__Logarithmic Scale:__ The decibel scale is a logarithmic scale
used to measure sound. Measured at some point *A*, the measurement
of the sound in decibels is

where *I*(*A*) is the sound intensity at the point *A*, and is a
reference intensity chosen so that the threshold of human hearing is
approximately 0 decibels.

__ExponentialGrowth:__ The simplest model for growth is exponential, where
it is assumed that
*y*'(*t*) is proportional to *y*. That is,

Separating the variables and integrating (see section 4.4 of the text), we have

so that

(If *y* is a positive quantity, we may drop the absolute value signs
around *y*.) Solving for *y*

which we may write in the form , where *A* is an arbitrary
positive constant. (Why?)

__Exponential Decay__ In a sample of a radioactive material, the
rate at which atoms decay is proportional to the amount of material present.
That is,

where *k*>0 is a constant. This is the same equation as in exponential growth,
except that -*k* replaces *k*. The solution is

where is a positive constant. Physically, is the amount of
material present at *t*=0.

Radioactivity is often expressed in terms of an element's half-life.
For example, the half-life of carbon-14 is 5730 years. This statement means
that for any given sample of , after 5730 years, half of it
will have undergone decay.
So, if the half-life is of an element Z is *c* years, it must be
that , so that and .

__Logistic Growth:__
One of the consequences of exponential growth is that
as . However, in some situations there is a limit *B*
to how large *y* can get. For example, the population of bacteria in
a laboratory culture, where the food supply is limited. In such situations,
the rate of growth slows as the population reaches the carrying capacity.
One useful model is the logistic growth model. It assumes that
the rate of growth is proportional to the product of the population
and the difference between the population and its upper limit. Thus we
model the growth with the differential equation

In the exercises you will use maple to solve this equation and work with
an example.

Tue Dec 10 14:03:34 EST 1996