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SYSTEMS – A GUIDE TO TERMS
Examples of systems:
Ecosystems, weather systems, food production systems, sewage treatment systems,
photosynthesis, education systems, information distribution systems, your
body………
All systems have:
 Inputs
o usually energy or materials or both
 Outputs
o usually energy, materials or both
o may be useful eg a crop
o may be a problem eg pollution
When a system is in equilibrium:
inputs = outputs
eg The temperature of our planet will remain constant, provided that
energy inputs = energy outputs.
If we reduce the outputs of energy by increasing levels of greenhouse gases which
trap heat energy, the temperature rises (global warming).
There are 3 main types of systems:
System
Energy
Materials
type
Inputs and outputs
Inputs and outputs
open
closed
Inputs and outputs
Cycle round within
system
isolated
No inputs or outputs
No inputs or
outputs
Examples
Most common type of
system - includes most
ecosystems, food
production systems and
many others
Earth is almost a closed
system; aquarium with
sealed lid; biosphere
domes
An abstract concept difficult to find real life
examples – possibly the
universe is an example
Systems and the Laws of Thermodynamics
The First Law of Thermodynamics tells us:
“Energy is never created or destroyed, only changed from one form to another”
This means that when energy flows through a system, the total amount of energy
never changes. For example in an ecosystem light energy from the sun is changed to
stored chemical energy and eventually it all turns to heat energy, but no energy is
“lost”.
The Second Law of Thermodynamics tells us that entropy, which is a measure of
the amount of disorder or chaos in a system always increases. All energy changes
release some energy as heat, and eventually all the energy in the universe will have
turned to low grade heat energy. This spreading out of energy increases the amount of
disorder and eventually (billions of years hence) we shall be left with a lukewarm
universe and no way of doing any useful work
Feedback mechanisms in systems
 feedback occurs when the output of a system influences the inputs and hence
affects the state of the system
 positive feedback changes a system to a new state. In most natural systems it
is considered a bad thing, but it is not always so! The term “vicious cycle”
usually refers to positive feedback.
o Ex. rising global temperatures melt frozen ground at high latitudes;
waterlogged ground releases methane gas; methane adds to green
house effect, increasing global warming further; more frozen ground
melts releasing more methane.
o Ex. teacher encourages student with positive feedback comments on
his/her work; student associates learning with being praised so wants to
learn more and produces a higher standard of work (a new state!).
Student gains confidence from achievement, passes exams and leaves
for university.

negative feedback returns a system to its original state. In many natural
systems homeostasis acts as a form of negative feedback . In most natural
systems, negative feedback is considered a good thing, because it restores
equilibrium.
o Ex. as a population of mice increases, there is less food to go around,
so some animals starve, reducing population back to sustainable levels
o Ex. if global temperatures rise because of increased levels of
greenhouse gases, evaporation rates from the sea surface will increase;
clouds may increase and more snow may fall in high latitudes. This
would increase the amount of sunlight reflected back to space, so the
planet would cool down to its original temperature
o Ex. a teacher writes only negative comments on a student’s work. The
student feels they will never succeed, so puts less and less effort into
studying, and eventually gives up and drops out of college and returns
to an uneducated state, turns to a life of crime...
o Most negative feedback mechanisms involving populations of
organisms depend on density dependent factors ie with increasing
population size, food and other resources are spread more thinly but