Systems and Models

What is a system? What is a model?

Feedback Mechanisms

Transfer vs. Transform

Laws of Thermodynamics

Systems

• A system is a collection of well-organised and well-integrated elements with perceptible attributes which establish relationshipsamong them within a defined space delimited by a boundary which necessarily transforms energy for its own functioning.

A Natural System - Ecosytem

• An ecosystem is a system whose organized and integratedelements (parts) transform (change) energy which is usedin the transformation process and recycling of matter in anattempt to preserve its structure and guarantee the survivalof all its biotic and abiotic characteristics.

• Although we tend to isolate systems by deleting/mergingthe boundaries, in reality such boundaries may not be exactor even real. Furthermore, one systems is always inconnection with another system with which it exchangesboth matter and energy.

• Question: How does this hold true for the universe?

Types of Systems

There are three types of systems based on

whether they exchange energy and/or matter:

Isolated System

System

It exchanges neither energy nor matter

Closed System

Energy System Energy

It only exchanges energy.

Open System

Energy Energy

System

Matter Matter

It exchanges both energy and matter.

Equilibria• The steady state is a common property of most open systems in

nature whereby the system state fluctuates around a certain pointwithout much change of its fundamental identity.

• Static equilibrium = no change at all.

• Dynamic equilibrium = a continuous move from one point toanother with the same magnitude, so no net change reallyhappens.

• Living systems (e.g. the human body, a plant, a population oftermites, a community of plants, animals and decomposers in theTropical Rainforest) neither remain static nor undergo harmonicfluctuations, instead living systems fluctuate almost unpredictablybut always around a mid value which is called the “steady state”.

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static equilibrium

Dynamic Equilibriu,

Steady State

Feedback Mechanisms (+/-)

The reaction of particular component elementsof the systems againts disturbing agents isconsider a feedback mechanism.

Two Types of Feedback Mechanisms:

Positive and Negative

Positive Feedback

• Positive feedback leads to increasing changein a system.

• Positive feedback amplifies or increaseschange; it leads to exponential deviation awayfrom an equilibrium.– For example, due to Global Warming high

temperatures increase evaporation leading tomore water vapour in the atmosphere. Watervapour is a greenhouse gas which traps more heatworsening Global Warming.

– In positive feedback, changes are reinforced. Thistakes ecosystems to new positions.

Negative Feedback• Negative feedback is a self-regulating method of control

leading to the maintenance of a steady state equilibrium.• Negative feedback counteracts deviations from the steady

state equilibrium point.• Negative feedback tends to damp down, neutralise or

counteract any deviation from an equilibrium, and promotesstability.– In this example, when the Hare population increases, the Lynx

population increases too in response to the increase in food offer which illustrates both Bottom-Up regulation and Positive Feedback.

– However, when the Lynx population increases too much, the large number of lynxes will pray more hares reducing the number of hares. As hares become fewer, some lynxes will die of starvation regulating the number of lynx in the population. This illustrates both Top-Down and negative Feedback regulation.

Transfer vs. Transformations

• Transfers normally flow through a system from onecompartment to another and involve a change inlocation.

– For example, precipitation involves the change in location ofwater from clouds to sea or ground. Similarly, liquid water inthe soil is transferred into the plant body through roots inthe same liquid form.

• Transformations lead to an interaction within a system in the formation of a new end product, or involve a change of state. – For example, the evaporation of sea water involves

the absorption of heat energy from the air so it can change into water vapour.

– In cell respiration, carbon in glucose changes to carbon in carbon dioxide. Ammonia (NH3) in the soil are absorbed by plant roots and in the plant nitrates are transformed into Amino acids. During photosynthesis carbon in the form of CO2 is changed into carbon in the form of Glucose (C6H12O6).These are just some example of transformations.

Laws of Thermodynamics

Two laws:

1st -

Models• A model is an artificial construction designed

to represent the properties, behavior orrelationships between individual parts of thewhole being studied or the order in which tostudy it under controlled conditions and tomake predictions about its functioning whenone or more elements and/or conditions arechanged.

• A model is a representation of a part of thereal world which helps us understandcomplexities large and small.

Limitations of Models• Models are simplifications of real systems. They can be used as tools to better

understand a system and to make predictions of what will happen to all of the

system components following a disturbance or a change in any one of them.

– The human brain cannot keep track of an array of complex interactions all at

one time, but it can easily understand individual interactions one at a time.

– Models are proposed representations of how a system is structured, which can

be rejected in light of contradictory evidence. (hypothesis)

• No model is a 'perfect' representation of the system because, as mentioned

above, all models are simplifications and in some cases over simplified.

– human subjectivity may lead to humans to make models biased by scholar

background, disregard of the relevance of some components or simply by a

limited perception or understanding of the reality which is to be modeled.