## System Dynamics Modules

*First Order Linear Positive Feedback*

A first-order system contains only one stock (level or accumulator).
Systems are described as linear systems when the algebraic expressions
controlling rate(s) of flow into the system, that is, creating
a change (at any point in time) in the state
variable ‘*S*’ *STATE OF SYSTEM* are simply the product of
a constant and the current value of *S*. Rate of
change of state is the product of the current state
of the system and a constant.

A first-order positive feedback
loop has the effect (each time we calculate it) of
adding to *S* an amount (where the constant is a
Fractional Growth Factor ‘*g*’) which is larger than the previous
time we calculated it. Consider a series of calculations at
times *t0*, * t1*, *t2* ... *tn*, with the initial
value of *S* being *S0* and a *Fractional Growth Factor*
*g* operating on the inflow.

The description of the module is
at

The module coded in Powersim *Studio* is
at .

A PowerSim Example using this module is
at .

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