundary. Hence, energy passes across the Earth's system boundary, but not mass, making it a closed system.

The interface between systems is not always easy to identify. Others are easier to recognize. The interface between the hydrosphere and lithosphere at a shoreline is easy to recognize as a definite planar boundary between a solid and fluid. The interface between the atmosphere and hydrosphere is less easy to discern as the hydrosphere comprises both liquid water of the surface and water held in the air.

Earth as a system

Figure ES.10 The Earth System (after Christopherson, 2005)

The Earth system (left) is a complex functioning system that includes all the components of the various "spheres" like the solid Earth or geosphere (lithosphere in Figure ES.10), the gaseous envelope surrounding the Earth that is the atmosphere, biosphere comprised of all living organisms and the hydrosphere or "water sphere". The Earth system diagram shows arrows representing flows of energy and mass that connect and intertwine the four spheres. At the top, solar energy drives many of the environmental processes operating in the four spheres. Additional sources of energy to drive Earth systems come from the Earth's internal heat engine and the gravitational attraction of the moon. Though the Earth system is a closed system, the "spheres" that comprise it are open. That is, there is a constant cycling of energy and mass between the hydrosphere, lithosphere, atmosphere, and biosphere.

National Geographic's "Science of Summer" looks at several physical processes operating in the Earth system.

Self Assessment

What kind of system is a terrarium that has a tightly sealed glass lid? Why?

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