Throwing Stones at Bottles

We’ve almost finished assembling our system. When completely constructed, it will, through a multi-step process, take pressure from dry ice and output electricity; all that’s left is the acquisition of an air ratchet with a greater RPM than the one we currently possess. In the meantime, we had some fun determining the pressure a Pepsi bottle can withstand, by placing different amounts of dry ice in three bottles, sealing them, and waiting to see if they burst from the pressure.James 2a Our first bottle contained 50 grams of dry ice and exploded within four hours. The bottle we tested at the same time as that one contained about 37 grams, and it appeared that those 37 grams were insufficient to burst it. We had to manually release the pressure by unscrewing the cap, and it seems that some seal was created by the pressure because the cap was very difficult to remove. James 2bA third bottle that we placed later than the aforementioned two suffered a pressure leak. We were unaware of this and spent several tense minutes trying to release the presumed pressure without endangering ourselves (including our employing such scientifically sound methods as throwing small stones at the bottle). Eventually, we recognized that the carbon dioxide had in fact vacated the bottle, and were immensely relieved.

Why must gases be so complex?

177_0285We’ve been researching various gas and fluid laws relevant to our project, including the Ideal Gas Law, the combined gas law, and Bernoulli’s principle. Even in their (at first glance) cryptic forms, these formulae make numerous assumptions about gasses, such as that they are composed of non-interacting massless particles, which is only ever an approximation of the messiness of the real world. We’ll have to take many of these laws into account when deciding various parameters for our engine, such as the range of amounts of dry ice we’ll use and how much pressure we should expect after the gas flows through a tube. Much calculation is still required before we can apply these laws to a physical engine.