Oct. 28, 2011

Science Dad on the Weight of the Atmosphere

by Vince Harriman

Click to enlarge images

As much as Beckett loves science, he also loves magic. So we decided to combine the two and see what kind of science-based magic tricks we could have fun with. My first question for Beckett was how much does air weigh? It seems like an impossible question for a young kid -- first, it doesn't seem like air weighs anything, then second, it seems impossible to try to weigh it. So we decided to see if we could find other ways to feel the weight of the air. Remember that both science and magic is best done with adult supervision -- and adult participation!

We started with a classic magic trick: Can you pick up a coin in a pool of water without getting your fingers wet? The set up is simple: put a coin on a plate, light a candle, and submerge the coin with water. Challenge the audience to pick up the coin without getting fingers wet. Finally, place a glass over the candle -- but not the coin -- and wait until the candle uses up all the oxygen in the glass:

What happens is that as the candle burns all the oxygen that is in the glass, two things happen: the glass is fills with carbon dioxide, and the candle heats the water vapor in the air so it becomes warmer than room temperature. When the candle goes out and is no longer heating the air and water vapor, the moisture in the warm air condenses on the inside of the glass and becomes liquid. As it does so, the air pressure in the glass goes down and the air pressure of the atmosphere outside the glass now pushes the water into the glass -- the area of lower pressure. Why? Because even though we can't feel the weight of air, it does have weight that exerts pressure. In fact, air pressure at sea level is about 14.7 pounds per square inch (14.7psi). Just lowering the pressure in the glass a little is enough to move the water.

Now that we could actually see air pushing down, we decided to see if air could also push something up. For this trick, all you need is a glass of water and a sturdy (not so absorbant) piece of paper or card stock. Becket began by demonstrating the strength of gravity -- pouring water from one glass to another and then dropping the paper. Then he placed the paper on the glass and carefully turned it upside down, and that's when the magic happens:

The way this trick works is remarkably similar to the first: when turning the glass over, a tiny bit of water squeezes out, creating a lower pressure in the glass than in the atmosphere outside the glass. There is also a tiny bit of surface tension and water molecule cohesion holding the paper to the glass, but the water is sitting on top of the high pressure air below it!

So just how much does air weigh if you just want to weigh it? We decided to find out. The easiest way to weigh air would be to put some in a bottle and throw it on a scale! Beckett found the family scale while I was getting a compressor from the basement. We started by weighing the empty tank:

The empty air tank

While it is hard to read the scale, without air it weighed 28.2 pounds. We plugged in the compressor in and pumped it up to 150 pounds per square inch then weighed it again:

the filled tank

When we filled it up, it weighed .4 pounds more! The air in the tank was pressurized to ten times the air pressure of the atmosphere, and all those extra molecules of nitrogen, oxygen, carbon dioxide and, in our case, candle smoke added up to almost half a pound. Now we were not using a scientific scale, so we weighed several times and took an average. The photograph shows the scale when we were actually on the average, but you can see that there is a measurable difference between an empty tank and a full tank.


About Vince Harriman

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.

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