Next: Exercises Up: ExponentialLogarithmic, and Logistic Previous: Introduction

## Background

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.

Next: Exercises Up: ExponentialLogarithmic, and Logistic Previous: Introduction

Sean O Anderson
Tue Dec 10 14:03:34 EST 1996