11.10.07

Atomic Mouse Traps

I'm a TA for a freshman seminar called "Beyond Fossil Fuels" in which we discuss energy and sources of energy. It is my job to come up with an interesting demonstration to illustrate the concept we are studying that day. Today we talked about atomic energy. I decided to use a demo that I had seen on Mr. Wizard years ago: The Mouse Trap Bomb. The story goes that this demo was developed by Walt Disney in the 40's to educate the public on how an atomic bomb works by demonstrating an atomic reaction. I don't know if this is true but I have found some journal articles from the American Journal of Physics that I thought were useful:

1. Sutton, R. M., American Journal of Physics, 1947, 15, 427-428
2. Manley, J. H., American Journal of Physics, 1948, 16, 119-120
3. H. D. Rathergeber, American Journal of Physics, 1963, 31, 62

This demo loosely represents what happens during nuclear fission. The mouse traps represent fissionable atoms while the ping pong balls represent neutrons. When a free neutron (ping pong ball) encounters a fissionable atom (mouse trap with ping pong ball) it induces fission and releases more neutrons (ping pong balls).

This video is the result of several hours of setting up traps and having them go off at all the wrong times.




Ideally the traps would be fixed to the surface so that only ping pong balls would set off other traps. As these traps went off not only did we record video but we also made a separate audio recording. It can be seen here plotted from the .wav file in Mathematica.

We next recorded one trap going off which was then plotted in the same manner. In theory the many mouse trap explosion could be recreated by taking linear combinations of the single mouse trap and overlapping as time goes by.


Using those two data sets and Mathematica's awesome computing skills we checked to see if we could plot the number of traps firing vs time. To do this we did a correlation between the two data sets to shows the degree to which the two data sets are linearly related. The plot below shows the amount of correlation (the number of traps firing) vs time.



As you can see the rate at which the traps are firing increases rapidly and then dies off. This is very different from a normal chemical reaction (A+B-->C+D where A and B are reactants and C and D are products) . The rate of a chemical reaction is proportional to the concentration of the reactants. The reaction is fastest the instant the chemicals are mixed and slows down over time because the reactants are consumed over time and their concentration falls. The fission reaction of can be written as follows A+B --> C+D+2A. Where A is a neutron, B is a fissionable atom, C and D are the products of the split atom B. So the reaction produces starting material exponentially. The reaction dies off because we are using up B.


So while we may just enjoy the demo because we like seeing ping pong balls fly across the room, there is so much more we can see in it. And that's what makes it a great demo.

2 comments:

Sam said...

Fantastic! That sounds like a fun class. Are there any publicly available transcripts of that seminar? Since chemical engineering became a part of my life, I've been fond of all things energy-related.

David said...

It is a very fun class but unfortunately there are no transcripts for the class that I can send out.