Decay of Beer Foam
- An example of exponential decay that is easy to visualise.
- Practice at plotting and interpreting log graphs.
- An example of how mathematical models can be applied to different situations.
- To have fun pretending to have beer in class.
The problem with experiments on radioactive decay is that you need a source that has a reasonably small half life, I used to have a Protactinium generator which stopped producing any radiation several years ago and I have just discovered should be sent away for disposal. This was very convenient since it was activated by shaking and had a short half life. The problem was that I only had one (and one GM tube) so it would have to be done as part of a circus of 8 experiments. I also find that this sort of experiment doesn't help students to gain much insight into what is happening, all they see is the numbers on the meters and nothing else, what I want them to see is the nuclei decaying randomly, this could be done using a simulation but beer is much more fun albeit non alcoholic beer.
This practical uses very simple apparatus, a measuring cylinder and some strips of paper. The best size cylinder seems to be the medium sized 500ml variety, these allow a sizeable head and give a large change in height for a small change in volume. For ethical and financial (beer is very expensive in Norway) reasons I use non alcoholic beer (Clausthaler), this works very well but you could use ordinary beer. The size of the pen used to mark the levels is quite important but its best to let the students work that out for themselves. This method is best carried out in pairs or threes, if students have to work alone then you could use some sort of "5s beeper".
500ml measuring cylinder
paper strips and tape
The alternative and better method using video analysis also requires a video camera or webcam. Using this method reduces the amoint of beer needed since the whole class can videom the same cylinder of beer at the same time.
When the beer is poured everything happens rather quickly so if students aren't prepared they will miss the first 3 or 4 measurements, this could mess up the results since the first part of the time is the part that fits the exponential equation best. If you use warm beer and shake the bottle you will get a big enough head of foam from half a bottle, this allows for two runs from one bottle.
- A much better way to do this experiment is by videoing the beer. A lot of students have video cameras so could try this method if they think of it.
Conclusion and Evaluation
You can't compare the result with a data book value but the half life should be something like 50s depending on the type and condition of the beer. It might be an idea to use some sort of Peer review (NOS) to give some idea as to the validity of the results, after all this is how it works in real science. If the whole class gets a value of 43±1s then the value is quite well supported.
The first part of the data when the level is changing fast is often the best part, towards the end there can be no detectable change for 15 or 20 seconds. It seems that that the foam that sticks to the sides lasts longer than the foam in the middle but I'm not sure why.
The marking of the the zero (high point) at the end of the experiment is a bit dubious since there is still beer foam in the tube when this is done. This however is simply an arbitrary zero and shouldn't effect the outcome. Students may like to adjust their measurements after they see the results.
- comparing half life of different drinks
- constructing simulation in Excel to represent the random decay of nuclei