If you missed it, here is the first Children’s Museum of Houston “O Wow Moments” video that appeared on KPRC Channel 2 News. My “O Wow” moments will be shown each week on Saturdays around 7:15am, so be sure to get up early (or set your DVR/Tivo) to record it!
In this one, we are discussing air pressure. We have many miles of air sitting on top of us. The actual amount varies depending on temperature, etc. – air is a fluid like water, after all, so it does sort of “slosh” around. But, on average, 99% of our air extends about 31 miles above sea level (in comparison, Mt. Everest is only 5.5 miles). With all this air pressing down, we experience about 14.7 pounds of pressure per square inch. Based on the surface area of your average 8 year old, that’s equivalent to the weight of a school bus! So why aren’t we squished? Simple – our bodies push back – we evolved with a certain fluid pressure internally that presses back against the 14.7 lbs/sq. in. In fact, we get altitude sickness when we’re up too high because of that pressure!
What to Do:
- Place about a tablespoon of water inside an empty aluminum can. No need to wrap with foil like we did. We were just covering the brand.
- Place it on the stove and turn the heat to high.
- While the can is heating, fill a bowl with cold water – the colder, the better.
- When you see steam, use tongs to quickly flip the can over upside down into the water. Do NOT touch the can until it has a chance to cool!
- If it doesn’t crush, you may have either taken too long to flip it, didn’t fully submerge the hole into the water, or pressed the can too far against the bottom. Reheat it and try it again.
What we did here was to heat up the air. Heated air expands because warmer air can exert more pressure. Think of it this way – the molecules moving inside the can move faster and faster when heated. All this motion causes them to press harder against each other and the sides of the can. BUT, due to the opening on top, some of the molecules leave the can so the pressure never really rises inside the can. If it were sealed, the pressure would rise until it burst.
When the can is flipped over, we seal the hole. At the same time, the air inside cools, so the molecules aren’t pressing as hard against the sides of the can and the pressure inside drops. But, with the hole sealed, the outside air can’t get inside to equalize the pressure, so the air simply crushes the can (and shoves water up into it). Had the can just been left out to cool, it wouldn’t have crushed.