EQUILIBRIUM AND THRESHOLD
CLASS LECTURE
BY
DR SOMABHATTACHARYA
EQUILIBRIUM
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Is a state of balance in any system created by a variety of forces in which the state will remain
unchanged through time unless the controlling forces change.
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Many open system are self regulatory in the sense that, when change is introduced through one of the
input adjustment occurs via a negative feedback so as to absorb the effects of the change
DIFFERENT TYPES OF EQUILIBRIUM
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STATIC EQUILIBRIUM
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No observable change
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This is a state achieved by a body or system when the balance of forces acting upon it requires no
movement in order to maintain or restore the balance
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Thus the body or system remained stationary in respect of its surroundings
STABLE EQUILIBRIUM
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After disturbance return to its former state
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i.e. a condition in a system when it has a tendency to recover its original state after being disturbed by
external forces
UNSTABLE EQUILIBRIUM
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Minor disturbances trigger change to a completely new equilibrium
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A condition in a system when a small disturbance leads to an even greater disturbance. This condition
is usually terminated by the achievement of a new stage of stable equilibrium.
METASTABLE EQUILIBRIUM
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Absorbs minor disturbances but when these exceed to a critical value triggers to completely new
equilibrium
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i.e. a condition in which a stable equilibrium will prevail untill a suitable catalyst or trigger carries the
system state over a threshold into a new equilibrium regime
STEADY STATE EQUILIBRIUM
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Long term stability but short term fluctuations which are often of a random nature.
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The condition in an open system in which properties are invariant when considered with reference to a
given time scale, but wherein its instantaneous condition may oscillate owing to the presence of
interacting variables.
THERMODYNAMIC EQUILIBRIUM
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Progressive diminution of free energy i.e. towards a condition of maximum entropy
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There will be an equal probability of encountering given states, events or energy levels throughout the
system
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Additionally there will be a slow decay of free energy leading to an extinction of energy differences
capable of performing work within the system
DYNAMIC EQUILIBRIUM
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Regular long term trend on which are superimposed short term fluctuations, often of a random nature
DYNAMIC METASTABLE EQUILIBRIUM
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A long term trend but with potential threshold that when exceeded will initiate a new dynamic
equilibrium
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Owing to the long term trend the value of critical threshold may change with time
OTHER EQUILIBRIUM CONDITIONS
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POISED EQUILIBRIUM: the condition occurring when the opposing forces are balanced
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QUASI EQUILIBRIUM: a state of near equilibrium reached when a system moves towards a steady state
equilibrium but where absolute equilibrium is never reached in the face of a constantly changing gross
energy environment.
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STATIONARY EQUILIBRIUM: the condition occurring when opposite velocities are balanced
THRESHOLD
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THRESHOLD is a transformation from one state to another i.e. it is a condition marking the transition
from one state of operation of a system to another thereby complicating the self-regulation
mechanism of negative feedback
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The passage of a system across a threshold is often irreversible and even the change of one variable
may force the entire system to adjust to a radically different equilibrium
INTRINSIC AND EXTRINSIC THRESHOLD
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The conditions of transformation from one state to another have been divided into two types:
i) Intrinsic Threshold
ii) Extrinsic Threshold
INTRINSIC THRESHOLD
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Implies that the changes can take place within the system without a change in an external variable.
Here the geomorphic system itself evolves to a critical condition
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Example – surging glaciers, river channel patters, river terrace changes (see dictionary- Goudie
EXTINSIC THRESHOLD
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An extrinsic threshold describes an abrupt change which is triggered by a progressive change in an
external variable
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Obvious extrinsic threshold are those arising from climatic, tectonic or land use alternations which
affect the inputs of either energy or mass or both into the system
RELAXATION TIME
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IS THE TIME TAKEN BY A SYSTEM TO SET IT INTO A NEW EQUILIBRIUM CONDITION AFTER BEING
THROWN OUT OF BALANCE FROM THE EXISTING EQUILIBRIUM CONDITION