Subject: Utilizing STELLA in High School

Posted by Prof. Dr. Niall Palfreyman on 4/11/2006
In Reply To:Utilizing STELLA in High School Posted by Alex Leus on 4/11/2006

Message:

Hi Alex,

For an introduction to SD I make sure I know the topics which interest the people I'm talking to: if they're interested in biology I go that route, and for physics I take another route. I then build a quick model within about 10 minutes which yields useful results in that area of interest.

One example I've often found successful is a simplified version of Will Glass-Husain's alcohol degradation model: a glass of wine contains about 4ml of alcohol, and I've just drunk 3 glasses of the stuff. The problem to solve is: when will I be able to drive back home? The legal limit for driving in Germany is 0.05% blood alcohol, and my body contains about 6 litres of blood. My liver processes 1ml of alcohol at a rate of about 0.3ml/hr. I also prepare beforehand a partial Stella model containing one empty stock (Blood alcohol), one empty converter (legal limit) and a graph containing both of these components, both prescaled to a maximum of 12ml (the initial value for Blood alcohol). I then let the participants calculate the amount of blood alcohol corresponding to the legal limit, and also how much blood alcohol my body now contains, and we enter these values in the prepared model. I ask how alcohol is degraded in the body (in the liver), and enter this as an outflow from Blood alcohol named degradation. Then I motivate the idea of the rate of alcohol degradation being proportional to the current level of Blood alcohol, and give the figure of 0.3 ml/hr. We connect Blood alcohol to degradation, and hard-code the 0.3 ml/hr into the expression for the flow. Finally, I ask participants what they expect to see if we run the model, and a variety of answers come back (linear decay, exponential decay), and we speculate how long it will take for Blood alcohol to fall below the legal limit. We then run the model and discover how long I have to wait before driving.

This will run through in 10 minutes once you have it off pat (and being sure to use the prepared model). In the de-briefing we discuss further applications of this model, e.g.: How often would I have to inject insulin as a diabetic?

Good luck with your presentation!
Niall Palfreyman.