# Imperial College/Courses/Spring2008/Synthetic Biology/Computer Modelling Practicals/Exponential Decay Model

<html> <body> <!-- Start of StatCounter Code --> <script type="text/javascript"> var sc_project=3315875; var sc_invisible=0; var sc_partition=36; var sc_security="779debd0"; </script> <script type="text/javascript" src="http://www.statcounter.com/counter/counter_xhtml.js"></script><noscript><div class="statcounter"><a class="statcounter" href="http://www.statcounter.com/"><img class="statcounter" src="http://c37.statcounter.com/3315875/0/779debd0/0/" alt="web metrics" /></a></div></noscript> <!-- End of StatCounter Code --> </body> </html>

**Exponential Decay Model**

A quantity is said to be subject to exponential decay if it decreases at a rate proportional to its value. Symbolically, this can be expressed as the following differential equation, where N is the quantity and λ is a positive number called the decay constant.

The equation that describes exponential decay is

Integrating, we have

- where D is the constant of integration.

where . If we evaluate this equation at , we see that .

so, we have :

### Half-Life

A more intuitive characteristic of exponential decay for many people is the time required for the decaying quantity to fall to one half of its initial value. This time is called the **half-life**, and often denoted by the symbol . The half-life can be written in terms of the decay constant, or the mean lifetime, as:

When , the exponential is equal to 1, and is equal to . As approaches infinity, the exponential approaches zero. In particular, there is a time such that

Substituting into the formula above, we have